pii: sp- 00211-16
http://dx.doi.org/10.5665/sleep.6304
SLEEP- DISORDERED BREATHING
Arousal Intensity is a Distinct Pathophysiological Trait in Obstructive Sleep Apnea Jason Amatoury, PhD1; Ali Azarbarzin, PhD5; Magdy Younes, MD, PhD2,3; Amy S. Jordan, PhD4; Andrew Wellman, MD, PhD5; Danny J. Eckert, PhD1 1 Neuroscience Research Australia (NeuRA), and the School of Medical Sciences, University of New South Wales, Sydney, NSW, Australia; 2YRT Ltd, Winnipeg, Manitoba, Canada; 3Sleep Disorders Centre, University of Manitoba, Winnipeg, Manitoba, Canada; 4Institute for Breathing and Sleep, and Melbourne School of Physiological Sciences, University of Melbourne, Melbourne, Victoria, Australia; 5Division of Sleep Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA
Study Objectives: Arousals from sleep vary in duration and intensity. Accordingly, the physiological consequences of different types of arousals may also vary. Factors that influence arousal intensity are only partly understood. This study aimed to determine if arousal intensity is mediated by the strength of the preceding respiratory stimulus, and investigate other factors mediating arousal intensity and its role on post-arousal ventilatory and pharyngeal muscle responses. Methods: Data were acquired in 71 adults (17 controls, 54 obstructive sleep apnea patients) instrumented with polysomnography equipment plus genioglossus and tensor palatini electromyography (EMG), a nasal mask and pneumotachograph, and an epiglottic pressure sensor. Transient reductions in CPAP were delivered during sleep to induce respiratory-related arousals. Arousal intensity was measured using a validated 10-point scale. Results: Average arousal intensity was not related to the magnitude of the preceding respiratory stimuli but was positively associated with arousal duration, time to arousal, rate of change in epiglottic pressure and negatively with BMI (R2 > 0.10, P ≤ 0.006). High (> 5) intensity arousals caused greater ventilatory responses than low (≤ 5) intensity arousals (10.9 [6.8–14.5] vs. 7.8 [4.7–12.9] L/min; P = 0.036) and greater increases in tensor palatini EMG (10 [3–17] vs. 6 [2–11]%max; P = 0.031), with less pronounced increases in genioglossus EMG. Conclusions: Average arousal intensity is independent of the preceding respiratory stimulus. This is consistent with arousal intensity being a distinct trait. Respiratory and pharyngeal muscle responses increase with arousal intensity. Thus, patients with higher arousal intensities may be more prone to respiratory control instability. These findings are important for sleep apnea pathogenesis. Keywords: arousal threshold, genioglossus, tensor palatini, sleep-disordered breathing, respiratory physiology Citation: Amatoury J, Azarbarzin A, Younes M, Jordan AS, Wellman A, Eckert DJ. Arousal intensity is a distinct pathophysiological trait in obstructive sleep apnea. SLEEP 2016;39(12):2091–2100. Significance As per American Academy of Sleep Medicine scoring rules, cortical arousals from sleep are quantified as an all or none phenomenon, yet not all cortical arousals are the same. The concept of quantifying the intensity of arousals has recently been described, but until now the factors influencing arousal intensity and its potential role in sleep apnea pathogenesis were unclear. This study demonstrates a new concept in OSA pathogenesis. Specifically, arousal intensity is a distinct trait and an important mediator of ventilatory and pharyngeal dilator muscle responses to arousal. Thus, there is a need to reconsider the current all or none approach to defining cortical arousals from sleep.
INTRODUCTION Cortical arousals from sleep occur frequently in sleep-disordered breathing. In adult obstructive sleep apnea (OSA), the most common sleep-related breathing disorder, cortical arousals often occur in association with the end of respiratory events.1,2 Arousals fragment sleep, contribute to excessive daytime sleepiness, and are involved in the pathogenesis of OSA.2–4 According to the American Academy of Sleep Medicine scoring rules,5 cortical arousals are scored as all or none phenomena, defined as a sudden increase in electroencephalogram (EEG) frequency lasting > 3 seconds when preceded with ≥ 10 seconds of stable sleep.5 However, not all cortical arousals that meet this definition are the same. Indeed, arousals vary in duration and intensity.2,6,7 Accordingly, the physiological and potential next day consequences of different types of arousals may also vary. In support of this concept, Younes demonstrated that airflow responses to cortical arousal at the end of an obstructive event are higher with increasing arousal intensity when scored visually using a 0 to 4 scale.2 An unnecessarily large increase in ventilation following arousal can contribute to subsequent hypocapnia, destabilization of the ventilatory control system, and the cyclical breathing pattern that occurs in OSA.2,8,9 More recently, Azabarzin and colleagues used an SLEEP, Vol. 39, No. 12, 2016
automated method to quantify arousal intensity over a wider range (0 to 9).6,7 Average arousal intensities were found to vary between individuals and average arousal intensity strongly correlated with heart rate responses to arousal.6,7 Thus, while comprehensive assessments of minute ventilation per se and its key determinants, such as dilator muscle activity, have not been performed, the physiological consequences of arousal appear to be stronger with increased arousal intensity. The factors that influence average arousal intensity in OSA are only partly understood.6,7 A high propensity for arousal (low respiratory arousal threshold) is a phenotypic contributor to OSA.2,3,10 Whether the strength of the respiratory stimulus prior to arousal (i.e., arousal threshold) and other associated factors (e.g., hypoxemia and the degree of airflow limitation) influence arousal intensity remains unknown. This is important because if arousal intensity is independent of the strength of the preceding respiratory stimulus, it would indicate that arousal intensity is a distinct trait. Similarly, it remains unclear if non-respiratory stimulus related factors, including anthropometric characteristics, importantly influence average arousal intensity. Thus, this study aimed to determine if average arousal intensity is related to the strength of the preceding respiratory
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Table 1—Anthropometric and sleep parameters for the study participants. Gender (M/F) Age (years) BMI (kg/m2) Mean AHI (events/h sleep) Range AHI (events/h sleep)
All Subjects (n = 71) 48/23 45.1 ± 11.9 32.8 ± 6.8 37.3 ± 32 0–112
Controls (n = 17) 8/9 37 ± 12 26 ± 3 4±3 0–10
OSA Patients (n = 54) 40/14 47 ± 11* 35 ± 6* 48 ± 30* 11–112
Data are mean ± SD, where applicable. *P < 0.05 for control vs. OSA. BMI, body mass index. AHI, apnea-hypopnea index.
stimulus. Secondary aims were to examine non-respiratory stimulus related factors that determine arousal intensity and the role of arousal intensity on post-arousal ventilatory responses and upper airway dilator muscle activity. METHODS Data for the current study were acquired as part of a larger study in 75 participants to quantify the key pathophysiological traits causing OSA.10 The current findings have not been previously reported. The experimental methodology has been described in detail previously.10 Participants Data were obtained in 71 individuals (47 men) including 17 healthy controls (AHI ≤ 10 events/h sleep) and 54 patients with OSA (AHI > 10 events/h sleep) who had been treated with CPAP for ≥ 3 months (Table 1). Four subjects were excluded from the original data set10 as they did not meet the criteria for one or more of the current study analyses. All subjects provided informed written consent and the protocol was approved by the Partners’ Healthcare Institutional Review Board. Experimental Design and Measurements Initially, participants underwent standard overnight polysomnography off CPAP to quantify the AHI.10 Approximately 1–2 weeks later, a detailed overnight upper airway physiology study was performed. Electroencephalograms (EEGs), electroculograms (EOGs), and chin electromyogram (EMG) were acquired for scoring of sleep stages and arousals. Genioglossus and tensor palatini EMG recordings (EMGgg and EMGtp) were made using fine-wire intramuscular electrodes (Cooner Wire Company, Chatsworth, CA).10,11 Epiglottic pressure (Pepi) was measured using a transducer-tipped pressure catheter (Millar MCP-500; Millar Instruments, Houston, TX), as described previously.10 A nasal CPAP mask was fitted and mask pressure and airflow were measured with differential pressure transducers (Validyne Corporation, Northbridge, CA) and a pneumotachograph (Hans Rudolf 3700A; Hans Rudolf Inc., Kansas City, MO).10 Protocol Participants were studied supine on CPAP. Transient reductions in CPAP from the holding pressure were applied for ≤ 3 minutes during stable sleep to cause varying degrees of upper airway collapse to induce respiratory-related arousals.10 SLEEP, Vol. 39, No. 12, 2016
Data Analysis Figure 1 shows an example CPAP reduction and quantification of several of the key study measurements. All reductions in CPAP that terminated in a respiratory-related arousal were included in the analyses. To be considered a respiratory-related arousal, reductions in CPAP that resulted in cortical arousals had to: (1) last > 10 s prior to arousal and (2) be associated with an increase in nadir negative Pepi > 2 cm H2O within the 30 s preceding cortical arousal. Arousal intensity was measured on a scale between 0 and 9 (most intense) using a validated automated wavelet transformation method (Figure 2).6,7 For arousals > 15 s, only the first 15 s was used to calculate arousal intensity. Arousals were divided into low (≤ 5) and high (> 5) intensity categories, similar to previous methodology.12 Pre-arousal respiratory stimulus related variables considered to contribute to arousal were quantified (Figure 1), including: CPAP reduction magnitude (∆CPAP); time to arousal (following reduction in CPAP); respiratory arousal threshold, defined as the nadir epiglottic pressure immediately preceding arousal3,10; rate of change of nadir epiglottic pressure (∆Pepi/∆t) from the first breath following CPAP reduction to the breath immediately prior to arousal; minute ventilation (Vi) and peak inspiratory flow (PIF), measured from the breath immediately preceding arousal; and the minimum blood arterial oxygen saturation caused by the reduction in CPAP (NadirSaO2). Steady state loop gain (sensitivity of the ventilatory control system) was also quantified according to methodology previously described.10 Arousal duration, body mass index (BMI), OSA status, sex, and age were also recorded to enable comparison of these non-stimulus related variables with arousal intensity. Raw EMGgg and EMGtp signals were rectified, movingtime-averaged (100-ms window) and expressed as a percent of maximum for each subject (Figure 1).11 To determine the influence of arousal intensity on respiratory and muscle responses during NREM sleep, Pepi, Vi, PIF, and peak and tonic EMGgg and EMGtp for three breaths prior to arousal (breaths −3 to −1) and 3 breaths following arousal (breaths +1 to +3) were quantified and compared between low and high arousal intensities. Statistical Approach Univariate and multiple linear regression analyses were performed (SPSS, v23, IBM Corp.) to assess the influence of prearousal stimuli and non-stimulus related variables on arousal intensity. Pre-arousal stimuli (∆CPAP, time to arousal, arousal threshold, ∆Pepi/∆t, Vi, PIF and NadirSaO2) and non-stimulus related variables (arousal duration, OSA status, sex, age and
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Figure 1—Raw data example illustrating a transient reduction in CPAP (∆CPAP; sudden drop in mask pressure, Pmask) and quantification of several of the key outcome variables. A respiratory-related cortical arousal during the CPAP reduction is shown highlighted on the EEG trace. Vertical bar highlight indicates the breath immediately preceding arousal where arousal threshold [nadir epiglottic pressure (Pepi)] was quantified (as shown), in addition to pre-arousal peak inspiratory flow (PIF) and minute ventilation. The rate of change of Pepi (∆Pepi/∆t) was quantified from the first breath following CPAP reduction to the arousal threshold breath (slope of dashed line shown on Pepi channel). PIF, peak and tonic genioglossus (GG) and tensor palatini (TP) EMG (EMGgg and EMGtp) are indicated for a single breath following arousal. MTA = 100 millisecond moving-time-average of the rectified raw EMG. SaO2 = oxygen saturation. NadirSaO2 = minimum blood arterial oxygen saturation caused by the reduction in CPAP. Direction of inspiration indicated on Pepi and Flow traces.
Figure 2—Three examples of cortical arousal (highlighted in gray) with different arousal intensity (AI), analyzed using an automated validated tool from C3/A2 and O2/A1 EEGs.
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BMI) were included in separate models. A final multiple regression analysis was performed including all variables with backward correction applied to eliminate the variable with the highest P value until only significant variables remained. Two-way ANOVA was used to determine the effect of breath number and arousal intensity on ventilatory and muscle parameters. Unpaired two-tailed t-tests/Mann-Whitney tests were performed for between group comparisons for the key study outcomes (Prism, v6.05, Graphpad Software Inc.). Data are reported as mean ± SD or median (25th–75th percentile). Statistical significance was inferred at P < 0.05. RESULTS Table 1 summarizes the anthropometric and sleep characteristics of the study participants. A total of 487 reductions in CPAP that resulted in a respiratory-related cortical arousal were analyzed across all subjects, with a median of 6 (3–8) per subject. The average reduction in CPAP was similar for controls and OSA patients (6.0 ± 1.8 vs. 7.3 ± 2.5 cm H2O; P = 0.053). However, the absolute level of CPAP during the drop was significantly greater in the OSA patients compared to controls (4.1 ± 2.7 vs. −0.7 ± 2.4 cm H2O; P < 0.0001), as
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Arousal Intensity The frequency distribution of average arousal intensity per subject is shown in Figure 3. Most subjects fell within the arousal intensity band of 5–6 (25.4%). No subject was classified with an arousal intensity of 0 or 1. Only 4.2% of subjects fell within the highest arousal intensity band of 8–9. The average arousal intensity for the group was 5.3 ± 1.5. Arousal intensity tended to be higher in control subjects compared to patients with OSA. However, this difference was not statistically significant (5.9 ± 1.6 vs. 5.1 ± 1.5, P = 0.08). There was also no difference in arousal intensity between NREM and REM sleep (5.3 ± 1.6 vs. 5.5 ± 2.2, P = 0.48). The average arousal intensity for the low (≤ 5) versus high (> 5) intensity arousal categories was 3.4 ± 0.9 vs. 7.4 ± 0.8 (P < 0.0001). Factors Influencing Arousal Intensity Pre-Arousal Respiratory Stimulus Related Factors
Pre-arousal stimulus data, including for low and high arousal intensity categories, are summarized in Table 2. Other than arousal threshold10 and rate of change of nadir epiglottic pressure, there were no differences in any of the pre-arousal stimulus variables between healthy controls and patients with OSA during NREM sleep (P > 0.1). CPAP reduction, time to arousal, arousal threshold, ∆Pepi/∆t, Vi, PIF, and NadirSaO2 did not differ between high and low arousal intensity categories (P > 0.25, Table 2). Univariate regression analysis also revealed no association between arousal intensity and CPAP reduction, arousal threshold (Figure 4A), Vi, PIF or NadirSaO2 during NREM sleep (P > 0.5). However, longer time to arousal and lower ∆Pepi/∆t correlated with increased arousal intensity (Figure 4B, 4C). Time to arousal did not correlate with either Vi or PIF during the breath prior to arousal (P > 0.88). However, time to arousal was inversely correlated with ∆Pepi/∆t
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were the CPAP holding levels (11.4 ± 3.1 vs. 5.3 ± 1.4 cm H2O; P < 0.0001). Data during REM sleep were obtained in 26 participants, while data from CPAP reductions during NREM sleep were acquired in all 71 participants.
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Arousal Intensity Figure 3—Frequency distribution of average arousal intensity (n = 71).
Table 2—Pre-arousal respiratory stimulus related factors. Variable ∆CPAP (cm H2O) Arousal Threshold (cm H2O) ∆Pepi/∆t (cm H2O/s) Vi (L/min) PIF (L/s) Time to Arousal (s) NadirSaO2 (%)
Total 7.0 ± 2.4 −15.5 (−20.2 to −11.2) −0.40 (−0.62 to −0.27) 4.6 (3.8 to 6.4) 0.32 (0.24 to 0.40) 32.8 (25.6 to 54.4) 89.3 (87.1 to 91.2)
Low AI 6.8 ± 2.6 −14.8 (−19.9 to −10.7) −0.41 (−0.62 to −0.25) 4.3 (3.0 to 6.8) 0.28 (0.21 to 0.43) 32.6 (24.7 to 55.7) 89.8 (88.0 to 91.1)
High AI 7.1 ± 2.3 −15.2 (−21.5 to −11.0) −0.45 (−0.62 to −0.26) 5.1 (2.5 to 6.5) 0.28 (0.18 to 0.44) 34.1 (25.2 to 53.7) 89.1 (86.8 to 91.1)
Group arousal intensity (AI) data (Total) and separated according to low and high AI during NREM sleep. Low and high AI values are calculated from subject average intensities for ≤ 5 and > 5, respectively. ∆CPAP = average reduction in continuous positive airway pressure delivered to induce airflow limitation/ arousal. Arousal Threshold = nadir epiglottic pressure immediately prior to arousal.3,10 ∆Pepi/∆t = rate of change of nadir epiglottic pressure (first breath following CPAP reduction to the breath immediately prior to arousal). Vi = minute ventilation and PIF = peak inspiratory flow during the breath immediately prior to arousal. Time to Arousal = time taken from the reduction in CPAP to arousal. NadirSaO2 = minimum blood arterial oxygen saturation caused by the reduction in CPAP. There were no differences between low and high AI values for any variable (P > 0.25). Data are mean ± SD or median (25th–75th percentile).
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Figure 4—Each symbol represents mean data for each subject and the univariate linear regression model (solid line) showing the relationship between average arousal intensity and (A) respiratory arousal threshold (nadir epiglottic pressure immediately prior to arousal), (B) time to arousal following reduction in CPAP, (C) ∆Pepi/∆t (rate of change of nadir epiglottic pressure, from first breath following CPAP reduction to arousal threshold breath), and (D) BMI (body mass index). Arousal intensity was not associated with arousal threshold (A). However, higher arousal intensities correlated with longer time to arousal (B), lower ∆Pepi/∆t (C) and lower BMI (D). Note linear regression model equations shown only for significant associations (B–D).
(R 2 = 0.17, P < 0.001). There was a borderline significant inverse relationship between steady state loop gain10 (acquired in a subset of individuals; n = 53) and arousal intensity (R 2 = 0.08, P = 0.047). Time to arousal and ∆Pepi/∆t were not associated with loop gain (R 2 < 0.017, P < 0.349). When pre-arousal stimulus variables were included in the multiple regression model (Table 3), the collective association was weak and of borderline significance (adjusted R 2 = 0.11, P = 0.049). Time to arousal remained the only significant variable (Table 3). Non-Stimulus Related Factors
Arousals that were less than 15 seconds accounted for 65% ± 27% of the total proportion of arousals. The proportion was similar for healthy controls and OSA patients (63% ± 36% vs. 66% ± 24%, P = 0.7). Arousal duration (for arousals < 15 s) was longer in healthy controls compared to patients with OSA (10.3 ± 2.5 vs. 8.9 ± 1.9 s; P = 0.03), as well as between low and high arousal intensity categories during NREM sleep (8.6 ± 2.0 vs. 10.1 ± 2.4 s; P < 0.001). Univariate analysis also confirmed that increased arousal intensity (for arousal durations < 15s) was associated with longer arousal duration during NREM sleep (R 2 = 0.13, P = 0.003). Increased arousal intensity correlated with lower BMI (Figure 4D), but not age (R 2 = 0.03, P = 0.174) or AHI (R 2 = 0.01, P = 0.377). Multiple regression analysis that included arousal duration, subject group (control/OSA), age, gender and BMI, revealed SLEEP, Vol. 39, No. 12, 2016
a significant collective association with arousal intensity (adjusted R 2 = 0.17, P = 0.005). BMI, however, was no longer significant, leaving arousal duration as the sole contributing variable (Table 3). All Factors
When all pre-arousal respiratory related stimulus and nonstimulus related factors were included in combined multiple regression analysis with backward elimination, arousal duration and time to arousal were the only variables that remained significant (Table 4; adjusted R 2 = 0.24, P < 0.001). Ventilatory and Upper Airway Dilator Muscle Responses to Arousal during NREM Sleep Breath-by-breath ventilatory and upper airway dilator muscle EMG parameters, separated according to low and high arousal intensities, are shown in Figure 5. Peak Inspiratory Flow and Minute Ventilation
There was a significant effect of breath number and arousal intensity on PIF (ANOVA P < 0.0001) and Vi (ANOVA P < 0.03). There was also a significant interaction effect between breath number and arousal intensity on PIF (ANOVA P = 0.013). PIF during the pre-arousal breaths was similar (−3 to −2) (P > 0.1, Figure 5A). PIF during breath −3 was slightly higher during low versus high intensity arousals (P = 0.03, Figure 5A). Following arousal, PIF was significantly higher than the breath preceding arousal (breath −1) for all post-arousal breaths (+1 to
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Table 3—Multiple linear regression analyses investigating factors contributing to arousal intensity. Variable Pre–Arousal Respiratory Related Stimulus Factors ∆CPAP (cm H2O) Arousal Threshold (cm H2O) ∆Pepi/∆t (cm H2O/s) Vi (L/min) PIF (L/s) Time to Arousal (s) NadirSaO2 (%) Non-Stimulus Related Factors Arousal Duration (s) OSA Male sex Age (years) BMI (kg/m2)
β
SE of β
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−0.02 0.05 1.43 −0.09 0.30 0.02 −0.03
0.09 0.04 0.90 0.15 2.36 0.01 0.05
−0.026 −0.21 0.30 −0.02 0.03 0.29 −0.09
0.85 0.27 0.12 0.96 0.90 0.048 0.51
0.28 −0.67 0.39 0.04 0.03
0.11 0.70 0.49 0.02 0.04
0.31 −0.15 0.24 0.10 −0.12
0.01 0.34 0.42 0.06 0.40
P value
Pre-arousal respiratory related stimulus factors and non-stimulus related factors analyzed separately to investigate contributors to average arousal intensity. Time to arousal and arousal duration were the only factors found to significantly contribute to arousal intensity (bolded in table). β and βstd are the non-standardized and standardized beta weights, respectively. SE = standard error. ∆CPAP = average reduction in continuous positive airway pressure delivered to induce airflow limitation/arousal. Arousal Threshold = nadir epiglottic pressure immediately prior to arousal. ∆Pepi/∆t = rate of change of nadir epiglottic pressure (first breath following CPAP reduction to the breath immediately prior to arousal). Vi = minute ventilation and PIF = peak inspiratory flow during the breath immediately prior to arousal. Time to Arousal = time taken from the reduction in CPAP to arousal. NadirSaO2 = minimum blood arterial oxygen saturation caused by the reduction in CPAP.
Table 4—Final multiple linear regression analysis following backward elimination to determine factors contributing to arousal intensity Variable Arousal Duration (s) Time to Arousal (s)
β 0.404 0.020
SE of β 0.097 0.007
βstd 0.455 0.320
P value < 0.001 0.005
Arousal duration and time to arousal were the only significant factors remaining following multiple linear regression analysis with backward correction, which included all pre-arousal respiratory related stimulus and non-stimulus factors.
+3; P < 0.001, Figure 5A). PIF was higher on the first and second breath (+1 and +2) following high intensity arousals compared with low intensity arousals (P < 0.009, Figure 5A). Vi was also greater during high versus low intensity arousals for the first breath following arousal (P = 0.03, Figure 5B). Between-breath comparisons for Vi were similar to PIF (Figure 5B). Genioglossus and Tensor Palatini Muscle Activity
There was a significant effect of breath number and arousal intensity on peak EMGgg (ANOVA P < 0.002), but not tonic EMGgg (ANOVA P = 0.7). During low and high arousal intensities, peak and tonic EMGgg increased from breath −3 to −1 (P < 0.03, Figure 5C and 5D). Peak EMGgg increased during the breath following arousal (+1) (P < 0.001, Figure 5C) in both arousal intensity categories. However, the increased EMG was only maintained above the −1 breath value on breaths +2 and +3 during high intensity arousals (P < 0.015, Figure 5C). Tonic EMGgg did not increase in the breaths following arousal for both high and low arousal intensities (P > 0.9, Figure 5D). There was a significant effect of breath number and arousal intensity (ANOVA P < 0.0005) for peak and tonic EMGtp. There was also a significant interaction effect between breath SLEEP, Vol. 39, No. 12, 2016
number and arousal intensity for peak and tonic EMGtp (ANOVA P < 0.007). Prior to arousal, tonic but not peak EMGtp was higher with high arousal intensities (breath −3 vs. −1; P = 0.035, Figure 5F). Peak and tonic EMGtp were higher than breath −1 for all three breaths following arousal (P < 0.02, Figure 5E, 5F). In addition, peak and tonic EMGtp were greater following high arousal intensities compared to low arousal intensities for all post arousal breaths (P < 0.05, Figure 5E, 5F). Epiglottic Pressure
Negative epiglottic pressure increased in the breaths prior to arousal and then decreased (became more positive) during the breaths following arousal (P < 0.001 vs. breath −1, all comparisons; Figure 5G). Nadir epiglottic pressure swings were similar between low and high arousal intensities (P > 0.9, Figure 5G). DISCUSSION The main findings of this study are that the key trigger for cortical arousal during airway narrowing (respiratory arousal threshold) and other important respiratory stimulus measures, including magnitude of airflow limitation and hypoxemia, do
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Figure 5—Ventilatory and upper airway muscle activity responses during NREM sleep for low (≤ 5; closed symbols) and high (> 5; open symbols) arousal intensities (AIs) for the three breaths prior to (−3 to −1) and for the three breaths following arousal (+1 to +3). All data are median (interquartile range) and expressed as a change (∆) from the pre-arousal breath (−1). PIF = peak inspiratory flow (A). Vi = minute ventilation (B). EMGgg Peak and EMGgg Tonic = peak inspiratory and tonic genioglossus muscle activity, respectively (C-D). EMGtp Peak and EMGtp Tonic = peak and tonic tensor palatini muscle activity, respectively (E,F). Pepi = epiglottic pressure (G). ^ P < 0.05 vs. −1 Breath for low AI condition. # P < 0.05 vs. −1 Breath for high AI condition. *P < 0.05 for high AI vs. low AI.
not influence average arousal intensity. In addition, higher arousal intensities result in approximately 40% greater increases in airflow and minute ventilation and approximately two times greater upper airway muscle activation. Consistent with arousal intensity being a distinct trait and a contributor to OSA pathogenesis, these findings indicate that average arousal intensity is independent of the preceding respiratory stimulus and that the physiological responses to arousal vary according to arousal intensity. SLEEP, Vol. 39, No. 12, 2016
Factors Influencing Arousal Intensity Pre-Arousal Respiratory Stimulus Related Factors
The lack of association between the magnitude of the negative epiglottic pressure immediately prior to arousal (respiratory arousal threshold) and average arousal intensity indicates that arousal intensity is independent of the key trigger for respiratory-related arousal. Rather than a graded arousal response according to the magnitude of respiratory stimuli, this finding
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is more consistent with arousal intensity being a distinct trait. However, other factors in addition to nadir pharyngeal pressure prior to an arousal, such as blood gas disturbances, can also trigger respiratory-related arousals.3,13–15 Indeed, increased duration of the preceding period of flow-limitation (time from the onset of the CPAP drop) was associated with increased arousal intensity. However, the degree of hypoxemia and the level of ventilatory impairment (minute ventilation and peak inspiratory flow) just prior to arousal were not associated with arousal intensity and were quantitatively similar between high and low intensity arousals. Furthermore, the effect of time to arousal on arousal intensity was only minor and the correlation between both parameters weak (time to arousal explained only approximately 15% of the variance in arousal intensity). Indeed, on average, the difference between low and high intensity arousals was less than a breath cycle (1.5s) in duration. Nonetheless, the notion that the time in which it takes to arouse from sleep, independent of the magnitude of the respiratory stimulus, influences the intensity of arousal is intriguing and may relate to differences in chemosensitivity. Consistent with this concept, the rate of change in nadir epiglottic pressure was associated with arousal intensity in the univariate analysis. Loop gain may be expected to influence the rate of change in nadir epiglottic pressure (and its relationship with arousal intensity). However, the lack of association between loop gain and the rate of change in nadir epiglottic pressure during the reduction in CPAP is likely due to our measure of loop gain being performed in the steady state. Nonetheless, steady state loop gain was independently associated with arousal intensity in the univariate analysis, although this parameter only explained a small proportion of the variance. Non-Stimulus Related Factors
The finding that arousal intensity increases with increasing arousal duration is consistent with the findings of an earlier study.6 The relationship between arousal intensity and duration was not a result of collinearity between the variables (as confirmed by Belsley collinearity diagnostics [MATLAB] between arousal intensity wavelet analysis parameters and arousal duration, performed on the original training data set used in the development of the wavelet analysis for arousal intensity scoring). In addition, this study confirms the initial finding involving ten OSA patients that arousal intensity does not differ between healthy controls and patients with OSA, or between NREM and REM sleep.6 Thus, while arousal intensity varies between individuals, the presence or absence of OSA and different sleep stages do not appear to systematically influence arousal intensity. However, the duration of arousals lasting < 15 s were longer in controls versus OSA patients. Similar to the respiratory arousal threshold3 and heart rate responses to arousal,6,7 this may reflect, at least in part, increased sleep pressure and cortical adaptation to repetitive respiratory loading in the OSA patients. In addition, the OSA patients in the current study had been treated with CPAP for at least 3 months. Thus, arousal intensities may still differ in untreated OSA patients. The current study also extends the findings of the earlier study by determining that neither age nor sex are important determinants of arousal intensity.7 SLEEP, Vol. 39, No. 12, 2016
Interestingly, a lower BMI was associated with higher arousal intensities in the univariate analysis. This finding is consistent with a blunted arousal response with obesity as is believed to occur in those with an obesity hypoventilation phenotype.3 While this observation is interesting, this association was not present in the multivariate analyses. Thus, additional investigation into the mechanisms of arousal is required to better understand the potential role of body habitus on arousal intensity. In the collective model, arousal duration and time to arousal were the only significant determinants of arousal intensity. Thus, the lack of association with the primary pre-arousal respiratory stimulus related factors combined with the fact that many arousal attributes, including average arousal intensity,7 are highly reproducible among subjects, strongly suggests that arousal intensity is a distinct phenotypic trait. Effects of Arousal Intensity on Ventilatory and Upper Airway Dilator Muscle Activity The pattern of changes in minute ventilation and dilator muscle activity following arousal, for both arousal intensity categories, are similar to those previously presented by Jordan and colleagues9 for ASDA (American Sleep Disorders Association) scored arousals. Increases in ventilation and dilator muscle activity following cortical arousal, according to standard definitions of arousal, have also been reported in other studies.8,12,16–18 Consistent with increased airflow with higher intensity arousals using a 0–4 point visual scale reported by Younes,2 the magnitude of the ventilatory and peak flow responses in the current study was greater with higher intensity arousals. Arousal intensity was associated with increases in tensor palatini muscle activity with less pronounced increases in peak but not tonic genioglossus muscle activity in the current study. Single motor unit studies of genioglossus indicate that unlike phasic motor neurons, which are actively recruited, tonic motor units are often inhibited during the immediate post-arousal period.17,19 This may explain, at least in part, the lack of an effect of arousal intensity on tonic genioglossus activity in the current study. This differs, however, to the findings of Younes and colleagues who performed brief reductions in CPAP (3 breaths) following prior CO2 stimulation in seven OSA patients, who also had appreciable genioglossus activity after discharge, and noted that tonic genioglossus muscle activity tended to be higher for the three breaths following the return to CPAP with higher intensity arousals.12 These apparent discrepancies are likely explained by differences in the experimental methodology. Differences include the use of CO2 and reintroduction of CPAP to predominantly complete obstructions in the prior study whereas the current study tended to examine less severe respiratory events and CPAP was not reintroduced for the postarousal analyses. Prior to the present study, the effects of arousal on tensor palatini activity had not been published.20 The current finding of increased multiunit tensor palatini activity with greater arousal intensities is consistent with more pronounced reintroduction of wakefulness drive and greater sleep/wake dependence of the tensor palatini muscle compared to genioglossus.11,21–24 In contrast, the genioglossus muscle is more
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responsive to subtle changes in respiratory stimuli and sleep stage effects.11,21–24 An increased ventilatory response to arousal with higher arousal intensities may contribute to OSA pathogenesis by destabilizing breathing control and contributing to subsequent upper airway collapse upon the return to sleep.2,8 Specifically, heightened arousal intensity leading to increased hyperventilation may lead to excessive reductions in carbon dioxide levels and, hence, hypoventilation and ventilatory control instability upon the return to sleep. Conversely, the novel finding that increased tensor palatini muscle activation occurs with higher arousal intensities for at least three breaths following arousal is likely to stabilize the upper airway. However, a relative lack of influence of arousal intensity on genioglossus muscle activity may contribute to overall discoordination or muscle asynchrony between the dilator muscles that could contribute to subsequent airway closure as highlighted by the recent findings of Dotan and colleagues.25 Specifically, activation of tensor palatini in the absence of coupled genioglossus activation may cause counterproductive motion and airway narrowing.26 CONCLUSIONS The findings from this study indicate that average arousal intensity is independent of the magnitude of the pre-arousal respiratory stimulus and that higher intensity arousals cause greater ventilatory and tensor palatini muscle responses following arousal, with comparatively modest increases in peak but not tonic genioglossus muscle activity. While recent findings have highlighted the potential importance of arousal in sleep apnea pathogenesis, the current findings demonstrate that not all cortical arousals or their consequences are the same. Thus, these data combined with prior studies support the concept that arousal intensity is a distinct trait and an independent contributor to OSA pathogenesis. REFERENCES 1. Rees K, Spence DP, Earis JE, Calverley PM. Arousal responses from apneic events during non-rapid-eye-movement sleep. Am J Respir Crit Care Med 1995;152:1016–21. 2. Younes M. Role of arousals in the pathogenesis of obstructive sleep apnea. Am J Respir Crit Care Med 2004;169:623–33. 3. Eckert DJ, Younes MK. Arousal from sleep: implications for obstructive sleep apnea pathogenesis and treatment. J Appl Physiol 2014;116:302–13. 4. Younes M, Hanly PJ. Immediate postarousal sleep dynamics: an important determinant of sleep stability in obstructive sleep apnea. J Appl Physiol 2016;120:801–8. 5. Berry RB, Budhiraja R, Gottlieb DJ, et al. Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual for the Scoring of Sleep and Associated Events. Deliberations of the Sleep Apnea Definitions Task Force of the American Academy of Sleep Medicine. J Clin Sleep Med 2012;8:597–619. 6. Azarbarzin A, Ostrowski M, Hanly P, Younes M. Relationship between arousal intensity and heart rate response to arousal. Sleep 2014;37:645–53. 7. Azarbarzin A, Ostrowski M, Younes M, et al. Arousal responses during overnight polysomnography and their reproducibility in healthy young adults. Sleep 2015;38:1313–21. 8. Jordan AS, Eckert DJ, Catcheside PG, McEvoy RD. Ventilatory response to brief arousal from non–rapid eye movement sleep is greater in men than in women. Am J Respir Crit Care Med 2003;168:1512–9.
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9. Jordan AS, Eckert DJ, Wellman A, Trinder JA, Malhotra A, White DP. Termination of respiratory events with and without cortical arousal in obstructive sleep apnea. Am J Respir Crit Care Med 2011;184:1183–91. 10. Eckert DJ, White DP, Jordan AS, Malhotra A, Wellman A. Defining phenotypic causes of obstructive sleep apnea. Identification of novel therapeutic targets. Am J Respir Crit Care Med 2013;188:996–1004. 11. Carberry JC, Jordan AS, White DP, Wellman A, Eckert DJ. Upper airway collapsibility (Pcrit) and pharyngeal dilator muscle activity are sleep-stage dependent. Sleep 2016;39:511–21. 12. Younes M, Loewen A, Ostrowski M, Hanly P. Short-term potentiation in the control of pharyngeal muscles in obstructive apnea patients. Sleep 2014;37:1833–49. 13. Xiao SC, He BT, Steier J, Moxham J, Polkey MI, Luo YM. Neural respiratory drive and arousal in patients with obstructive sleep apnea hypopnea. Sleep 2015;38:941–9. 14. Ayas NT, Brown R, Shea SA. Hypercapnia can induce arousal from sleep in the absence of altered respiratory mechanoreception. Am J Respir Crit Care Med 2000;162:1004–8. 15. Amatoury J, Jordan A, Wellman A, White D, Eckert D. New insights into the mechanisms of respiratory load-induced arousal: role of breath timing and respiratory load compensation [abstract]. Sleep Biol Rhythms 2015;13:A171. 16. Horner RL, Rivera MP, Kozar LF, Phillipson EA. The ventilatory response to arousal from sleep is not fully explained by differences in CO2 levels between sleep and wakefulness. J Physiol (Lond) 2001;534:881–90. 17. Wilkinson V, Malhotra A, Nicholas CL, et al. Discharge patterns of human genioglossus motor units during arousal from sleep. Sleep 2010;33:379–87. 18. Jordan AS, Cori JM, Dawson A, et al. Arousal from sleep does not lead to reduced dilator muscle activity or elevated upper airway resistance on return to sleep in healthy individuals. Sleep 2014;38:53–9. 19. Trinder J, Jordan AS, Nicholas CL. Discharge properties of upper airway motor units during wakefulness and sleep. Prog Brain Res 2014;212:59–75. 20. Yeo A, Catcheside PG, George K, Thomson K, McEvoy RD. Prolonged post-arousal upper airway dilator muscle activity in men with obstructive sleep apnoea on continuous positive airway pressure [abstract]. Sleep Biol Rhythms 2007;5:A133. 21. Horner RL, Hughes SW, Malhotra A. State-dependent and reflex drives to the upper airway: basic physiology with clinical implications. J Appl Physiol 2014;116:325–36. 22. Lo YL, Jordan AS, Malhotra A, et al. Influence of wakefulness on pharyngeal airway muscle activity. Thorax 2007;62:799–805. 23. Carberry JC, Hensen H, Fisher LP, et al. Mechanisms contributing to the response of upper-airway muscles to changes in airway pressure. J Appl Physiol 2015;118:1221–8. 24. Worsnop C, Kay A, Pierce R, Kim Y, Trinder J. Activity of respiratory pump and upper airway muscles during sleep onset. J Appl Physiol 1998;85:908–20. 25. Dotan Y, Pillar G, Schwartz AR, Oliven A. Asynchrony of lingual muscle recruitment during sleep in obstructive sleep apnea. J Appl Physiol 2015;118:1516–24. 26. Brown EC, Cheng S, McKenzie DK, Butler JE, Gandevia SC, Bilston LE. Tongue and lateral upper airway movement with mandibular advancement. Sleep 2013;36:397–404.
ACKNOWLEDGMENTS The authors thank the Brigham and Women’s Sleep Disorders Research Program staff for technical support and Professor Rob Herbert (NeuRA) for statistical advice. Author contributions: Dr. Amatoury and Dr. Eckert developed the study concepts and wrote the manuscript. Dr. Amatoury performed the data analyses. Dr. Azarbarzin performed the arousal intensity analyses. Dr. Eckert and Dr. Jordan collected the data. All authors provided important insight on data interpretation and contributed to the final version of the manuscript
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SUBMISSION & CORRESPONDENCE INFORMATION
(NHMRC) Centre of Research Excellence (CRE) (1060992) and National Institute of Health (5R01HL048531, PI Andrew Wellman). Dr. Amatoury is supported by a NeuroSleep NHMRC CRE Postdoctoral Fellowship (1060992). Dr. Eckert is supported by a NHMRC RD Wright Fellowship (1049814). Dr. Younes is majority owner of YRT Ltd. Dr. Wellman has received research support from Philips Respironics. Dr. Azarbarzin is a former employee of YRT Ltd. The arousal intensity metric applied in this manuscript was developed under YRT Ltd by Dr. Younes and Dr. Azarbarzin. Dr. Jordan has received research support from Philips Respironics. The work was performed at the Division of Sleep Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA.
Submitted for publication April, 2016 Submitted in final revised form June, 2016 Accepted for publication August, 2016 Address correspondence to: Dr. Jason Amatoury, Neuroscience Research Australia (NeuRA), PO Box 1165, Randwick, NSW, Australia 2031; Tel: +61 2 9399 1834; Fax: +61 2 9399 1027; Email: [email protected]
DISCLOSURE STATEMENT This was not an industry supported study. This work was supported by the NeuroSleep National Health and Medical Research Council of Australia
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http://dx.doi.org/10.5665/sleep.6304
SLEEP- DISORDERED BREATHING
Arousal Intensity is a Distinct Pathophysiological Trait in Obstructive Sleep Apnea Jason Amatoury, PhD1; Ali Azarbarzin, PhD5; Magdy Younes, MD, PhD2,3; Amy S. Jordan, PhD4; Andrew Wellman, MD, PhD5; Danny J. Eckert, PhD1 1 Neuroscience Research Australia (NeuRA), and the School of Medical Sciences, University of New South Wales, Sydney, NSW, Australia; 2YRT Ltd, Winnipeg, Manitoba, Canada; 3Sleep Disorders Centre, University of Manitoba, Winnipeg, Manitoba, Canada; 4Institute for Breathing and Sleep, and Melbourne School of Physiological Sciences, University of Melbourne, Melbourne, Victoria, Australia; 5Division of Sleep Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA
Study Objectives: Arousals from sleep vary in duration and intensity. Accordingly, the physiological consequences of different types of arousals may also vary. Factors that influence arousal intensity are only partly understood. This study aimed to determine if arousal intensity is mediated by the strength of the preceding respiratory stimulus, and investigate other factors mediating arousal intensity and its role on post-arousal ventilatory and pharyngeal muscle responses. Methods: Data were acquired in 71 adults (17 controls, 54 obstructive sleep apnea patients) instrumented with polysomnography equipment plus genioglossus and tensor palatini electromyography (EMG), a nasal mask and pneumotachograph, and an epiglottic pressure sensor. Transient reductions in CPAP were delivered during sleep to induce respiratory-related arousals. Arousal intensity was measured using a validated 10-point scale. Results: Average arousal intensity was not related to the magnitude of the preceding respiratory stimuli but was positively associated with arousal duration, time to arousal, rate of change in epiglottic pressure and negatively with BMI (R2 > 0.10, P ≤ 0.006). High (> 5) intensity arousals caused greater ventilatory responses than low (≤ 5) intensity arousals (10.9 [6.8–14.5] vs. 7.8 [4.7–12.9] L/min; P = 0.036) and greater increases in tensor palatini EMG (10 [3–17] vs. 6 [2–11]%max; P = 0.031), with less pronounced increases in genioglossus EMG. Conclusions: Average arousal intensity is independent of the preceding respiratory stimulus. This is consistent with arousal intensity being a distinct trait. Respiratory and pharyngeal muscle responses increase with arousal intensity. Thus, patients with higher arousal intensities may be more prone to respiratory control instability. These findings are important for sleep apnea pathogenesis. Keywords: arousal threshold, genioglossus, tensor palatini, sleep-disordered breathing, respiratory physiology Citation: Amatoury J, Azarbarzin A, Younes M, Jordan AS, Wellman A, Eckert DJ. Arousal intensity is a distinct pathophysiological trait in obstructive sleep apnea. SLEEP 2016;39(12):2091–2100. Significance As per American Academy of Sleep Medicine scoring rules, cortical arousals from sleep are quantified as an all or none phenomenon, yet not all cortical arousals are the same. The concept of quantifying the intensity of arousals has recently been described, but until now the factors influencing arousal intensity and its potential role in sleep apnea pathogenesis were unclear. This study demonstrates a new concept in OSA pathogenesis. Specifically, arousal intensity is a distinct trait and an important mediator of ventilatory and pharyngeal dilator muscle responses to arousal. Thus, there is a need to reconsider the current all or none approach to defining cortical arousals from sleep.
INTRODUCTION Cortical arousals from sleep occur frequently in sleep-disordered breathing. In adult obstructive sleep apnea (OSA), the most common sleep-related breathing disorder, cortical arousals often occur in association with the end of respiratory events.1,2 Arousals fragment sleep, contribute to excessive daytime sleepiness, and are involved in the pathogenesis of OSA.2–4 According to the American Academy of Sleep Medicine scoring rules,5 cortical arousals are scored as all or none phenomena, defined as a sudden increase in electroencephalogram (EEG) frequency lasting > 3 seconds when preceded with ≥ 10 seconds of stable sleep.5 However, not all cortical arousals that meet this definition are the same. Indeed, arousals vary in duration and intensity.2,6,7 Accordingly, the physiological and potential next day consequences of different types of arousals may also vary. In support of this concept, Younes demonstrated that airflow responses to cortical arousal at the end of an obstructive event are higher with increasing arousal intensity when scored visually using a 0 to 4 scale.2 An unnecessarily large increase in ventilation following arousal can contribute to subsequent hypocapnia, destabilization of the ventilatory control system, and the cyclical breathing pattern that occurs in OSA.2,8,9 More recently, Azabarzin and colleagues used an SLEEP, Vol. 39, No. 12, 2016
automated method to quantify arousal intensity over a wider range (0 to 9).6,7 Average arousal intensities were found to vary between individuals and average arousal intensity strongly correlated with heart rate responses to arousal.6,7 Thus, while comprehensive assessments of minute ventilation per se and its key determinants, such as dilator muscle activity, have not been performed, the physiological consequences of arousal appear to be stronger with increased arousal intensity. The factors that influence average arousal intensity in OSA are only partly understood.6,7 A high propensity for arousal (low respiratory arousal threshold) is a phenotypic contributor to OSA.2,3,10 Whether the strength of the respiratory stimulus prior to arousal (i.e., arousal threshold) and other associated factors (e.g., hypoxemia and the degree of airflow limitation) influence arousal intensity remains unknown. This is important because if arousal intensity is independent of the strength of the preceding respiratory stimulus, it would indicate that arousal intensity is a distinct trait. Similarly, it remains unclear if non-respiratory stimulus related factors, including anthropometric characteristics, importantly influence average arousal intensity. Thus, this study aimed to determine if average arousal intensity is related to the strength of the preceding respiratory
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Table 1—Anthropometric and sleep parameters for the study participants. Gender (M/F) Age (years) BMI (kg/m2) Mean AHI (events/h sleep) Range AHI (events/h sleep)
All Subjects (n = 71) 48/23 45.1 ± 11.9 32.8 ± 6.8 37.3 ± 32 0–112
Controls (n = 17) 8/9 37 ± 12 26 ± 3 4±3 0–10
OSA Patients (n = 54) 40/14 47 ± 11* 35 ± 6* 48 ± 30* 11–112
Data are mean ± SD, where applicable. *P < 0.05 for control vs. OSA. BMI, body mass index. AHI, apnea-hypopnea index.
stimulus. Secondary aims were to examine non-respiratory stimulus related factors that determine arousal intensity and the role of arousal intensity on post-arousal ventilatory responses and upper airway dilator muscle activity. METHODS Data for the current study were acquired as part of a larger study in 75 participants to quantify the key pathophysiological traits causing OSA.10 The current findings have not been previously reported. The experimental methodology has been described in detail previously.10 Participants Data were obtained in 71 individuals (47 men) including 17 healthy controls (AHI ≤ 10 events/h sleep) and 54 patients with OSA (AHI > 10 events/h sleep) who had been treated with CPAP for ≥ 3 months (Table 1). Four subjects were excluded from the original data set10 as they did not meet the criteria for one or more of the current study analyses. All subjects provided informed written consent and the protocol was approved by the Partners’ Healthcare Institutional Review Board. Experimental Design and Measurements Initially, participants underwent standard overnight polysomnography off CPAP to quantify the AHI.10 Approximately 1–2 weeks later, a detailed overnight upper airway physiology study was performed. Electroencephalograms (EEGs), electroculograms (EOGs), and chin electromyogram (EMG) were acquired for scoring of sleep stages and arousals. Genioglossus and tensor palatini EMG recordings (EMGgg and EMGtp) were made using fine-wire intramuscular electrodes (Cooner Wire Company, Chatsworth, CA).10,11 Epiglottic pressure (Pepi) was measured using a transducer-tipped pressure catheter (Millar MCP-500; Millar Instruments, Houston, TX), as described previously.10 A nasal CPAP mask was fitted and mask pressure and airflow were measured with differential pressure transducers (Validyne Corporation, Northbridge, CA) and a pneumotachograph (Hans Rudolf 3700A; Hans Rudolf Inc., Kansas City, MO).10 Protocol Participants were studied supine on CPAP. Transient reductions in CPAP from the holding pressure were applied for ≤ 3 minutes during stable sleep to cause varying degrees of upper airway collapse to induce respiratory-related arousals.10 SLEEP, Vol. 39, No. 12, 2016
Data Analysis Figure 1 shows an example CPAP reduction and quantification of several of the key study measurements. All reductions in CPAP that terminated in a respiratory-related arousal were included in the analyses. To be considered a respiratory-related arousal, reductions in CPAP that resulted in cortical arousals had to: (1) last > 10 s prior to arousal and (2) be associated with an increase in nadir negative Pepi > 2 cm H2O within the 30 s preceding cortical arousal. Arousal intensity was measured on a scale between 0 and 9 (most intense) using a validated automated wavelet transformation method (Figure 2).6,7 For arousals > 15 s, only the first 15 s was used to calculate arousal intensity. Arousals were divided into low (≤ 5) and high (> 5) intensity categories, similar to previous methodology.12 Pre-arousal respiratory stimulus related variables considered to contribute to arousal were quantified (Figure 1), including: CPAP reduction magnitude (∆CPAP); time to arousal (following reduction in CPAP); respiratory arousal threshold, defined as the nadir epiglottic pressure immediately preceding arousal3,10; rate of change of nadir epiglottic pressure (∆Pepi/∆t) from the first breath following CPAP reduction to the breath immediately prior to arousal; minute ventilation (Vi) and peak inspiratory flow (PIF), measured from the breath immediately preceding arousal; and the minimum blood arterial oxygen saturation caused by the reduction in CPAP (NadirSaO2). Steady state loop gain (sensitivity of the ventilatory control system) was also quantified according to methodology previously described.10 Arousal duration, body mass index (BMI), OSA status, sex, and age were also recorded to enable comparison of these non-stimulus related variables with arousal intensity. Raw EMGgg and EMGtp signals were rectified, movingtime-averaged (100-ms window) and expressed as a percent of maximum for each subject (Figure 1).11 To determine the influence of arousal intensity on respiratory and muscle responses during NREM sleep, Pepi, Vi, PIF, and peak and tonic EMGgg and EMGtp for three breaths prior to arousal (breaths −3 to −1) and 3 breaths following arousal (breaths +1 to +3) were quantified and compared between low and high arousal intensities. Statistical Approach Univariate and multiple linear regression analyses were performed (SPSS, v23, IBM Corp.) to assess the influence of prearousal stimuli and non-stimulus related variables on arousal intensity. Pre-arousal stimuli (∆CPAP, time to arousal, arousal threshold, ∆Pepi/∆t, Vi, PIF and NadirSaO2) and non-stimulus related variables (arousal duration, OSA status, sex, age and
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Figure 1—Raw data example illustrating a transient reduction in CPAP (∆CPAP; sudden drop in mask pressure, Pmask) and quantification of several of the key outcome variables. A respiratory-related cortical arousal during the CPAP reduction is shown highlighted on the EEG trace. Vertical bar highlight indicates the breath immediately preceding arousal where arousal threshold [nadir epiglottic pressure (Pepi)] was quantified (as shown), in addition to pre-arousal peak inspiratory flow (PIF) and minute ventilation. The rate of change of Pepi (∆Pepi/∆t) was quantified from the first breath following CPAP reduction to the arousal threshold breath (slope of dashed line shown on Pepi channel). PIF, peak and tonic genioglossus (GG) and tensor palatini (TP) EMG (EMGgg and EMGtp) are indicated for a single breath following arousal. MTA = 100 millisecond moving-time-average of the rectified raw EMG. SaO2 = oxygen saturation. NadirSaO2 = minimum blood arterial oxygen saturation caused by the reduction in CPAP. Direction of inspiration indicated on Pepi and Flow traces.
Figure 2—Three examples of cortical arousal (highlighted in gray) with different arousal intensity (AI), analyzed using an automated validated tool from C3/A2 and O2/A1 EEGs.
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BMI) were included in separate models. A final multiple regression analysis was performed including all variables with backward correction applied to eliminate the variable with the highest P value until only significant variables remained. Two-way ANOVA was used to determine the effect of breath number and arousal intensity on ventilatory and muscle parameters. Unpaired two-tailed t-tests/Mann-Whitney tests were performed for between group comparisons for the key study outcomes (Prism, v6.05, Graphpad Software Inc.). Data are reported as mean ± SD or median (25th–75th percentile). Statistical significance was inferred at P < 0.05. RESULTS Table 1 summarizes the anthropometric and sleep characteristics of the study participants. A total of 487 reductions in CPAP that resulted in a respiratory-related cortical arousal were analyzed across all subjects, with a median of 6 (3–8) per subject. The average reduction in CPAP was similar for controls and OSA patients (6.0 ± 1.8 vs. 7.3 ± 2.5 cm H2O; P = 0.053). However, the absolute level of CPAP during the drop was significantly greater in the OSA patients compared to controls (4.1 ± 2.7 vs. −0.7 ± 2.4 cm H2O; P < 0.0001), as
Frequency (%)
25 20 15 10 5 0-1
1-2
2-3
3-4
4-5
5-6
6-7 7-8
Arousal Intensity The frequency distribution of average arousal intensity per subject is shown in Figure 3. Most subjects fell within the arousal intensity band of 5–6 (25.4%). No subject was classified with an arousal intensity of 0 or 1. Only 4.2% of subjects fell within the highest arousal intensity band of 8–9. The average arousal intensity for the group was 5.3 ± 1.5. Arousal intensity tended to be higher in control subjects compared to patients with OSA. However, this difference was not statistically significant (5.9 ± 1.6 vs. 5.1 ± 1.5, P = 0.08). There was also no difference in arousal intensity between NREM and REM sleep (5.3 ± 1.6 vs. 5.5 ± 2.2, P = 0.48). The average arousal intensity for the low (≤ 5) versus high (> 5) intensity arousal categories was 3.4 ± 0.9 vs. 7.4 ± 0.8 (P < 0.0001). Factors Influencing Arousal Intensity Pre-Arousal Respiratory Stimulus Related Factors
Pre-arousal stimulus data, including for low and high arousal intensity categories, are summarized in Table 2. Other than arousal threshold10 and rate of change of nadir epiglottic pressure, there were no differences in any of the pre-arousal stimulus variables between healthy controls and patients with OSA during NREM sleep (P > 0.1). CPAP reduction, time to arousal, arousal threshold, ∆Pepi/∆t, Vi, PIF, and NadirSaO2 did not differ between high and low arousal intensity categories (P > 0.25, Table 2). Univariate regression analysis also revealed no association between arousal intensity and CPAP reduction, arousal threshold (Figure 4A), Vi, PIF or NadirSaO2 during NREM sleep (P > 0.5). However, longer time to arousal and lower ∆Pepi/∆t correlated with increased arousal intensity (Figure 4B, 4C). Time to arousal did not correlate with either Vi or PIF during the breath prior to arousal (P > 0.88). However, time to arousal was inversely correlated with ∆Pepi/∆t
30
0
were the CPAP holding levels (11.4 ± 3.1 vs. 5.3 ± 1.4 cm H2O; P < 0.0001). Data during REM sleep were obtained in 26 participants, while data from CPAP reductions during NREM sleep were acquired in all 71 participants.
8-9
Arousal Intensity Figure 3—Frequency distribution of average arousal intensity (n = 71).
Table 2—Pre-arousal respiratory stimulus related factors. Variable ∆CPAP (cm H2O) Arousal Threshold (cm H2O) ∆Pepi/∆t (cm H2O/s) Vi (L/min) PIF (L/s) Time to Arousal (s) NadirSaO2 (%)
Total 7.0 ± 2.4 −15.5 (−20.2 to −11.2) −0.40 (−0.62 to −0.27) 4.6 (3.8 to 6.4) 0.32 (0.24 to 0.40) 32.8 (25.6 to 54.4) 89.3 (87.1 to 91.2)
Low AI 6.8 ± 2.6 −14.8 (−19.9 to −10.7) −0.41 (−0.62 to −0.25) 4.3 (3.0 to 6.8) 0.28 (0.21 to 0.43) 32.6 (24.7 to 55.7) 89.8 (88.0 to 91.1)
High AI 7.1 ± 2.3 −15.2 (−21.5 to −11.0) −0.45 (−0.62 to −0.26) 5.1 (2.5 to 6.5) 0.28 (0.18 to 0.44) 34.1 (25.2 to 53.7) 89.1 (86.8 to 91.1)
Group arousal intensity (AI) data (Total) and separated according to low and high AI during NREM sleep. Low and high AI values are calculated from subject average intensities for ≤ 5 and > 5, respectively. ∆CPAP = average reduction in continuous positive airway pressure delivered to induce airflow limitation/ arousal. Arousal Threshold = nadir epiglottic pressure immediately prior to arousal.3,10 ∆Pepi/∆t = rate of change of nadir epiglottic pressure (first breath following CPAP reduction to the breath immediately prior to arousal). Vi = minute ventilation and PIF = peak inspiratory flow during the breath immediately prior to arousal. Time to Arousal = time taken from the reduction in CPAP to arousal. NadirSaO2 = minimum blood arterial oxygen saturation caused by the reduction in CPAP. There were no differences between low and high AI values for any variable (P > 0.25). Data are mean ± SD or median (25th–75th percentile).
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A
B
9
6 5 4 3 2 1 0 -50
C
-40
-30
-20
-10
D
8
5 4 3 2
R2 = 0.16, P < 0.001 Y = 0.025*X + 4.2
0
30
60
90
120
150
180
45
50
Time to Arousal (s) 9 8
Arousal Intensity
Arousal Intensity
6
0
0
9 7 6 5 4 3 1
7
1
R2 < 0.001, P = 0.9
Arousal Threshold (cmH2O)
2
9 8
7
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Figure 4—Each symbol represents mean data for each subject and the univariate linear regression model (solid line) showing the relationship between average arousal intensity and (A) respiratory arousal threshold (nadir epiglottic pressure immediately prior to arousal), (B) time to arousal following reduction in CPAP, (C) ∆Pepi/∆t (rate of change of nadir epiglottic pressure, from first breath following CPAP reduction to arousal threshold breath), and (D) BMI (body mass index). Arousal intensity was not associated with arousal threshold (A). However, higher arousal intensities correlated with longer time to arousal (B), lower ∆Pepi/∆t (C) and lower BMI (D). Note linear regression model equations shown only for significant associations (B–D).
(R 2 = 0.17, P < 0.001). There was a borderline significant inverse relationship between steady state loop gain10 (acquired in a subset of individuals; n = 53) and arousal intensity (R 2 = 0.08, P = 0.047). Time to arousal and ∆Pepi/∆t were not associated with loop gain (R 2 < 0.017, P < 0.349). When pre-arousal stimulus variables were included in the multiple regression model (Table 3), the collective association was weak and of borderline significance (adjusted R 2 = 0.11, P = 0.049). Time to arousal remained the only significant variable (Table 3). Non-Stimulus Related Factors
Arousals that were less than 15 seconds accounted for 65% ± 27% of the total proportion of arousals. The proportion was similar for healthy controls and OSA patients (63% ± 36% vs. 66% ± 24%, P = 0.7). Arousal duration (for arousals < 15 s) was longer in healthy controls compared to patients with OSA (10.3 ± 2.5 vs. 8.9 ± 1.9 s; P = 0.03), as well as between low and high arousal intensity categories during NREM sleep (8.6 ± 2.0 vs. 10.1 ± 2.4 s; P < 0.001). Univariate analysis also confirmed that increased arousal intensity (for arousal durations < 15s) was associated with longer arousal duration during NREM sleep (R 2 = 0.13, P = 0.003). Increased arousal intensity correlated with lower BMI (Figure 4D), but not age (R 2 = 0.03, P = 0.174) or AHI (R 2 = 0.01, P = 0.377). Multiple regression analysis that included arousal duration, subject group (control/OSA), age, gender and BMI, revealed SLEEP, Vol. 39, No. 12, 2016
a significant collective association with arousal intensity (adjusted R 2 = 0.17, P = 0.005). BMI, however, was no longer significant, leaving arousal duration as the sole contributing variable (Table 3). All Factors
When all pre-arousal respiratory related stimulus and nonstimulus related factors were included in combined multiple regression analysis with backward elimination, arousal duration and time to arousal were the only variables that remained significant (Table 4; adjusted R 2 = 0.24, P < 0.001). Ventilatory and Upper Airway Dilator Muscle Responses to Arousal during NREM Sleep Breath-by-breath ventilatory and upper airway dilator muscle EMG parameters, separated according to low and high arousal intensities, are shown in Figure 5. Peak Inspiratory Flow and Minute Ventilation
There was a significant effect of breath number and arousal intensity on PIF (ANOVA P < 0.0001) and Vi (ANOVA P < 0.03). There was also a significant interaction effect between breath number and arousal intensity on PIF (ANOVA P = 0.013). PIF during the pre-arousal breaths was similar (−3 to −2) (P > 0.1, Figure 5A). PIF during breath −3 was slightly higher during low versus high intensity arousals (P = 0.03, Figure 5A). Following arousal, PIF was significantly higher than the breath preceding arousal (breath −1) for all post-arousal breaths (+1 to
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Table 3—Multiple linear regression analyses investigating factors contributing to arousal intensity. Variable Pre–Arousal Respiratory Related Stimulus Factors ∆CPAP (cm H2O) Arousal Threshold (cm H2O) ∆Pepi/∆t (cm H2O/s) Vi (L/min) PIF (L/s) Time to Arousal (s) NadirSaO2 (%) Non-Stimulus Related Factors Arousal Duration (s) OSA Male sex Age (years) BMI (kg/m2)
β
SE of β
βstd
−0.02 0.05 1.43 −0.09 0.30 0.02 −0.03
0.09 0.04 0.90 0.15 2.36 0.01 0.05
−0.026 −0.21 0.30 −0.02 0.03 0.29 −0.09
0.85 0.27 0.12 0.96 0.90 0.048 0.51
0.28 −0.67 0.39 0.04 0.03
0.11 0.70 0.49 0.02 0.04
0.31 −0.15 0.24 0.10 −0.12
0.01 0.34 0.42 0.06 0.40
P value
Pre-arousal respiratory related stimulus factors and non-stimulus related factors analyzed separately to investigate contributors to average arousal intensity. Time to arousal and arousal duration were the only factors found to significantly contribute to arousal intensity (bolded in table). β and βstd are the non-standardized and standardized beta weights, respectively. SE = standard error. ∆CPAP = average reduction in continuous positive airway pressure delivered to induce airflow limitation/arousal. Arousal Threshold = nadir epiglottic pressure immediately prior to arousal. ∆Pepi/∆t = rate of change of nadir epiglottic pressure (first breath following CPAP reduction to the breath immediately prior to arousal). Vi = minute ventilation and PIF = peak inspiratory flow during the breath immediately prior to arousal. Time to Arousal = time taken from the reduction in CPAP to arousal. NadirSaO2 = minimum blood arterial oxygen saturation caused by the reduction in CPAP.
Table 4—Final multiple linear regression analysis following backward elimination to determine factors contributing to arousal intensity Variable Arousal Duration (s) Time to Arousal (s)
β 0.404 0.020
SE of β 0.097 0.007
βstd 0.455 0.320
P value < 0.001 0.005
Arousal duration and time to arousal were the only significant factors remaining following multiple linear regression analysis with backward correction, which included all pre-arousal respiratory related stimulus and non-stimulus factors.
+3; P < 0.001, Figure 5A). PIF was higher on the first and second breath (+1 and +2) following high intensity arousals compared with low intensity arousals (P < 0.009, Figure 5A). Vi was also greater during high versus low intensity arousals for the first breath following arousal (P = 0.03, Figure 5B). Between-breath comparisons for Vi were similar to PIF (Figure 5B). Genioglossus and Tensor Palatini Muscle Activity
There was a significant effect of breath number and arousal intensity on peak EMGgg (ANOVA P < 0.002), but not tonic EMGgg (ANOVA P = 0.7). During low and high arousal intensities, peak and tonic EMGgg increased from breath −3 to −1 (P < 0.03, Figure 5C and 5D). Peak EMGgg increased during the breath following arousal (+1) (P < 0.001, Figure 5C) in both arousal intensity categories. However, the increased EMG was only maintained above the −1 breath value on breaths +2 and +3 during high intensity arousals (P < 0.015, Figure 5C). Tonic EMGgg did not increase in the breaths following arousal for both high and low arousal intensities (P > 0.9, Figure 5D). There was a significant effect of breath number and arousal intensity (ANOVA P < 0.0005) for peak and tonic EMGtp. There was also a significant interaction effect between breath SLEEP, Vol. 39, No. 12, 2016
number and arousal intensity for peak and tonic EMGtp (ANOVA P < 0.007). Prior to arousal, tonic but not peak EMGtp was higher with high arousal intensities (breath −3 vs. −1; P = 0.035, Figure 5F). Peak and tonic EMGtp were higher than breath −1 for all three breaths following arousal (P < 0.02, Figure 5E, 5F). In addition, peak and tonic EMGtp were greater following high arousal intensities compared to low arousal intensities for all post arousal breaths (P < 0.05, Figure 5E, 5F). Epiglottic Pressure
Negative epiglottic pressure increased in the breaths prior to arousal and then decreased (became more positive) during the breaths following arousal (P < 0.001 vs. breath −1, all comparisons; Figure 5G). Nadir epiglottic pressure swings were similar between low and high arousal intensities (P > 0.9, Figure 5G). DISCUSSION The main findings of this study are that the key trigger for cortical arousal during airway narrowing (respiratory arousal threshold) and other important respiratory stimulus measures, including magnitude of airflow limitation and hypoxemia, do
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Figure 5—Ventilatory and upper airway muscle activity responses during NREM sleep for low (≤ 5; closed symbols) and high (> 5; open symbols) arousal intensities (AIs) for the three breaths prior to (−3 to −1) and for the three breaths following arousal (+1 to +3). All data are median (interquartile range) and expressed as a change (∆) from the pre-arousal breath (−1). PIF = peak inspiratory flow (A). Vi = minute ventilation (B). EMGgg Peak and EMGgg Tonic = peak inspiratory and tonic genioglossus muscle activity, respectively (C-D). EMGtp Peak and EMGtp Tonic = peak and tonic tensor palatini muscle activity, respectively (E,F). Pepi = epiglottic pressure (G). ^ P < 0.05 vs. −1 Breath for low AI condition. # P < 0.05 vs. −1 Breath for high AI condition. *P < 0.05 for high AI vs. low AI.
not influence average arousal intensity. In addition, higher arousal intensities result in approximately 40% greater increases in airflow and minute ventilation and approximately two times greater upper airway muscle activation. Consistent with arousal intensity being a distinct trait and a contributor to OSA pathogenesis, these findings indicate that average arousal intensity is independent of the preceding respiratory stimulus and that the physiological responses to arousal vary according to arousal intensity. SLEEP, Vol. 39, No. 12, 2016
Factors Influencing Arousal Intensity Pre-Arousal Respiratory Stimulus Related Factors
The lack of association between the magnitude of the negative epiglottic pressure immediately prior to arousal (respiratory arousal threshold) and average arousal intensity indicates that arousal intensity is independent of the key trigger for respiratory-related arousal. Rather than a graded arousal response according to the magnitude of respiratory stimuli, this finding
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is more consistent with arousal intensity being a distinct trait. However, other factors in addition to nadir pharyngeal pressure prior to an arousal, such as blood gas disturbances, can also trigger respiratory-related arousals.3,13–15 Indeed, increased duration of the preceding period of flow-limitation (time from the onset of the CPAP drop) was associated with increased arousal intensity. However, the degree of hypoxemia and the level of ventilatory impairment (minute ventilation and peak inspiratory flow) just prior to arousal were not associated with arousal intensity and were quantitatively similar between high and low intensity arousals. Furthermore, the effect of time to arousal on arousal intensity was only minor and the correlation between both parameters weak (time to arousal explained only approximately 15% of the variance in arousal intensity). Indeed, on average, the difference between low and high intensity arousals was less than a breath cycle (1.5s) in duration. Nonetheless, the notion that the time in which it takes to arouse from sleep, independent of the magnitude of the respiratory stimulus, influences the intensity of arousal is intriguing and may relate to differences in chemosensitivity. Consistent with this concept, the rate of change in nadir epiglottic pressure was associated with arousal intensity in the univariate analysis. Loop gain may be expected to influence the rate of change in nadir epiglottic pressure (and its relationship with arousal intensity). However, the lack of association between loop gain and the rate of change in nadir epiglottic pressure during the reduction in CPAP is likely due to our measure of loop gain being performed in the steady state. Nonetheless, steady state loop gain was independently associated with arousal intensity in the univariate analysis, although this parameter only explained a small proportion of the variance. Non-Stimulus Related Factors
The finding that arousal intensity increases with increasing arousal duration is consistent with the findings of an earlier study.6 The relationship between arousal intensity and duration was not a result of collinearity between the variables (as confirmed by Belsley collinearity diagnostics [MATLAB] between arousal intensity wavelet analysis parameters and arousal duration, performed on the original training data set used in the development of the wavelet analysis for arousal intensity scoring). In addition, this study confirms the initial finding involving ten OSA patients that arousal intensity does not differ between healthy controls and patients with OSA, or between NREM and REM sleep.6 Thus, while arousal intensity varies between individuals, the presence or absence of OSA and different sleep stages do not appear to systematically influence arousal intensity. However, the duration of arousals lasting < 15 s were longer in controls versus OSA patients. Similar to the respiratory arousal threshold3 and heart rate responses to arousal,6,7 this may reflect, at least in part, increased sleep pressure and cortical adaptation to repetitive respiratory loading in the OSA patients. In addition, the OSA patients in the current study had been treated with CPAP for at least 3 months. Thus, arousal intensities may still differ in untreated OSA patients. The current study also extends the findings of the earlier study by determining that neither age nor sex are important determinants of arousal intensity.7 SLEEP, Vol. 39, No. 12, 2016
Interestingly, a lower BMI was associated with higher arousal intensities in the univariate analysis. This finding is consistent with a blunted arousal response with obesity as is believed to occur in those with an obesity hypoventilation phenotype.3 While this observation is interesting, this association was not present in the multivariate analyses. Thus, additional investigation into the mechanisms of arousal is required to better understand the potential role of body habitus on arousal intensity. In the collective model, arousal duration and time to arousal were the only significant determinants of arousal intensity. Thus, the lack of association with the primary pre-arousal respiratory stimulus related factors combined with the fact that many arousal attributes, including average arousal intensity,7 are highly reproducible among subjects, strongly suggests that arousal intensity is a distinct phenotypic trait. Effects of Arousal Intensity on Ventilatory and Upper Airway Dilator Muscle Activity The pattern of changes in minute ventilation and dilator muscle activity following arousal, for both arousal intensity categories, are similar to those previously presented by Jordan and colleagues9 for ASDA (American Sleep Disorders Association) scored arousals. Increases in ventilation and dilator muscle activity following cortical arousal, according to standard definitions of arousal, have also been reported in other studies.8,12,16–18 Consistent with increased airflow with higher intensity arousals using a 0–4 point visual scale reported by Younes,2 the magnitude of the ventilatory and peak flow responses in the current study was greater with higher intensity arousals. Arousal intensity was associated with increases in tensor palatini muscle activity with less pronounced increases in peak but not tonic genioglossus muscle activity in the current study. Single motor unit studies of genioglossus indicate that unlike phasic motor neurons, which are actively recruited, tonic motor units are often inhibited during the immediate post-arousal period.17,19 This may explain, at least in part, the lack of an effect of arousal intensity on tonic genioglossus activity in the current study. This differs, however, to the findings of Younes and colleagues who performed brief reductions in CPAP (3 breaths) following prior CO2 stimulation in seven OSA patients, who also had appreciable genioglossus activity after discharge, and noted that tonic genioglossus muscle activity tended to be higher for the three breaths following the return to CPAP with higher intensity arousals.12 These apparent discrepancies are likely explained by differences in the experimental methodology. Differences include the use of CO2 and reintroduction of CPAP to predominantly complete obstructions in the prior study whereas the current study tended to examine less severe respiratory events and CPAP was not reintroduced for the postarousal analyses. Prior to the present study, the effects of arousal on tensor palatini activity had not been published.20 The current finding of increased multiunit tensor palatini activity with greater arousal intensities is consistent with more pronounced reintroduction of wakefulness drive and greater sleep/wake dependence of the tensor palatini muscle compared to genioglossus.11,21–24 In contrast, the genioglossus muscle is more
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responsive to subtle changes in respiratory stimuli and sleep stage effects.11,21–24 An increased ventilatory response to arousal with higher arousal intensities may contribute to OSA pathogenesis by destabilizing breathing control and contributing to subsequent upper airway collapse upon the return to sleep.2,8 Specifically, heightened arousal intensity leading to increased hyperventilation may lead to excessive reductions in carbon dioxide levels and, hence, hypoventilation and ventilatory control instability upon the return to sleep. Conversely, the novel finding that increased tensor palatini muscle activation occurs with higher arousal intensities for at least three breaths following arousal is likely to stabilize the upper airway. However, a relative lack of influence of arousal intensity on genioglossus muscle activity may contribute to overall discoordination or muscle asynchrony between the dilator muscles that could contribute to subsequent airway closure as highlighted by the recent findings of Dotan and colleagues.25 Specifically, activation of tensor palatini in the absence of coupled genioglossus activation may cause counterproductive motion and airway narrowing.26 CONCLUSIONS The findings from this study indicate that average arousal intensity is independent of the magnitude of the pre-arousal respiratory stimulus and that higher intensity arousals cause greater ventilatory and tensor palatini muscle responses following arousal, with comparatively modest increases in peak but not tonic genioglossus muscle activity. While recent findings have highlighted the potential importance of arousal in sleep apnea pathogenesis, the current findings demonstrate that not all cortical arousals or their consequences are the same. Thus, these data combined with prior studies support the concept that arousal intensity is a distinct trait and an independent contributor to OSA pathogenesis. REFERENCES 1. Rees K, Spence DP, Earis JE, Calverley PM. Arousal responses from apneic events during non-rapid-eye-movement sleep. Am J Respir Crit Care Med 1995;152:1016–21. 2. Younes M. Role of arousals in the pathogenesis of obstructive sleep apnea. Am J Respir Crit Care Med 2004;169:623–33. 3. Eckert DJ, Younes MK. Arousal from sleep: implications for obstructive sleep apnea pathogenesis and treatment. J Appl Physiol 2014;116:302–13. 4. Younes M, Hanly PJ. Immediate postarousal sleep dynamics: an important determinant of sleep stability in obstructive sleep apnea. J Appl Physiol 2016;120:801–8. 5. Berry RB, Budhiraja R, Gottlieb DJ, et al. Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual for the Scoring of Sleep and Associated Events. Deliberations of the Sleep Apnea Definitions Task Force of the American Academy of Sleep Medicine. J Clin Sleep Med 2012;8:597–619. 6. Azarbarzin A, Ostrowski M, Hanly P, Younes M. Relationship between arousal intensity and heart rate response to arousal. Sleep 2014;37:645–53. 7. Azarbarzin A, Ostrowski M, Younes M, et al. Arousal responses during overnight polysomnography and their reproducibility in healthy young adults. Sleep 2015;38:1313–21. 8. Jordan AS, Eckert DJ, Catcheside PG, McEvoy RD. Ventilatory response to brief arousal from non–rapid eye movement sleep is greater in men than in women. Am J Respir Crit Care Med 2003;168:1512–9.
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9. Jordan AS, Eckert DJ, Wellman A, Trinder JA, Malhotra A, White DP. Termination of respiratory events with and without cortical arousal in obstructive sleep apnea. Am J Respir Crit Care Med 2011;184:1183–91. 10. Eckert DJ, White DP, Jordan AS, Malhotra A, Wellman A. Defining phenotypic causes of obstructive sleep apnea. Identification of novel therapeutic targets. Am J Respir Crit Care Med 2013;188:996–1004. 11. Carberry JC, Jordan AS, White DP, Wellman A, Eckert DJ. Upper airway collapsibility (Pcrit) and pharyngeal dilator muscle activity are sleep-stage dependent. Sleep 2016;39:511–21. 12. Younes M, Loewen A, Ostrowski M, Hanly P. Short-term potentiation in the control of pharyngeal muscles in obstructive apnea patients. Sleep 2014;37:1833–49. 13. Xiao SC, He BT, Steier J, Moxham J, Polkey MI, Luo YM. Neural respiratory drive and arousal in patients with obstructive sleep apnea hypopnea. Sleep 2015;38:941–9. 14. Ayas NT, Brown R, Shea SA. Hypercapnia can induce arousal from sleep in the absence of altered respiratory mechanoreception. Am J Respir Crit Care Med 2000;162:1004–8. 15. Amatoury J, Jordan A, Wellman A, White D, Eckert D. New insights into the mechanisms of respiratory load-induced arousal: role of breath timing and respiratory load compensation [abstract]. Sleep Biol Rhythms 2015;13:A171. 16. Horner RL, Rivera MP, Kozar LF, Phillipson EA. The ventilatory response to arousal from sleep is not fully explained by differences in CO2 levels between sleep and wakefulness. J Physiol (Lond) 2001;534:881–90. 17. Wilkinson V, Malhotra A, Nicholas CL, et al. Discharge patterns of human genioglossus motor units during arousal from sleep. Sleep 2010;33:379–87. 18. Jordan AS, Cori JM, Dawson A, et al. Arousal from sleep does not lead to reduced dilator muscle activity or elevated upper airway resistance on return to sleep in healthy individuals. Sleep 2014;38:53–9. 19. Trinder J, Jordan AS, Nicholas CL. Discharge properties of upper airway motor units during wakefulness and sleep. Prog Brain Res 2014;212:59–75. 20. Yeo A, Catcheside PG, George K, Thomson K, McEvoy RD. Prolonged post-arousal upper airway dilator muscle activity in men with obstructive sleep apnoea on continuous positive airway pressure [abstract]. Sleep Biol Rhythms 2007;5:A133. 21. Horner RL, Hughes SW, Malhotra A. State-dependent and reflex drives to the upper airway: basic physiology with clinical implications. J Appl Physiol 2014;116:325–36. 22. Lo YL, Jordan AS, Malhotra A, et al. Influence of wakefulness on pharyngeal airway muscle activity. Thorax 2007;62:799–805. 23. Carberry JC, Hensen H, Fisher LP, et al. Mechanisms contributing to the response of upper-airway muscles to changes in airway pressure. J Appl Physiol 2015;118:1221–8. 24. Worsnop C, Kay A, Pierce R, Kim Y, Trinder J. Activity of respiratory pump and upper airway muscles during sleep onset. J Appl Physiol 1998;85:908–20. 25. Dotan Y, Pillar G, Schwartz AR, Oliven A. Asynchrony of lingual muscle recruitment during sleep in obstructive sleep apnea. J Appl Physiol 2015;118:1516–24. 26. Brown EC, Cheng S, McKenzie DK, Butler JE, Gandevia SC, Bilston LE. Tongue and lateral upper airway movement with mandibular advancement. Sleep 2013;36:397–404.
ACKNOWLEDGMENTS The authors thank the Brigham and Women’s Sleep Disorders Research Program staff for technical support and Professor Rob Herbert (NeuRA) for statistical advice. Author contributions: Dr. Amatoury and Dr. Eckert developed the study concepts and wrote the manuscript. Dr. Amatoury performed the data analyses. Dr. Azarbarzin performed the arousal intensity analyses. Dr. Eckert and Dr. Jordan collected the data. All authors provided important insight on data interpretation and contributed to the final version of the manuscript
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SUBMISSION & CORRESPONDENCE INFORMATION
(NHMRC) Centre of Research Excellence (CRE) (1060992) and National Institute of Health (5R01HL048531, PI Andrew Wellman). Dr. Amatoury is supported by a NeuroSleep NHMRC CRE Postdoctoral Fellowship (1060992). Dr. Eckert is supported by a NHMRC RD Wright Fellowship (1049814). Dr. Younes is majority owner of YRT Ltd. Dr. Wellman has received research support from Philips Respironics. Dr. Azarbarzin is a former employee of YRT Ltd. The arousal intensity metric applied in this manuscript was developed under YRT Ltd by Dr. Younes and Dr. Azarbarzin. Dr. Jordan has received research support from Philips Respironics. The work was performed at the Division of Sleep Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA.
Submitted for publication April, 2016 Submitted in final revised form June, 2016 Accepted for publication August, 2016 Address correspondence to: Dr. Jason Amatoury, Neuroscience Research Australia (NeuRA), PO Box 1165, Randwick, NSW, Australia 2031; Tel: +61 2 9399 1834; Fax: +61 2 9399 1027; Email: [email protected]
DISCLOSURE STATEMENT This was not an industry supported study. This work was supported by the NeuroSleep National Health and Medical Research Council of Australia
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