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ORIGINAL ARTICLE
Evaluation of the role of lung volume and airway size and shape in supine-predominant obstructive sleep apnoea patients SIMON A. JOOSTEN,1,2 SCOTT A. SANDS,3 BRADLEY A. EDWARDS,3,4 KAIS HAMZA,5 ANTHONY TURTON,1 KENNETH K. LAU,6 MARCUS CROSSETT,6 PHILIP J. BERGER2 AND GARUN S. HAMILTON1,7 1
Monash Lung and Sleep, Monash Health, Monash Medical Centre, 2Ritchie Centre, Monash Institute of Medical Research, 4 Department of Physiology and School of Psychological Sciences, Schools of 5Mathematical Sciences and 7Clinical Sciences, Monash University, 6Department of Diagnostic Imaging, Monash Health, Melbourne, Australia, and 3Division of Sleep Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts, USA
ABSTRACT Background and objective: This study aimed to evaluate the involvement of airway cross-sectional area and shape, and functional residual capacity (FRC), in the genesis of obstructive sleep apnoea (OSA) in patients with supine-predominant OSA. Methods: Three groups were recruited: (i) supine OSA, defined as a supine apnoea–hyponoea index (AHI) at least twice that of the non-supine AHI; (ii) rapid eye movement (REM) OSA, defined as REM AHI at least twice the non-REM AHI and also selected to have supine AHI less than twice that of the non-supine AHI (i.e. to be non-positional); and (iii) no OSA, defined as an AHI less than five events per hour. The groups were matched for age, gender and body mass index. Patients underwent four-dimensional computed tomography scanning of the upper airway in the supine and lateral decubitus positions. FRC was measured in the seated, supine and lateral decubitus positions. Results: Patients with supine OSA demonstrated a significant decrease in FRC of 340 mL (P = 0.026) when moving from the lateral to supine position compared to controls with no OSA, and REM OSA patients. We found no differences between groups in upper airway size and shape. However, all groups showed a significant change in airway shape with the velopharyngeal airway adopting a more elliptoid shape (with the long axis laterally oriented), with reduced anteroposterior diameter in the supine position. Conclusions: A fall in FRC when moving lateral to supine in supine OSA patients may be an important triggering factor in the generation of OSA in this patient group. Key words: apnoea, functional residual capacity, lung volume measurement, sleep, supine position. Correspondence: Simon A. Joosten, Monash Lung and Sleep, Monash Health, Monash Medical Centre, 246 Clayton Road, Clayton, Vic. 3168, Australia. Email: [email protected] Received 20 November 2014; invited to revise 6 January 2015; revised 29 January 2015; accepted 16 February 2015 (Associate Editor: Safwan Badr). Article first published online: 4 May 2015 © 2015 Asian Pacific Society of Respirology
SUMMARY AT A GLANCE There is a significant reduction in lung volume when moving from the lateral to supine position in supine-predominant obstructive sleep apnoea patients, which may be an important triggering factor in their airway obstruction. Additionally, the upper airway is more elliptoid in shape when supine in both sleep apnoea patients and controls.
Abbreviations: AHI, apnoea–hyponoea index; ANOVA, one-way analysis of variance; AP, anteroposterior; BMI, body mass index; CSA, cross-sectional area; CT, computed tomography; FRC, functional residual capacity; OSA, obstructive sleep apnoea; REM, rapid eye movement.
INTRODUCTION Obstructive sleep apnoea (OSA) is due to repetitive collapse of the upper airway during sleep. The majority of patients with OSA have mild–moderate disease1 with intermittent periods of OSA mixed with periods of stable breathing, which may be driven by a change in body position. Supine-predominant OSA, when defined as more than twice the number of respiratory events in supine compared with non-supine positions, is present in approximately 60% of the OSA population.2,3 An obvious inference from the existence of supinepredominant OSA is that at least one of the underlying mechanisms of airway collapse is worsened in the supine position. A number of pathogenic factors may predispose to OSA, including unfavourable airway anatomy,4 an inability of muscles to open or stiffen the airway during sleep,5,6 an oversensitive ventilatory control system (i.e. high loop gain),7 a low respiratory arousal threshold8 and a low lung volume.9 Importantly, unfavourable airway anatomy (pharyngeal critical closing pressure, Pcrit, less than −2 cmH2O) is Respirology (2015) 20, 819–827 doi: 10.1111/resp.12549
820 seen in over 80% of patients with OSA.10 The propensity to collapse is related to airway size or airway shape (elliptical vs circular). According to Laplace’s law a more rounded airway is inherently more stable than an elliptically shaped one as the transmural pressure gradient required to collapse the airway varies inversely with the radius of curvature. Furthermore, the orientation of an elliptically shaped airway is important, as collapse is most likely to occur at the end segments of the long axis.11,12 Although airway size and shape are affected by body position, there is disagreement as to how they are altered. Studies in awake OSA patients consistently report no significant change in pharyngeal cross-sectional area (CSA) moving from lateral to supine.13–16 The majority of airway shape studies in awake OSA patients demonstrate that the velopharyngeal shape approximates a laterally oriented ellipse in the supine position,13,14,17,18 whereas others suggest the shape is more circular when supine.4,19 Limiting their value, existing studies have not always used controls, and only one matched subjects for age, gender and body mass index (BMI).13 Crucially, only one existing study has targeted patients with supinepredominant OSA14 and no studies to date have assessed differences in CSA and shape when moving from the lateral to supine position in supinepredominant versus non-position dependent OSA patients and matched controls. A key determinate of airway shape is lung volume (in particular functional residual capacity, FRC), which influences airway collapsibility20 through caudal tracheal displacement and subsequent upper airway tissue pressure changes.21 Both BMI and body position have been shown to influence FRC. The interaction between BMI and FRC is complex and the BMI of a subject will influence how much lung volume changes with changing body position, at least in the non-obese range of BMIs.22 In normal BMI subjects without OSA, FRC falls significantly when moving from the lateral to supine position,23,24 but this is unlikely to be the case in the morbidly obese (BMI >35 kg/m2) as there is no change when moving from the sitting to the supine position.25 Patients with supine OSA more commonly have a BMI in the overweight range (25–30 kg/m2)3 compared with nonpositional OSA patients who are more likely to have a BMI in the obese range (>30 kg/m2)3; however, there are no published data investigating the effect of body position changes on lung volume in OSA patients. Given the current literature, we performed a study to test three hypotheses to explain the existence of supine-predominant OSA. We hypothesize that when supine-predominant OSA patients assume the supine position they will experience (i) a decrease in airway CSA; (ii) a deformation of the upper airway into a more elliptoid shape; and (iii) a decrease in FRC. To evaluate the involvement of airway CSA, airway shape and FRC in the genesis of supinepredominant OSA we compared each factor in subjects with supine-predominant OSA to those in two matched control groups: one with non-positional OSA and one comprising normal controls without OSA. Respirology (2015) 20, 819–827
SA Joosten et al.
METHODS Ethics approval was obtained from the Human Research Ethics Committee of Monash Health. Patients were recruited based on polysomnogram results (polysomnograms staged and scored in accordance with American Academy of Sleep Medicine guidelines),26 if they experienced >30 min supine sleep, >30 min non-supine sleep and >15 min of rapid eye movement (REM) sleep and had an apnoea and hypopnoea index (AHI) more than five events per hour. Subjects were recruited into three groups: 1 Supine-predominant OSA patients: – Supine AHI (AHIS) to non-supine AHI (AHINS) ratio greater than 2:1. Additionally, patients were required to have REM AHI (AHIREM) to non-REM AHI (AHINREM) ratio of less than 2:1 to ensure there was no sleep stage effect. – During the recruitment of this study we published data demonstrating that the definition of supine OSA with the greatest level of repeatability from night-to-night is one which incorporates a supine AHI to non-supine AHI of >4:1.27 Although we set out initially to recruit subjects with a ratio of >2:1 for this study, all subjects included, in fact, also have a supine to non-supine ratio of >4:1. 2 Control patients without OSA: – Overall AHI less than five per hour. Patients were also required to have an AHI less than five per hour in supine sleep, in non-supine sleep, in REM sleep and in non-REM sleep. 3 REM-based OSA patients with no body positionality to their respiratory events: – Required to have the AHIS to AHINS ratio
ORIGINAL ARTICLE
Evaluation of the role of lung volume and airway size and shape in supine-predominant obstructive sleep apnoea patients SIMON A. JOOSTEN,1,2 SCOTT A. SANDS,3 BRADLEY A. EDWARDS,3,4 KAIS HAMZA,5 ANTHONY TURTON,1 KENNETH K. LAU,6 MARCUS CROSSETT,6 PHILIP J. BERGER2 AND GARUN S. HAMILTON1,7 1
Monash Lung and Sleep, Monash Health, Monash Medical Centre, 2Ritchie Centre, Monash Institute of Medical Research, 4 Department of Physiology and School of Psychological Sciences, Schools of 5Mathematical Sciences and 7Clinical Sciences, Monash University, 6Department of Diagnostic Imaging, Monash Health, Melbourne, Australia, and 3Division of Sleep Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts, USA
ABSTRACT Background and objective: This study aimed to evaluate the involvement of airway cross-sectional area and shape, and functional residual capacity (FRC), in the genesis of obstructive sleep apnoea (OSA) in patients with supine-predominant OSA. Methods: Three groups were recruited: (i) supine OSA, defined as a supine apnoea–hyponoea index (AHI) at least twice that of the non-supine AHI; (ii) rapid eye movement (REM) OSA, defined as REM AHI at least twice the non-REM AHI and also selected to have supine AHI less than twice that of the non-supine AHI (i.e. to be non-positional); and (iii) no OSA, defined as an AHI less than five events per hour. The groups were matched for age, gender and body mass index. Patients underwent four-dimensional computed tomography scanning of the upper airway in the supine and lateral decubitus positions. FRC was measured in the seated, supine and lateral decubitus positions. Results: Patients with supine OSA demonstrated a significant decrease in FRC of 340 mL (P = 0.026) when moving from the lateral to supine position compared to controls with no OSA, and REM OSA patients. We found no differences between groups in upper airway size and shape. However, all groups showed a significant change in airway shape with the velopharyngeal airway adopting a more elliptoid shape (with the long axis laterally oriented), with reduced anteroposterior diameter in the supine position. Conclusions: A fall in FRC when moving lateral to supine in supine OSA patients may be an important triggering factor in the generation of OSA in this patient group. Key words: apnoea, functional residual capacity, lung volume measurement, sleep, supine position. Correspondence: Simon A. Joosten, Monash Lung and Sleep, Monash Health, Monash Medical Centre, 246 Clayton Road, Clayton, Vic. 3168, Australia. Email: [email protected] Received 20 November 2014; invited to revise 6 January 2015; revised 29 January 2015; accepted 16 February 2015 (Associate Editor: Safwan Badr). Article first published online: 4 May 2015 © 2015 Asian Pacific Society of Respirology
SUMMARY AT A GLANCE There is a significant reduction in lung volume when moving from the lateral to supine position in supine-predominant obstructive sleep apnoea patients, which may be an important triggering factor in their airway obstruction. Additionally, the upper airway is more elliptoid in shape when supine in both sleep apnoea patients and controls.
Abbreviations: AHI, apnoea–hyponoea index; ANOVA, one-way analysis of variance; AP, anteroposterior; BMI, body mass index; CSA, cross-sectional area; CT, computed tomography; FRC, functional residual capacity; OSA, obstructive sleep apnoea; REM, rapid eye movement.
INTRODUCTION Obstructive sleep apnoea (OSA) is due to repetitive collapse of the upper airway during sleep. The majority of patients with OSA have mild–moderate disease1 with intermittent periods of OSA mixed with periods of stable breathing, which may be driven by a change in body position. Supine-predominant OSA, when defined as more than twice the number of respiratory events in supine compared with non-supine positions, is present in approximately 60% of the OSA population.2,3 An obvious inference from the existence of supinepredominant OSA is that at least one of the underlying mechanisms of airway collapse is worsened in the supine position. A number of pathogenic factors may predispose to OSA, including unfavourable airway anatomy,4 an inability of muscles to open or stiffen the airway during sleep,5,6 an oversensitive ventilatory control system (i.e. high loop gain),7 a low respiratory arousal threshold8 and a low lung volume.9 Importantly, unfavourable airway anatomy (pharyngeal critical closing pressure, Pcrit, less than −2 cmH2O) is Respirology (2015) 20, 819–827 doi: 10.1111/resp.12549
820 seen in over 80% of patients with OSA.10 The propensity to collapse is related to airway size or airway shape (elliptical vs circular). According to Laplace’s law a more rounded airway is inherently more stable than an elliptically shaped one as the transmural pressure gradient required to collapse the airway varies inversely with the radius of curvature. Furthermore, the orientation of an elliptically shaped airway is important, as collapse is most likely to occur at the end segments of the long axis.11,12 Although airway size and shape are affected by body position, there is disagreement as to how they are altered. Studies in awake OSA patients consistently report no significant change in pharyngeal cross-sectional area (CSA) moving from lateral to supine.13–16 The majority of airway shape studies in awake OSA patients demonstrate that the velopharyngeal shape approximates a laterally oriented ellipse in the supine position,13,14,17,18 whereas others suggest the shape is more circular when supine.4,19 Limiting their value, existing studies have not always used controls, and only one matched subjects for age, gender and body mass index (BMI).13 Crucially, only one existing study has targeted patients with supinepredominant OSA14 and no studies to date have assessed differences in CSA and shape when moving from the lateral to supine position in supinepredominant versus non-position dependent OSA patients and matched controls. A key determinate of airway shape is lung volume (in particular functional residual capacity, FRC), which influences airway collapsibility20 through caudal tracheal displacement and subsequent upper airway tissue pressure changes.21 Both BMI and body position have been shown to influence FRC. The interaction between BMI and FRC is complex and the BMI of a subject will influence how much lung volume changes with changing body position, at least in the non-obese range of BMIs.22 In normal BMI subjects without OSA, FRC falls significantly when moving from the lateral to supine position,23,24 but this is unlikely to be the case in the morbidly obese (BMI >35 kg/m2) as there is no change when moving from the sitting to the supine position.25 Patients with supine OSA more commonly have a BMI in the overweight range (25–30 kg/m2)3 compared with nonpositional OSA patients who are more likely to have a BMI in the obese range (>30 kg/m2)3; however, there are no published data investigating the effect of body position changes on lung volume in OSA patients. Given the current literature, we performed a study to test three hypotheses to explain the existence of supine-predominant OSA. We hypothesize that when supine-predominant OSA patients assume the supine position they will experience (i) a decrease in airway CSA; (ii) a deformation of the upper airway into a more elliptoid shape; and (iii) a decrease in FRC. To evaluate the involvement of airway CSA, airway shape and FRC in the genesis of supinepredominant OSA we compared each factor in subjects with supine-predominant OSA to those in two matched control groups: one with non-positional OSA and one comprising normal controls without OSA. Respirology (2015) 20, 819–827
SA Joosten et al.
METHODS Ethics approval was obtained from the Human Research Ethics Committee of Monash Health. Patients were recruited based on polysomnogram results (polysomnograms staged and scored in accordance with American Academy of Sleep Medicine guidelines),26 if they experienced >30 min supine sleep, >30 min non-supine sleep and >15 min of rapid eye movement (REM) sleep and had an apnoea and hypopnoea index (AHI) more than five events per hour. Subjects were recruited into three groups: 1 Supine-predominant OSA patients: – Supine AHI (AHIS) to non-supine AHI (AHINS) ratio greater than 2:1. Additionally, patients were required to have REM AHI (AHIREM) to non-REM AHI (AHINREM) ratio of less than 2:1 to ensure there was no sleep stage effect. – During the recruitment of this study we published data demonstrating that the definition of supine OSA with the greatest level of repeatability from night-to-night is one which incorporates a supine AHI to non-supine AHI of >4:1.27 Although we set out initially to recruit subjects with a ratio of >2:1 for this study, all subjects included, in fact, also have a supine to non-supine ratio of >4:1. 2 Control patients without OSA: – Overall AHI less than five per hour. Patients were also required to have an AHI less than five per hour in supine sleep, in non-supine sleep, in REM sleep and in non-REM sleep. 3 REM-based OSA patients with no body positionality to their respiratory events: – Required to have the AHIS to AHINS ratio
