Am J Physiol Heart Circ Physiol 307: H397–H404, 2014. First published May 30, 2014; doi:10.1152/ajpheart.00194.2014.

Blunted cerebral blood flow velocity in response to a nitric oxide donor in postural tachycardia syndrome Andrew T. Del Pozzi, Akash Pandey, Marvin S. Medow, Zachary R. Messer, and Julian M. Stewart Departments of Pediatrics and Physiology, New York Medical College, Center for Hypotension, Hawthorne, New York Submitted 21 March 2014; accepted in final form 26 May 2014

Del Pozzi AT, Pandey A, Medow MS, Messer ZR, Stewart JM. Blunted cerebral blood flow velocity in response to a nitric oxide donor in postural tachycardia syndrome. Am J Physiol Heart Circ Physiol 307: H397–H404, 2014. First published May 30, 2014; doi:10.1152/ajpheart.00194.2014.—Cognitive deficits are characteristic of postural tachycardia syndrome (POTS). Intact nitrergic nitric oxide (NO) is important to cerebral blood flow (CBF) regulation, neurovascular coupling, and cognitive efficacy. POTS patients often experience defective NO-mediated vasodilation caused by oxidative stress. We have previously shown dilation of the middle cerebral artery in response to a bolus administration of the NO donor sodium nitroprusside (SNP) in healthy volunteers. In the present study, we hypothesized a blunted middle cerebral artery response to SNP in POTS. We used combined transcranial Doppler-ultrasound to measure CBF velocity and near-infrared spectroscopy to measure cerebral hemoglobin oxygenation while subjects were in the supine position. The responses of 17 POTS patients were compared with 12 healthy control subjects (age: 14 –28 yr). CBF velocity in POTS patients and control subjects were not different at baseline (75 ⫾ 3 vs. 71 ⫾ 2 cm/s, P ⫽ 0.31) and decreased to a lesser degree with SNP in POTS patients (to 71 ⫾ 3 vs. 62 ⫾ 2 cm/s, P ⫽ 0.02). Changes in total and oxygenated hemoglobin (8.83 ⫾ 0.45 and 8.13 ⫾ 0.48 ␮mol/kg tissue) were markedly reduced in POTS patients compared with control subjects (14.2 ⫾ 1.4 and 13.6 ⫾ 1.6 ␮mol/kg tissue), primarily due to increased venous efflux. The data indicate reduced cerebral oxygenation, blunting of cerebral arterial vasodilation, and heightened cerebral venodilation. We conclude, based on the present study outcomes, that decreased bioavailability of NO is apparent in the vascular beds, resulting in a downregulation of NO receptor sites, ultimately leading to blunted responses to exogenous NO. near-infrared spectroscopy; orthostatic intolerance; autonomic nervous system ORTHOSTATIC INTOLERANCE (OI) can be defined by the inability to tolerate upright posture, because the signs and symptoms are relieved by lying down (42). Typical symptoms include impaired consciousness, cognitive deficits, lightheadedness, headache, fatigue, weakness, nausea, abdominal pain, sweating, tremulousness, and exercise intolerance (25), which are related to deficient cerebral perfusion and sympathetic activation. Postural orthostatic tachycardia syndrome (POTS) is the most common form of chronic OI (2), defined by the symptoms of OI with concomitant excessive upright tachycardia. We have previously demonstrated impaired cerebral blood flow (CBF) in POTS (34). Nitric oxide (NO) is integral to CBF regulation (22) and neurovascular coupling (7, 8, 10) through its effects on smooth muscle vasodilation. Thus, using near-infrared spectroscopy (NIRS) combined with transcranial Doppler-ultrasound (TCD),

Address for reprint requests and other correspondence: J. M. Stewart, Center for Hypotension, 19 Bradhurst Ave., Suite 1600 S., Hawthorne, NY 10532 (e-mail: [email protected]). http://www.ajpheart.org

we showed that a bolus injection of sodium nitroprusside (SNP), a NO donor, to healthy volunteers resulted in a robust increase in cerebral total hemoglobin (THb) and oxyhemoglobin (HbO2) due to increased arterial vasodilation. This included a robust increase in the diameter of the middle cerebral artery (MCA), as shown in animals (37, 49) as well as in humans using TCD (51) and positron emission tomography (PET) (60). Because deficits in NO bioavailability as well as decrements in NO-dependent vasodilation have been linked to oxidative stress and ANG II signaling in POTS (50), we hypothesized that the CBF response to an administered NO donor in POTS patients might be blunted compared with healthy volunteers. METHODS

Patients and control subjects. Seventeen POTS patients and 12 control subjects were compared. Characteristics of the study participants are shown in Table 1. Potential POTS patients for the study were selected based on referral to our clinic for possible POTS, all having symptoms lasting longer than 6 mo; diagnosis was confirmed in our laboratory by means of completing a screening head-up tilt (HUT) test. The criteria for POTS in adults is a heart rate (HR) increase of ⱖ30 beats/min within 10 min of orthostasis; in children, a higher threshold is used of a HR increase of ⱖ40 beats/min or an absolute HR of ⱖ120 beats/min (26, 48). Only patients who fit the aforementioned POTS criteria were enrolled in the study. POTS enrollees were either therapeutically naïve (⬃85%) or were weaned off medication over 2 wk before the experiments. After withdrawal from medication, POTS patients were medication free for a minimum of 2 wk. Healthy control subjects had no history of OI. Exclusionary criteria for participation in the study included any infectious or systemic disease, including diseases of the central nervous system, autonomic, endocrine, respiratory, metabolic, or cardiovascular diseases, competitive athletic training, and use of nicotine or any other chronic medication, excluding oral contraceptives. In addition, any subject with a history of fainting or experiencing syncope during a screening HUT test was excluded from participation. This study was approved by the Committee for the Protection of Human Subjects (Institutional Review Board) of New York Medical College and conformed with The Declaration of Helsinki. All subjects 18 yr and older signed informed consent before participation, and those ⬍18 yr old gave assent and their legal guardians signed informed consent forms. Instrumentation. All participants arrived for testing at 9:30 AM after a 2-h fast. Participants were then instructed about the day’s experiment and outfitted with the instrumentation. An intravenous catheter was placed in the left antecubital vein. Beat-to-beat arterial blood pressure was measured using an oscillometric calibrated Finometer (FMS, Amsterdam, The Netherlands). TCD (Multigon, Yonkers, NY) insonated the right MCA, and the signal was optimized for depth and signal strength. Respiratory plethysmography (Respitrace 200, NIMS, North Bay Village, FL) measured changes in respiration. A combined capnograph and integrated pulse oximeter (Smith Medical PM, Waukesha, WI) was used to measure end-tidal CO2 and

0363-6135/14 Copyright © 2014 the American Physiological Society

H397

H398

BLUNTED RESPONSE TO NO DONOR IN POTS

Table 1. Demographic characteristics for control subjects and POTS patients Control Subjects

Age, yr Weight, kg Height, cm Hemoglobin, g/dl

POTS Patients

Men

Women

Men

Women

23 ⫾ 3 84 ⫾ 11 178 ⫾ 6 14.5 ⫾ 0.6

23 ⫾ 4 63 ⫾ 5* 165 ⫾ 4* 13.1 ⫾ 0.6*

20 ⫾ 5 78 ⫾ 7 176 ⫾ 5 14.1 ⫾ 0.9

21 ⫾ 4 58 ⫾ 7* 167 ⫾ 8* 13.2 ⫾ 0.8*

Data are shown means ⫾ SD; n ⫽ 12 control subjects and 17 postural tachycardia syndrome (POTS) patients. *P ⬍ 0.05, men compared with women. There were no significant differences when control subjects were compared with POTS patients.

arterial O2 saturation. An ECG measured HR from the beat-to-beat cardiac electrical intervals. Continuous-wave spatially resolved NIRS (Oxymon MKII, Artinis) was used to monitor changes in HbO2, deoxyhemoglobin (Hb), and THb over the same volume of cerebral tissue throughout the protocol (40). Infrared signals produced by the cutaneous bed were resolved through spatial resolution, ensuring that the infrared signals from the brain were the only signals analyzed. NIRS was sampled at 50 Hz. Using an integrated digital-to-analog converter, the sampled NIRS signal was reconstructed as an analog signal. The modified Beer-Lambert law was used to calculate micromolar changes in tissue HbO2 and Hb across time using optical densities from near-infrared light at 780 and 850 nm. Depending on the subject characteristics, different differential path-length factors were used for each individual participant (13). We were aware that the volume of cerebral tissue illuminated may differ between the participants; however, within each participant, that volume was assumed constant. To index changes in blood volume within the illuminated brain volume, changes in THb were obtained by adding the changes in HbO2 to the changes in Hb. This was done because only changes in Hb and HbO2 could be measured. To define baseline values, averages of Hb and HbO2 during quiet rest were used. Because these are noninvasive measurements, we assumed that the notable changes in TCD representing CBF velocity (CBFv) were also taking place in the conduit vessels feeding the tissue that was illuminated by NIRS. All other analog signals were digitized at 200 Hz with custom signal-processing software and were analyzed offline by the same trained researcher. Protocol. After instrumentation, subjects remained supine for 30 min to acclimate to their surroundings and the equipment. After the 30-min acclimation period, a 10-min baseline period was recorded. The modified Oxford maneuver was completed by first administering a bolus injection of 100 ␮g of sodium nitroprusside (SNP) followed by 150 ␮g phenylephrine (PE) after the subjects’ systolic blood pressure (SBP) dropped by 30 mmHg or 1 min after the injection of SNP, whichever occurred first (46, 51); all variables were continuously recorded. We noted an approximate decrease in SBP of 15–30 mmHg below baseline with SNP administration and a subsequent increase in SBP of ⬃15–30 mmHg above baseline by the administration of PE. Changes in cerebral hemoglobin concentration were measured by NIRS while CBFv of the MCA was measured by TCD. The modified Oxford method was performed twice for each subject with a 30-min recovery period in between. Data analysis. Mean arterial pressure (MAP) was calculated from SBP and diastolic blood pressure (DBP) as follows: MAP ⫽ 1/3 ⫻ SBP ⫹ 2/3 ⫻ DBP. Mean CBFv was calculated from systolic CBFv and diastolic CBFv as follows: mean CBFv ⫽ 1/3 ⫻ systolic CBFv ⫹ 2/3 ⫻ diastolic CBFv and was verified by automated multibeat time averaging. NIRS generates relative changes in concentrations of HbO2 and Hb that, when summed, yield THb. Because all participants remained supine and at rest during the entire study, we assumed that

cerebral O2 consumption remained constant during the drug administration phase (39). Hemoglobin mass-balance considerations for the sample volume defined by equations for the conservation of mass of HbO2, Hb, and THb per unit sampled mass of cerebral tissue (in ␮mol/kg tissue) (38, 51) were used. Additionally, we worked on the assumption that venous saturation (Sv) and arterial saturation (Sa) remained unaffected by bolus injection of SNP (17) and PE (5). By determining Sa and Sv, we were able to calculate changes in arterial inflow (⌬Qa) and venous outflow (⌬Qv) (51). Sa was measured using pulse oximetry. To determine Sv, we used a form of venous occlusion plethysmography, right jugular venous occlusion, where gentle pressure was applied for 10 s to the right side of the patient’s neck, occluding their jugular vein with minimal hemodynamic disturbance to the patient during baseline measurements (12, 51, 61). This was measured only at baseline. Both THb and HbO2 increased linearly with time. Using least-squares methods, the slope of each increase was obtained. The ratio of HbO2 to THb was used to estimate Sv. Additionally, this was used for all subsequent calculations. Qv is equal to Qa ⫺ (dTHb/dt), where t is time (51); therefore, dHbO2 dt

⫺ Sv

dTHb dt

⫽ 共Sa ⫺ Sv兲⌬Qa

and dHbO2 dt

⫺ Sa

dTHb dt

⫽ 共Sa ⫺ Sv兲⌬Qa

Rearranging the above equations leads us to the following solution for ⌬Qa: dHbO2 ⌬Qa ⫽

dt

⫺ Sv

dTHb dt

共 S a ⫺ S v兲

and to the following solution for ⌬Qv: dHbO2 ⌬Qv ⫽

dt

⫺ Sa

dTHb dt

共 S a ⫺ S v兲

A complete explanation of the mathematic equations has been previously published and can be found in Stewart et al. (51). Statistics. Because there were no differences between the sexes, both male and female data were combined for group analysis. During the modified Oxford maneuver, measurements of arterial pressures, HR, CBFv, changes in Hb, HbO2, and THb via NIRS, and NIRSestimated changes in Qa and Qv (⌬Qa and ⌬Qv) were tabulated before SNP, at the peak response to SNP, and during the peak response to PE and were subsequently analyzed by a one-way ANOVA. When appropriate, post hoc evaluations were made using the Bonferroni procedure. Significance was set a priori, and differences were considered significant when P values of ⬍0.05 was achieved. All values are reported as means ⫾ SE. Results were calculated using SPSS 16 (SPSS, Chicago, IL). RESULTS

Hemodynamic responses to bolus SNP followed by bolus PE. Figure 1 is provided for illustrative purposes and shows a representative subject from both the control group and POTS group for TCD and NIRS tracings during the modified Oxford method. Comparative (control vs. POTS) hemodynamic responses to SNP and PE are shown in Table 1. Both control and POTS groups responded with a significant increase in HR after SNP (P ⬍ 0.001) and showed a significant decrease in HR after the PE bolus (P ⬍ 0.001).

AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00194.2014 • www.ajpheart.org

H399

BLUNTED RESPONSE TO NO DONOR IN POTS 100

Control

MAP

CBFv

100

POTS 80

CBFv (cm/s)

MAP (mmHg)

80

60

40

SNP

60

40

Phenylephrine 20

20

0

0

20

20 HbO2 Oxyhemoglobin (µM/kg)

Total Hemoglobin (µM/kg)

THb

10

0

-10

10

Fig. 1. Data presented from one representative subject from each group [postural tachycardia syndrome (POTS) and control] during the modified Oxford maneuver in which a bolus of sodium nitroprusside (SNP) was followed 1 min later by a bolus of phenylephrine (PE). Top left: mean arterial pressure (MAP) in both control and POTS groups decreased below baseline with SNP and increased above baseline with PE. Top right: cerebral blood flow velocity (CBFv) measured via transcranial Doppler-ultrasound. Note that the POTS patient did not fall off as much as that of the control subject. Bottom left: total hemoglobin (THb) per kilogram of brain tissue. Note that the control subject had a much larger increase in THb than that of the POTS patient. Bottom right: oxyhemoglobin (HbO2) per kilogram of brain tissue. Note that the POTS patient had a blunted response compared with the control subject.

0

-10 Time (seconds)

Time (seconds)

In addition, MAP, SBP, and DBP decreased significantly (P ⬍ 0.001) after SNP and increased significantly (P ⬍ 0.001) after PE in both groups. The respiratory rate of the control group was unaffected; however, the POTS group experienced a significant increase in respiratory rate (P ⬍ 0.001) after SNP infusion, which returned to baseline values after the PE bolus (P ⬍ 0.001). For the POTS group, end-tidal CO2 was relatively unchanged. Conversely, endtidal CO2 in the control group was increased after PE compared with SNP (P ⬍ 0.05). Neither group experienced a change in Sa during any phase of the protocol. After the SNP and PE bolus injections, Qa in the POTS group was significantly lower than that of the control group (P ⬍ 0.01). Additionally, Qv in the POTS group was greater than that of the control group (P ⬍ 0.01), as measured by NIRS (Table 2). ⌬THb in the POTS group was decreased significantly compared with the control group after SNP administration (8.83 ⫾ 0.45 vs.15.2 ⫾ 1.4 ␮mol/kg tissue, respectively, P ⬍ 0.01). ⌬HbO2 was decreased significantly in the POTS group compared with the control group after SNP adminis-

tration (8.13 ⫾ 0.46 vs.13.6 ⫾ 1.6 ␮mol/kg tissue, respectively, P ⬍ 0.01; Fig. 2). However, after the PE bolus, ⌬THb and ⌬HbO2 in the POTS and control groups were not statistically different (Fig. 2). Both groups experienced significant (P ⬍ 0.001) decreases in CBFv in response to SNP administration. However, the POTS group did not experience a drop in CBFv as large as the control group (change of ⫺4 ⫾ 2 vs. ⫺9 ⫾ 2 cm/s, respectively, P ⬍ 0.001); these results are shown in Fig. 2. DISCUSSION

One of the most debilitating symptoms of POTS is cognitive impairment (32, 44), sometimes referred to as “brain fog” (43). In past work, using an executive memory task in younger POTS patients, we demonstrated that POTS patients exhibit a progressive cognitive impairment during step-wise incremental orthostatic stress (36). Moreover, cognitive impairment was associated with neurovascular uncoupling (35, 52) such that the normal increase in CBF after neuronal activity (functional hyperemia) was absent in POTS patients (32, 52). Neurovas-

AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00194.2014 • www.ajpheart.org

H400

BLUNTED RESPONSE TO NO DONOR IN POTS

Table 2. Supine hemodynamic measurements before the Oxford maneuver and after bolus injections of SNP and PE Before the Oxford Maneuver Measurement

POTS group

Heart rate, beats/min Systolic blood pressure, mmHg Diastolic blood pressure, mmHg Mean arterial pressure, mmHg Respiratory rate, breaths/min End-tidal CO2, Torr Cerebral blood flow velocity in the middle cerebral artery, cm/s Estimated cerebral vascular resitance, mmHg 䡠 cm⫺1 䡠 s⫺1 Venous O2 saturation Arterial O2 saturation Change in NIRS arterial inflow, ␮mol 䡠 kg tissue⫺1 䡠 s⫺1 Change in NIRS venous outflow, ␮mol 䡠 kg tissue⫺1 䡠 s⫺1 Change in oxyhemoglobin, ␮mol/kg tissue Change in deoxyhemoglobin, ␮mol/kg tissue Change in total hemoglobin, ␮mol/kg tissue

73 ⫾ 2* 118 ⫾ 5 51 ⫾ 3 71 ⫾ 3 15 ⫾ 1 43 ⫾ 1

Control group

SNP

PE

POTS group

Control group

58 ⫾ 2 116 ⫾ 2 58 ⫾ 3 78 ⫾ 2 14 ⫾ 2 42 ⫾ 1

109 ⫾ 3* 94 ⫾ 5 36 ⫾ 3 52 ⫾ 3* 18 ⫾ 1* 41 ⫾ 1

81 ⫾ 2 102 ⫾ 4 41 ⫾ 3 61 ⫾ 3 15 ⫾ 1 39 ⫾ 1

61 ⫾ 3* 140 ⫾ 5 62 ⫾ 3 84 ⫾ 3 15 ⫾ 1 42 ⫾ 1

50 ⫾ 2 135 ⫾ 2 63 ⫾ 4 87 ⫾ 3 13 ⫾ 1 43 ⫾ 1

75 ⫾ 3

71 ⫾ 2

71 ⫾ 3*

62 ⫾ 2

78 ⫾ 4

75 ⫾ 3

0.96 ⫾ 0.07 0.63 ⫾ 0.03 0.98 ⫾ 0.02

1.10 ⫾ 0.08 0.64 ⫾ 0.02 0.97 ⫾ 0.01

0.75 ⫾ 0.06* 0.63 ⫾ 0.03 0.98 ⫾ 0.02

0.99 ⫾ 0.05 0.64 ⫾ 0.02 0.97 ⫾ 0.01

1.08 ⫾ 0.07 0.63 ⫾ 0.03 0.98 ⫾ 0.02

1.18 ⫾ 0.07 0.64 ⫾ 0.02 0.97 ⫾ 0.01

0

0

0.27 ⫾ 0.02

0.36 ⫾ 0.03

⫺0.26 ⫾ 0.02

0 0 0 0

0 0 0 0

⫺0.09 ⫾ 0.04 13.6 ⫾ 1.6 1.5 ⫾ 0.6 15.2 ⫾ 1.4

⫺0.11 ⫾ 0.06 ⫺2.15 ⫾ 0.95 ⫺0.316 ⫾ 0.46 ⫺1.383 ⫾ 0.69

⫺0.009 ⫾ 0.025 8.13 ⫾ 0.46* 0.419 ⫾ 0.46 8.83 ⫾ 0.45*

POTS group

Control group

⫺0.27 ⫾ 0.2 0.05 ⫾ 0.03 ⫺3.6 ⫾ 0.6 ⫺0.7 ⫾ 0.3 ⫺2.8 ⫾ 1.0

Values are means ⫾ SE. SNP, sodium nitroprusside; PE, phenylephrine. Near-infrared spectroscopy (NIRS) flow and oxyhemoglobin and deoxyhemoglobin were set to zero before the Oxford recordings. Venous O2 saturation was assumed constant during the modified Oxford maneuver, and therefore values during SNP and PE were estimated. *P ⬍ 0.05 compared with the control group.

NIRS Flows (µM/kg/sec) change CBFv (cm/sec)

Fig. 2. Top: ⌬THb (solid lines) and ⌬HbO2 (dashed lines) averaged over all subjects for control (black) and POTS (gray) groups during the modified Oxford maneuver. Results are shown for illustrative purposes; statistical comparisons are shown in Table 1. NIRS, near-infrared spectroscopy. Middle: corresponding changes in arterial inflow (⌬Qa; solid lines) and venous outflow (⌬Qv; dashed lines) for both control (black) and POTS (gray) groups during the modified Oxford maneuver. Bottom: averaged ⌬CBFv during the modified Oxford maneuver for control (black) and POTS (gray) groups. The first arrow indicates the administration of SNP, and the second arrow indicates the administration of PE.

NIRS (µM/kg tissue)

cular coupling depends on the integrity of the neurotransmitters that link neuronal activity to increased CBF. NO is one of the most important of these neurotransmitters. Therefore, impaired NO responses may have dire consequences for neurovascular coupling. We have previously shown that POTS patients experience a deficit in NO-dependent vasodilation within the peripheral microcirculation, linked to oxidative stress (28, 56). The 20

deficit in NO has been associated with increased plasma levels of ANG II (50). The increase in ANG II was attributed to a defect in angiotension-converting enzyme-2 (54) and is known to produce ROS via binding to ANG II type 1 receptors (AT1R) on NADPH oxidase (6, 62) and scavenging NO by superoxide. Additional oxidative stress produced locally can cause augmented adrenergic-mediated vasoconstriction (20, 24). THb-Control

SNP

HbO2- Control

10

THb-POTS HbO2- POTS

0 Phenylephrine

-10

0

100

200

300

400

500 Qa - Control

0.40

Qv - Control

Qa - POTS

Qv - POTS

0.00

-0.40

0

100

200

300

400

500

10 CBFv - Control

CBFv - POTS

0

-10

0

100

200

300

Time (seconds) AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00194.2014 • www.ajpheart.org

400

500

BLUNTED RESPONSE TO NO DONOR IN POTS

When present, systemic circulatory abnormalities are also found in the brain (34, 55) and may account for impaired CBF autoregulation in the upright POTS patient (34). Impairment of cerebral autoregulation is associated with a reduction in neurovascular coupling (52) as well as cognition (33). In the present study, we show that individuals diagnosed with POTS exhibit a blunted response to exogenous NO. This is shown in Fig. 2, where POTS subjects have reduced responses in THb and HbO2, concurrently showing an increase in ⌬Qa and a decrease in ⌬Qv while also exhibiting a blunted decrease in CBFv in response to SNP (see Table 2). Others, using NO synthase inhibitors, have stated that NO does not have an effect on the dimensions of the MCA measured using TCD (63). However, the present study shows that during exogenous NO administration, Qa and CBFv are directionally opposite, consistent with vascular dilation from NO donors. This is similar to our previous study using TCD (51) and those by others using PET scans (37, 49, 60), which showed a decrease in CBF under similar experimental conditions, indicating a dilation of the MCA in response to a NO donor. We (28) have previously demonstrated that POTS patients experience a reduction in the amount of bioavailable NO in response to local warming of the cutaneous microvasculature, which was related to reduced neuronal NO (of neuronal NO synthase origin) activity in POTS (53). This is of interest as NO from neuronal NO synthase is believed to interact with central ANG II in the regulation of central sympathetic outflow (27), in the myogenic response of cerebral autoregulation (15), and with nitergic interneurons, which participate in the functional hyperemia of the neurovascular coupling (11). Impaired NO responses may also potentially affect the regulation of proximal CBF through the parasympathetic nitrergic innervation of conduit cerebral vessels and pial arteries. Agassandian et al. (1) and Boysen et al. (4) have shown that direct projections from the cardiovascular nucleus tractus solitarii to the pontine preganglionic parasympathetic neurons, acting through the pterygopalantine ganglia, provide tonic dilatory influences on the cerebral vessels. This may provide a link between changes in central blood volume and arterial baroreflexes to influence cerebral vascular regulation via the extrinsic vascular innervation system (15). Thus, baroreflex unloading causes parasympathetic withdrawal; this is enhanced in POTS and may reduce NO-dependent dilation of the cerebral vasculature, contributing to cerebral vasoconstriction. In POTS, the sympathetic baroreflex is intact (29), whereas vagal withdrawal is enhanced (57). This conservation of the sympathetic baroreflex coupled with an enhanced vagal withdrawal implies attenuation of nitergic parasympathetic activity, leading to tonic cerebrovascular constriction and impaired myogenic dilation of the cerebral vessels (16). The present study shows significant reductions in MAP in POTS patients compared with control subjects, yet CBFv, while decreased, was higher in POTS patients than in control subjects. Previous investigations have shown a reduction of bioavailable NO in many POTS patients, and the present study may also imply a deficit in NO signaling. For example, the NO-cGMP receptor pathway (e.g., guanylyl cyclase) may be defective as exogenous NO did not produce the same response to CBFv as in the control group (9). The present study found results similar to others who noted that cerebral perfusion persisted after HUT (18, 41, 47). Her-

H401

mosillo et al. (18) compared CBFv of patients experiencing neurocardiogenic syncope with those diagnosed with POTS and showed that in POTS and syncope patients, CBFv did not differ significantly during HUT. They concluded that defects within the central nervous system play a crucial role in the pathology of POTS (18). Razumovsky et al. (41) investigated if chronic fatigue syndrome patients, who have an exaggerated risk for POTS, experienced an abnormal CBFv response compared with healthy control subjects during HUT. They found no difference in CBFv through all phases of the tilt; however, hypotension was apparent in ⬃61% of control subjects. Additionally, they noted that POTS was present in ⬃30% of healthy control subjects (41). They concluded that there was not a characteristic CBFv pattern in chronic fatigue syndrome (41). Schondorf et al. (47) also compared POTS patients with healthy control subjects during HUT and found no differences in CBFv responses to orthostatic stress in systolic CBFv. Additionally, they noted that diastolic CBFv did not differ between the two groups until late in the tilt. They concluded that cerebral perfusion in POTS patients did not differ from healthy control subjects (47). Other studies have shown significantly impaired CBF in POTS (19, 31). Novak et al. (31) compared POTS patients with healthy control subjects during HUT. They observed significant reductions in CBFv during all stages of the tilt and attributed these findings to concurrent hypocapnia and hyperventilation (31). Jacob et al. (19), who also used HUT, evaluated patients with OI compared with control subjects and found significant decrements in CBFv with increases in tilt angle in the OI group but not in the control group. They concluded that the regulation of cerebrovascular tone is impaired in OI (19). Lavi et al. (21) studied the role of NO in the regulation of CBF and noted a drop in end-tidal CO2 after a steady-state infusion of SNP to achieve a stable drop in blood pressure (5–10 mmHg). They concluded that NO affects basal cerebrovascular tone and is not involved in the blood pressuredependent portion of cerebral autoregulation but does affect the chemoregulatory mechanism of cerebral autoregulation (21). In contrast, subjects in the present study had no significant drop in end-tidal CO2 and the modified Oxford maneuver was used, which consisted of a bolus injection of SNP followed by a bolus injection of PE. The differences in the administration of SNP (bolus vs. continual infusion) could potentially explain the observed differences in end-tidal CO2 values. Additionally, Zhang et al. (63) investigated cerebral autoregulation after NO inhibition using NG-monomethyl-L-arginine (L-NMMA). Their results indicated that L-NMMA poorly crossed the blood-brain barrier, whereas NO crossed freely. They concluded that tonic production of NO did not alter cerebral autoregulation. Zhang et al. (63) noted no significant differences between end-tidal CO2 under conditions of NO inhibition. This is in agreement with the present study; however, Zhang et al. did not use a NO donor. This could potentially explain the differences in our conclusions. In the present study, during the administration of SNP, the POTS group experienced a decrease in MAP from 71 to 52 mmHg. A MAP of 52 mmHg falls below the described range of blood pressure where autoregulation is thought to be most effective (60 –160 mmHg) (21). Based on this, we would expect an interruption in cerebral autoregulation; however, this was not observed, as CBFv after SNP was not different than

AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00194.2014 • www.ajpheart.org

H402

BLUNTED RESPONSE TO NO DONOR IN POTS

CBFv before administration of this NO donor. Despite the fall in MAP, autoregulation was maintained. It is possible that what we observed was due to a decrease in the critical lower limit of the autoregulation curve in POTS. Alternatively, this may have been due to diminished NO-dependent vasodilation, for it has been suggested that for pressures that fall moderately below the lower limit of the autoregulation regulation curve, pharmacologically induced vasodilation could normalize CBF (58). The present study, however, was not designed to distinguish between these possibilities. The differences in CBF found in the present study could be explained by the differences in patient populations as well as differing methodologies. The present study required that all patients must fit the POTS criteria, having symptoms for a minimum of 6 mo. While POTS is the most common form of chronic OI (2), a key feature in POTS is that patients do not experience orthostatic hypotension. By requiring that all of our patients had POTS and by excluding subjects with OI, including vasovagal syncope in which hypotension is the hallmark, we reduced variability in our study population. The present study describes decreased cerebral circulation responsiveness to exogenous NO. Previous work (28, 53, 56) indicated reductions in the peripheral bioavailability of NO. Based on this, we conclude that deficits in NO in POTS may be manifold and that research into the mechanisms responsible for these decrements in NO is warranted. Limitations. The present study is limited by the use of TCD and NIRS, which are indirect methods for the estimation of changes in CBF, and they have low regional specificity. However, these methods have superior time resolution compared with functional MRI or PET scans. Thus, because of the experimental arrangement and the rapidity of changes during bolus administration of SNP and PE, the use of large-scale imaging is precluded. It is possible that interference from the cutaneous vascular bed could influence the results of NIRS; however, the infrared signals produced by the cutaneous bed were resolved through spatial resolution, assuring that the infrared signals from the brain were the only signals analyzed (3). Additionally, if cutaneous blood flow was interfering during the bolus injection of SNP, then an increase in blood flow, as measured by NIRS, would have been observed; no such increase was observed. We (54) have previously shown that a subset of POTS patients experience augmented vasoconstriction in the dependent periphery. This heightened vasoconstriction was due to increased ANG II and reduced angiotension converting enzyme-2. The present study, however, did not distinguish between the subsets of POTS patients. Any differences in peripheral vasoconstrictors would have little effect in the present study because peripheral ANG II has limited access to the central circulation (59).We also assumed that the MCA is uniform and cylindrical. Most studies using TCD to measure CBFv within conduit vessels use this assumption, and it has been a useful approximation (23, 30, 51). We assumed that the concentrations of THb, Hb, and HBO2 in the vessels illuminated by NIRS are the same as that in the MCA. However, this assumption applies only if other conduit arteries that might contribute to the illuminated sample would be equally affected by SNP and PE. Due to the rapidity of the modified Oxford maneuver, we assumed that Sv was unaffected by the bolus injection of SNP

(17) and PE (5). In addition, Sa and Sv are indirectly measured, and this too could possibly affect the results. However, Ross et al. (45), who studied noninvasive measurements of arterial O2 saturation, compared different pulse oximeters with direct arterial blood gas measurement and found no statistical differences among them. Finally, we assumed that the cerebral rate of O2 consumption and Sv of Hb during the modified Oxford maneuver remained constant, and, under the conditions of the present study, we feel that these assumptions are reasonable (5, 14). ACKNOWLEDGMENTS The authors thank Seli Dzogbeta for the help in data collection and analysis and Courtney Terilli for the help with subject recruitment and screening. GRANTS Funding for this work was provided by a grant from Dysautonomia International (to A. T. Del Pozzi) and by National Heart, Lung, and Blood Institute Grants RO1-HL-112736 and RO1-HL-074873 (to J. M. Stewart). DISCLOSURES No conflicts of interest, financial or otherwise, are declared by the author(s). AUTHOR CONTRIBUTIONS Author contributions: A.T.D.P. and J.M.S. conception and design of research; A.T.D.P., A.P., and Z.R.M. performed experiments; A.T.D.P. and J.M.S. analyzed data; A.T.D.P. and J.M.S. interpreted results of experiments; A.T.D.P., Z.R.M., and J.M.S. prepared figures; A.T.D.P. drafted manuscript; A.T.D.P., A.P., M.S.M., Z.R.M., and J.M.S. edited and revised manuscript; A.T.D.P., A.P., M.S.M., Z.R.M., and J.M.S. approved final version of manuscript. REFERENCES 1. Agassandian K, Fazan VP, Adanina V, Talman WT. Direct projections from the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons: a link to cerebrovascular regulation. J Comp Neurol 452: 242–254, 2002. 2. Benarroch EE. Postural tachycardia syndrome: a heterogeneous and multifactorial disorder. Mayo Clin Proc 87: 1214 –1225, 2012. 3. Boushel R, Langberg H, Olesen J, Gonzales-Alonzo J, Bulow J, Kjaer M. Monitoring tissue oxygen availability with near infrared spectroscopy (NIRS) in health and disease. Scand J Med Sci Sports 11: 213–222, 2001. 4. Boysen NC, Dragon DN, Talman WT. Parasympathetic tonic dilatory influences on cerebral vessels. Auton Neurosci 147: 101–104, 2009. 5. Brassard P, Seifert T, Wissenberg M, Jensen PM, Hansen CK, Secher NH. Phenylephrine decreases frontal lobe oxygenation at rest but not during moderately intense exercise. J Appl Physiol (1985) 108: 1472– 1478, 2010. 6. Chan SH, Hsu KS, Huang CC, Wang LL, Ou CC, Chan JY. NADPH oxidase-derived superoxide anion mediates angiotensin II-induced pressor effect via activation of p38 mitogen-activated protein kinase in the rostral ventrolateral medulla. Circ Res 97: 772–780, 2005. 7. Cholet N, Bonvento G, Seylaz J. Effect of neuronal NO synthase inhibition on the cerebral vasodilatory response to somatosensory stimulation. Brain Res 708: 197–200, 1996. 8. Cholet N, Seylaz J, Lacombe P, Bonvento G. Local uncoupling of the cerebrovascular and metabolic responses to somatosensory stimulation after neuronal nitric oxide synthase inhibition. J Cereb Blood Flow Metab 17: 1191–1201, 1997. 9. Denninger JW, Marletta MA. Guanylate cyclase and the. NO/cGMP signaling pathway. Biochim Biophys Acta 1411: 334 –350, 1999. 10. Dirnagl U, Lindauer U, Villringer A. Role of nitric oxide in the coupling of cerebral blood flow to neuronal activation in rats. Neurosci Lett 149: 43–46, 1993. 11. Duchemin S, Boily M, Sadekova N, Girouard H. The complex contribution of NOS interneurons in the physiology of cerebrovascular regulation. Front Neural Circuits 6: 51, 2012. 12. Elwell CE, Matcher SJ, Tyszczuk L, Meek JH, Delpy DT. Measurement of cerebral venous saturation in adults using near infrared spectroscopy. Adv Exp Med Biol 411: 453–460, 1997.

AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00194.2014 • www.ajpheart.org

BLUNTED RESPONSE TO NO DONOR IN POTS 13. Essenpreis M, Elwell CE, Cope M, van der Zee P, Arridge SR, Delpy DT. Spectral dependence of temporal point spread functions in human tissues. Appl Opt 32: 418 –425, 1993. 14. Griffiths DP, Cummins BH, Greenbaum R, Griffith HB, Staddon GE, Wilkins DG, Zorab JS. Cerebral blood flow and metabolism during hypotension induced with sodium nitroprusside. Br J Anaesth 46: 671– 679, 1974. 15. Hamel E. Perivascular nerves and the regulation of cerebrovascular tone. J Appl Physiol (1985) 100: 1059 –1064, 2006. 16. Harraz OF, Brett SE, Welsh DG. Nitric oxide suppresses vascular voltage-gated T-type Ca2⫹ channels through cGMP/PKG signaling. Am J Physiol Heart Circ Physiol 306: H279 –H285, 2014. 17. Henriksen L, Paulson OB, Lauritzen M. The effects of sodium nitroprusside on cerebral blood flow and cerebral venous blood gases. I. Observations in awake man during and following moderate blood pressure reduction. Eur J Clin Invest 12: 383–387, 1982. 18. Hermosillo AG, Jauregui-Renaud K, Kostine A, Marquez MF, Lara JL, Cardenas M. Comparative study of cerebral blood flow between postural tachycardia and neurocardiogenic syncope, during head-up tilt test. Europace 4: 369 –374, 2002. 19. Jacob G, Atkinson D, Jordan J, Shannon JR, Furlan R, Black BK, Robertson D. Effects of standing on cerebrovascular resistance in patients with idiopathic orthostatic intolerance. Am J Med 106: 59 –64, 1999. 20. Jacob G, Costa F, Shannon JR, Robertson RM, Wathen M, Stein M, Biaggioni I, Ertl A, Black B, Robertson D. The neuropathic postural tachycardia syndrome. N Engl J Med 343: 1008 –1014, 2000. 21. Lavi S, Egbarya R, Lavi R, Jacob G. Role of nitric oxide in the regulation of cerebral blood flow in humans: chemoregulation versus mechanoregulation. Circulation 107: 1901–1905, 2003. 22. Lindauer U, Megow D, Matsuda H, Dirnagl U. Nitric oxide: a modulator, but not a mediator, of neurovascular coupling in rat somatosensory cortex. Am J Physiol Heart Circ Physiol 277: H799 –H811, 1999. 23. Lindegaard KF, Lundar T, Wiberg J, Sjoberg D, Aaslid R, Nornes H. Variations in middle cerebral artery blood flow investigated with noninvasive transcranial blood velocity measurements. Stroke 18: 1025–1030, 1987. 24. Low PA. Autonomic neuropathies. Curr Opin Neurol 15: 605–609, 2002. 25. Low PA, Opfer-Gehrking TL, McPhee BR, Fealey RD, Benarroch EE, Willner CL, Suarez GA, Proper CJ, Felten JA, Huck CA, Prospective evaluation of clinical characteristics of orthostatic hypotension. Mayo Clin Proc 70: 617–622, 1995. 26. Low PA, Opfer-Gehrking TL, Textor SC, Benarroch EE, Shen WK, Schondorf R, Suarez GA, Rummans TA. Postural tachycardia syndrome (POTS). Neurology 45: S19 –25, 1995. 27. Macarthur H, Wilken GH, Westfall TC, Kolo LL. Neuronal and non-neuronal modulation of sympathetic neurovascular transmission. Acta Physiol (Oxf) 203: 37–45, 2011. 28. Medow MS, Minson CT, Stewart JM. Decreased microvascular nitric oxide-dependent vasodilation in postural tachycardia syndrome. Circulation 112: 2611–2618, 2005. 29. Muenter Swift N, Charkoudian N, Dotson RM, Suarez GA, Low PA. Baroreflex control of muscle sympathetic nerve activity in postural orthostatic tachycardia syndrome. Am J Physiol Heart Circ Physiol 289: H1226 –H1233, 2005. 30. Newell DW, Aaslid R, Lam A, Mayberg TS, Winn HR. Comparison of flow and velocity during dynamic autoregulation testing in humans. Stroke 25: 793–797, 1994. 31. Novak V, Spies JM, Novak P, McPhee BR, Rummans TA, Low PA. Hypocapnia and cerebral hypoperfusion in orthostatic intolerance. Stroke 29: 1876 –1881, 1998. 32. Ocon AJ. Caught in the thickness of brain fog: exploring the cognitive symptoms of chronic fatigue syndrome. Front Physiol 4: 63, 2013. 33. Ocon AJ, Kulesa J, Clarke D, Taneja I, Medow MS, Stewart JM. Increased phase synchronization and decreased cerebral autoregulation during fainting in the young. Am J Physiol Heart Circ Physiol 297: H2084 –H2095, 2009. 34. Ocon AJ, Medow MS, Taneja I, Clarke D, Stewart JM. Decreased upright cerebral blood flow and cerebral autoregulation in normocapnic postural tachycardia syndrome. Am J Physiol Heart Circ Physiol 297: H664 –H673, 2009. 35. Ocon AJ, Messer Z, Medow MS, Stewart JM. Increased pulsatile cerebral blood flow, cerebral vasodilation, and postsyncopal headache in adolescents. J Pediatr 159: 656 –662 e651, 2011.

H403

36. Ocon AJ, Messer ZR, Medow MS, Stewart JM. Increasing orthostatic stress impairs neurocognitive functioning in chronic fatigue syndrome with postural tachycardia syndrome. Clin Sci (Lond) 122: 227–238, 2012. 37. Okamura T, Ayajiki K, Toda N. Basilar arterial construction caused by intracisternal NG-nitro-L-arginine in anesthetized monkeys. Cardiovasc Res 30: 663–667, 1995. 38. Olufsen MS, Peskin CS, Kim WY, Pedersen EM, Nadim A, Larsen J. Numerical simulation and experimental validation of blood flow in arteries with structured-tree outflow conditions. Ann Biomed Eng 28: 1281–1299, 2000. 39. Pinaud M, Souron R, Lelausque JN, Gazeau MF, Lajat Y, Dixneuf B. Cerebral blood flow and cerebral oxygen consumption during nitroprusside-induced hypotension to less than 50 mmHg. Anesthesiology 70: 255–260, 1989. 40. Rasmussen P, Dawson EA, Nybo L, van Lieshout JJ, Secher NH, Gjedde A. Capillary-oxygenation-level-dependent near-infrared spectrometry in frontal lobe of humans. J Cereb Blood Flow Metab 27: 1082–1093, 2007. 41. Razumovsky AY, DeBusk K, Calkins H, Snader S, Lucas KE, Vyas P, Hanley DF, Rowe PC. Cerebral and systemic hemodynamics changes during upright tilt in chronic fatigue syndrome. J Neuroimaging 13: 57–67, 2003. 42. Robertson D. The epidemic of orthostatic tachycardia and orthostatic intolerance. Am J Med Sci 317: 75–77, 1999. 43. Ross AJ, Medow MS, Rowe PC, Stewart JM. What is brain fog? An evaluation of the symptom in postural tachycardia syndrome. Clin Auton Res 23: 305–311, 2013. 44. Ross AJ, Ocon AJ, Medow MS, Stewart JM. A double-blind, placebocontrolled, crossover study of the vascular effects of midodrine in neuropathic versus hyperadrenergic postural tachycardia syndrome. Clin Sci (Lond) 126: 289 –296, 2014. 45. Ross EM, Matteucci MJ, Shepherd M, Barker M, Orr L. Measuring arterial oxygenation in a high altitude field environment: comparing portable pulse oximetry with blood gas analysis. Wilderness Environ Med 24: 112–117, 2013. 46. Rudas L, Crossman AA, Morillo CA, Halliwill JR, Tahvanainen KU, Kuusela TA, Eckberg DL. Human sympathetic and vagal baroreflex responses to sequential nitroprusside and phenylephrine. Am J Physiol Heart Circ Physiol 276: H1691–H1698, 1999. 47. Schondorf R, Benoit J, Stein R. Cerebral autoregulation is preserved in postural tachycardia syndrome. J Appl Physiol 99: 828 –835, 2005. 48. Schondorf R, Low PA. Idiopathic postural orthostatic tachycardia syndrome: an attenuated form of acute pandysautonomia? Neurology 43: 132–137, 1993. 49. Schumann-Bard P, Touzani O, Young AR, Toutain J, Baron JC, Mackenzie ET, Schmidt EA. Cerebrovascular effects of sodium nitroprusside in the anaesthetized baboon: a positron emission tomographic study. J Cereb Blood Flow Metab 25: 535–544, 2005. 50. Stewart JM, Glover JL, Medow MS. Increased plasma angiotensin II in postural tachycardia syndrome (POTS) is related to reduced blood flow and blood volume. Clin Sci (Lond) 110: 255–263, 2006. 51. Stewart JM, Medow MS, DelPozzi A, Messer ZR, Terilli C, Schwartz CE. Middle cerebral O2 delivery during the modified Oxford maneuver increases with sodium nitroprusside and decreases during phenylephrine. Am J Physiol Heart Circ Physiol 304: H1576 –H1583, 2013. 52. Stewart JM, Medow MS, Messer ZR, Baugham IL, Terilli C, Ocon AJ. Postural neurocognitive and neuronal activated cerebral blood flow deficits in young chronic fatigue syndrome patients with postural tachycardia syndrome. Am J Physiol Heart Circ Physiol 302: H1185–H1194, 2012. 53. Stewart JM, Medow MS, Minson CT, Taneja I. Cutaneous neuronal nitric oxide is specifically decreased in postural tachycardia syndrome. Am J Physiol Heart Circ Physiol 293: H2161–H2167, 2007. 54. Stewart JM, Ocon AJ, Clarke D, Taneja I, Medow MS. Defects in cutaneous angiotensin-converting enzyme 2 and angiotensin-(1-7) production in postural tachycardia syndrome. Hypertension 53: 767–774, 2009. 55. Stewart JM, Ocon AJ, Medow MS. Ascorbate improves circulation in postural tachycardia syndrome. Am J Physiol Heart Circ Physiol 301: H1033–H1042, 2011. 56. Stewart JM, Taneja I, Glover J, Medow MS. Angiotensin II type 1 receptor blockade corrects cutaneous nitric oxide deficit in postural tachycardia syndrome. Am J Physiol Heart Circ Physiol 294: H466 –H473, 2008.

AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00194.2014 • www.ajpheart.org

H404

BLUNTED RESPONSE TO NO DONOR IN POTS

57. Stewart JM, Weldon A. Vascular perturbations in the chronic orthostatic intolerance of the postural orthostatic tachycardia syndrome. J Appl Physiol (1985) 89: 1505–1512, 2000. 58. Strandgaard S, Paulson OB. Cerebral autoregulation. Stroke 15: 413– 416, 1984. 59. Volicer L, Loew CG. Penetration of angiotensin II into the brain. Neuropharmacology 10: 631–636, 1971. 60. White RP, Hindley C, Bloomfield PM, Cunningham VJ, Vallance P, Brooks DJ, Markus HS. The effect of the nitric oxide synthase inhibitor L-NMMA on basal CBF and vasoneuronal coupling in man: a PET study. J Cereb Blood Flow Metab 19: 673–678, 1999.

61. Yoxall CW, Weindling AM, Dawani NH, Peart I. Measurement of cerebral venous oxyhemoglobin saturation in children by near-infrared spectroscopy and partial jugular venous occlusion. Pediatr Res 38: 319 – 323, 1995. 62. Yu Y, Zhong MK, Li J, Sun XL, Xie GQ, Wang W, Zhu GQ. Endogenous hydrogen peroxide in paraventricular nucleus mediating cardiac sympathetic afferent reflex and regulating sympathetic activity. Pflügers Arch 454: 551–557, 2007. 63. Zhang R, Wilson TE, Witkowski S, Cui J, Crandall GG, Levine BD. Inhibition of nitric oxide synthase does not alter dynamic cerebral autoregulation in humans. Am J Physiol Heart Circ Physiol 286: H863–H869, 2004.

AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00194.2014 • www.ajpheart.org

Copyright of American Journal of Physiology: Heart & Circulatory Physiology is the property of American Physiological Society and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.

Blunted cerebral blood flow velocity in response to a nitric oxide donor in postural tachycardia syndrome.

Cognitive deficits are characteristic of postural tachycardia syndrome (POTS). Intact nitrergic nitric oxide (NO) is important to cerebral blood flow ...
841KB Sizes 0 Downloads 4 Views