Effects of acetazolamide and dexamethasone on cerebral hemodynamics in hypoxia

Abstract

Previous attempts to detect global cerebral hemodynamic differences between those who develop headache, nausea, and fatigue following rapid exposure to hypoxia [acute mountain sickness (AMS)] and those who remain healthy have been inconclusive. In this study, we investigated the effects of two drugs known to reduce symptoms of AMS to determine if a common cerebral hemodynamic mechanism could explain the prophylactic effect within individuals. With the use of randomized, placebo-controlled, double-blind, crossover design, 20 healthy volunteers were given oral acetazolamide (250 mg), dexamethasone (4 mg), or placebo every 8 h for 24 h prior to and during a 10-h exposure to a simulated altitude of 4,875 m in a hypobaric chamber, which included 2 h of exercise at 50% of altitude-specific V̇o_2max. Cerebral hemodynamic parameters derived from ultrasound assessments of dynamic cerebral autoregulation and vasomotor reactivity were recorded 15 h prior to and after 9 h of hypoxia. AMS symptoms were scored using the Lake Louise Questionnaire (LLQ). It was found that both drugs prevented AMS in those who became ill on placebo (∼70% decrease in LLQ), yet a common cerebral hemodynamic mechanism was not identified. Compared with placebo, acetazolamide reduced middle cerebral artery blood flow velocity (11%) and improved dynamic cerebral autoregulation after 9 h of hypoxia, but these effects appeared independent of AMS. Dexamethasone had no measureable cerebral hemodynamic effects in hypoxia. In conclusion, global cerebral hemodynamic changes resulting from hypoxia may not explain the development of AMS.

people who travel to high altitude frequently suffer from headache, the central symptom of acute mountain sickness (AMS). Several million people experience symptoms of AMS annually (14). Unfortunately, the pathophysiology of headache in AMS remains unresolved. A prevalent hypothesis is that high-altitude headache results from changes in cerebral blood flow and oxygenation that disrupt the blood-brain barrier and lead to vasogenic cerebral edema (29). We recently tested the hypothesis that impairment of the ability to maintain cerebral blood flow as blood pressure fluctuates (cerebral autoregulation) is a key factor in the development of AMS. We showed that cerebral autoregulation was impaired by hypoxia, yet the degree of impairment was similar between subjects developing AMS and those remaining healthy, suggesting that autoregulation was not a causative factor in AMS (32). While these findings were robust, group comparisons are inherently less sensitive than intra-individual comparisons for detecting small, but potentially meaningful, changes in cerebral hemodynamics. We thus continued our investigation with longitudinal, intra-individual assessments. In addition, on subsequent visits, subjects were treated with drugs proven effective for AMS prophylaxis (16) to provide a more specific picture of cerebral hemodynamic adjustments that may offer protection against AMS.

We studied the effects of acetazolamide, a carbonic anhydrase enzyme inhibitor, and dexamethasone, a glucocorticoid steroid hormone, because they are the two most common and effective drugs used to prevent and treat AMS. Both drugs have been shown to alter cerebral hemodynamics, yet their specific mechanisms of action are unclear. Acetazolamide is thought to offset the stress of hypoxia by inducing metabolic acidosis (increased renal bicarbonate excretion) and cerebral tissue respiratory acidosis (inhibition of CO₂ hydration), which together stimulate central chemoreceptors, increase ventilation, and thus improve arterial oxygenation (20). Additionally, acetazolamide has been shown to dilate cerebral blood vessels (10) and therefore may alleviate AMS by increasing cerebral oxygen delivery (4). Dexamethasone is believed to prevent cerebral edema by maintaining blood-brain barrier integrity (11), but has also been shown to affect cerebral blood flow in patients with brain tumors (35, 21, 3). To the best of our knowledge, no studies have evaluated effects of dexamethasone on cerebral blood flow in healthy individuals exposed to acute hypoxia. Whether these drugs have a common effect on the regulation of cerebral blood flow was the central question of this study.

We hypothesized that both acetazolamide and dexamethasone reduce symptoms of AMS by improving control of cerebral blood flow and oxygenation. We reasoned that if these parameters are responsible for the development of AMS, subjects who are susceptible to AMS may exhibit less favorable cerebral hemodynamic control than those who remain healthy. By treating AMS-susceptible subjects with acetazolamide and dexamethasone we expected to detect intraindividual improvements in cerebral hemodynamic regulation that could offer new insight into the pathophysiology of AMS.

MATERIALS AND METHODS

Recruitment and screening.

Following institutional ethics approval, healthy volunteers who had resided at 1,650 m for at least 1 yr were screened to exclude those with recent (<1 mo) exposure to altitudes above 2,500 m, medical conditions affected by hypoxia, or poor aerobic fitness, as previously described (32). All subjects voluntarily provided written consent and were treated in accordance with the Declaration of Helsinki.

General study design.

Using a randomized, double-blind, placebo controlled, crossover design, we evaluated the effects of acetazolamide (250 mg/8 h), dexamethasone (4 mg/8 h) and placebo on cerebral hemodynamics under baseline, normoxic conditions (Pb ∼625 mmHg, 1,650 m) and during 10 h of exposure to hypobaric hypoxia (Pb 425 mmHg, ∼4,875 m) in an environmental chamber. Medications were begun 24 h prior to chamber decompression and continued during hypoxia. A minimum of 3 wk was scheduled between trials to washout potential carryover effects from drugs and hypoxia.

Protocol.

Study participants were familiarized with all measurement techniques during an abbreviated practice trial in normoxia before receiving study medication. Six hours after starting respective medication, subjects' baseline physiological measurements were assessed in normoxia. Subjects were seated upright, with their back, shoulders, and arms supported. Sensors were placed to continuously monitor arterial blood pressure (finger plethysmography: Nexfin HD, BMeye), middle cerebral artery blood flow velocity (transcranial Doppler: ST³, Spencer Technologies), frontal cortex oxygenation (near infrared spectroscopy: Oxymon MKIII, Artinis), finger pulse oximetry (Nellcor N-595), pulmonary ventilation (spirometry: UVM, Vacumed), end-tidal gas concentrations (fast response gas analyzer: O₂Cap, Oxigraf), and ECG (lead II: Bioamp, ADInstruments), as previously described (32). After steady-state, rhythmical breathing patterns were observed, resting data were recorded (200 Hz, Powerlab 16SP, ADInstruments) for 10 min to evaluate dynamic cerebral autoregulation, cerebral vascular resistance, and critical closing pressure. Additionally, spot measurements of basilar artery blood flow velocity were obtained during the last 2 min of rest with a second 2-MHz, handheld Doppler probe (ST³, Spencer Technologies) through the suboccipital window at penetration depths of ∼90 mm.

Next, subjects performed a modified rebreathing protocol to assess cerebral vasomotor reactivity to CO₂ (9). Briefly, individuals were coached to hyperventilate for 2 min to reduce Pet_CO2 below 20 mmHg, then, following two deep inspirations of a gas mixture providing 250 mmHg Pi_O2 and 60 mmHg Pi_CO2, subjects were instructed to breathe normally while expired air was circulated through a rebreathing circuit to slowly raise Pet_CO2 to each person's limit of CO₂ tolerance (>50 mmHg).

Subjects reported back to the lab on the following morning for testing in the hypobaric chamber. On reaching the target barometric pressure (425 mmHg), subjects performed four sets of 30-min cycling exercise (50% of altitude adjusted V̇o_2max) with 15 min of rest between sets. This exercise protocol was designed to simulate physical activity that typically accompanies ascent to high terrestrial altitude and has been shown to increase the severity of AMS symptoms (30). For the remainder of the day, subjects rested inside the chamber. Cerebral hemodynamic assessments, as described above, were repeated after 4 and 9 h of hypobaric hypoxia. Self-reported sections (headache, gastrointestinal, dizziness, fatigue) of the Lake Louise AMS Questionnaire (LLQ) were used to evaluate AMS symptoms at corresponding time points (28). Subjects with LLQ ≥ 3, including headache, at 9 h were classified as ill (AMS susceptible) and those with LLQ ≤ 2, or without headache, were classified as healthy (AMS resistant).

Hemodynamic analyses.

To quantitatively describe the effects of spontaneous changes in resting blood pressure on cerebral blood flow velocity, a cerebral autoregulation index (ARI) was determined using transfer function analysis and subsequent step response (see appendix), as previously described (32, 31). Critical closing pressure and resistance area product of cerebral blood vessels were estimated using the x-intercept and inverse slope of the relationship between cerebral blood flow velocity (CBFv) vs. arterial blood pressure (ABP) across beats (27). A cerebral vascular resistance index was calculated as the quotient of mean ABP/mean CBFv. Arterial pulse-wave modeling was used to estimate cardiac output (36). Cerebral vasomotor reactivity (CVMR) was defined as the slope of the linear fit between percent resting CBFv and Pet_CO2 during the modified rebreathing segment of the protocol (25). Additionally, the cerebrovascular conductance index (CVCi), which corrected CVMR for changes in mean ABP, was calculated from the slope of the linear fit between percent resting CBFv/ABP and Pet_CO2 (8).

Statistics.

Only subjects completing all three drug trials were included in the statistical analysis. Each variable of interest was analyzed using a mixed factor ANOVA, with AMS classification after 9 h of hypoxia on placebo (AMS susceptible vs. AMS resistant) analyzed with respect to drug (placebo, acetazolamide, and dexamethasone) over time (baseline and 9 h). Criteria for significance were set at P < 0.05 for main and interaction effects. t-Tests were used for post hoc analyses of differences between AMS status (independent) and across drug and time (paired) using more stringent criteria (P < 0.01) to control for type I error. Additionally, we identified subjects with the best (top quartile) and worst (bottom quartile) cerebral autoregulation scores after 9 h of hypoxia during the placebo run. Additional nonparametric tests were run on these groups to determine if those with worse autoregulation responded differently to acetazolamide and dexamethasone than those with better autoregulation. Friedman's ANOVA by ranks was used to evaluate individual responses to the drugs (P < 0.05) and Mann-Whitney U tests were used to evaluate differences between groups using P < 0.05 as the criteria for significance. Data are presented as means ± SD.

RESULTS

Twenty-nine volunteers met the inclusion criteria and completed at least the placebo and one drug trial. Nine subjects dropped out of the study due to the large time commitment required to obtain an additional trial (∼20 h over 2 days). Twenty subjects (16 men, 4 women) participated in all three trials and were used in the analysis.

Effects of acetazolamide and dexamethasone in normoxia.

In normoxia, acetazolamide reduced Pet_CO2 by 5% (Table 1) and impaired cerebral autoregulation (Table 2), but had no other detectable effects compared with placebo. Dexamethasone elevated heart rate (12%) and cardiac output (15%) and lowered Pet_CO2 (4%). The drop in Pet_CO2 was associated with reductions in middle cerebral and basilar artery flow velocity (17 and 9%, respectively), increased cerebral vascular resistance (20%), and improved cerebral autoregulation (Tables 1–3).

Table 1.

Resting cardiopulmonary variables

	V̇e, min	Vt, liters	RR, beats/min	PetO2, mmHg	PetCO2, mmHg	SpO2,%	HR, beats/min	Q, l/min	MABP, mmHg
Normoxia
Placebo
Mean	8.7	0.62	16	79.2	36.2	95	63	6.0	92
SD	2.0	0.17	3	4.8	3.2	1	9	1.1	12
Acetazolamide
Mean	9.6	0.66	15	81.8	34.5*	96	63	5.9	90
SD	1.7	0.13	4	3.2	2.3	1	12	0.9	12
Dexamethasone
Mean	10.2	0.66	16	81.1	34.8*	95	71*‡	7.0*‡	90
SD	2.4	0.15	3	3.7	3.2	1	13	1.3	8
Hypoxia
Placebo
Mean	12.6†	0.74†	17	44.6†	31.0†	78†	97†	8.3†	89
SD	3.3	0.19	4	5.1	3.3	8	11	1.6	13
Acetazolamide
Mean	13.3†	0.79†	18†	48.1*†	27.6*†	82*†	94†	7.5†	84
SD	2.8	0.23	4	3.0	2.0	5	11	1.7	12
Dexamethasone
Mean	13.1†	0.84†	17	43.1†‡	31.0†‡	77†‡	97†	9.1†§	85†
SD	2.6	0.22	4	4.6	2.3	8	10	1.9	12

Values are means ± SD, n =20. V̇e, minute ventilation; Vt, tidal volume; RR, respiratory rate; Pet_O2, partial pressure O₂; Pet_CO2, partial pressure CO₂; SpO₂, arterial saturation; HR, heart rate; Q, cardiac output; MABP, mean arterial blood pressure.

^*Different from PL (P < 0.01 to control for type I error);

^†different from normoxia (P < 0.01);

^‡different from Acetazolamide (P < 0.01).

Table 2.

Cerebral hemodynamic variables at rest and during rebreating

	Resting							Rebreathing
	MCA CBFv, cm/s	BA CBFv, cm/s	CVR mmHg · cm−1 · s−1	TSI, %	ARI	CrCp, mmHg	RAP, cm · s · −1mmHg−1	CVMR. %/mmHg	CVCi. %/mmHg2
Normoxia
Placebo
Mean	56.5	37.8	1.64	83	4.8	27	1.16	2.93	1.19
SD	6.6	8.0	0.27	15	1.1	11	0.26	0.82	0.50
Acetazolamide
Mean	52.4	37.3	1.77	84	4.2*	22	1.34	2.73	1.23
SD	8.3	9.8	0.39	8	0.8	10	0.31	0.96	0.57
Dexamethasone
Mean	‡	34.4*	1.96*	86	5.2*‡	27	1.36	3.21	1.50
SD	7.6	6.9	0.37	8	1.0	10	0.37	1.08	0.86
Hypoxia
Placebo
Mean	58.3	39.4	1.56	72†	3.2†	28	1.07	3.33	1.74†
SD	8.6	12.3	0.32	14	1.4	13	0.29	1.40	1.00
Acetazolamide
Mean	51.8*	39.6	1.66	72†	4.2*	24	1.16	3.54	1.09
SD	7.6	10.3	0.35	12	0.8	12	0.28	1.42	0.65
Dexamethasone
Mean	53.6†	39.4†	1.62†	71†	3.7†	27	1.12	3.62	1.82§
SD	8.8	8.4	0.31	14	0.8	16	0.40	1.14	0.72

Values are means ± SD; n = 20. MCA CBFv, middle cerebral artery flow velocity; BA CBFv, basilar artery flow velocity; CVR, cerebral vascular resistance index; TSI, cerebral tissue oxygenation index (NIRS); ARI, autoregulation index; CrCP, critical closing pressure; RAP, resistance area product; CVMR, cerebral vasomotor reactivity to CO₂; CVCi, cerebrovascular conductance index.

^*Different from Placebo (P < 0.01 to control for type I error);

^†different from normoxia (P < 0.01);

^‡different from Acetazolamide (P < 0.01).

Table 3.

Transfer function analyses across low frequencies (<0.10 Hz)

	PSD CBFv, cm · s−1 · Hz−1	PSD ABP, mmHg2/Hz	Coherence	Gain, %/%	Phase, rad
Normoxia
Placebo
Mean	6.29	10.57	0.56	0.55	0.59
SD	3.86	3.59	0.11	0.10	0.28
Acetazolamide
Mean	5.50	9.56	0.56	0.54	0.43
SD	5.00	5.64	0.09	0.17	0.19
Dexamethasone
Mean	4.53	10.56	0.52	0.46	0.65‡
SD	2.63	4.94	0.10	0.11	0.27
Hypoxia
Placebo
Mean	12.94	11.33	0.66	0.81†	0.31†
SD	12.14	7.67	0.14	0.20	0.17
Acetazolamide
Mean	7.28	9.47	0.57*	0.67	0.48
SD	4.08	5.92	0.13	0.25	0.26
Dexamethasone
Mean	6.44†	8.95	0.63†	0.71†	0.33†
SD	3.33	5.63	0.10	0.26	0.16

Values are means ± SD; n = 20. PSD CBFv and PSD ABP, power spectal density of cerebral blood flow velocity and arterial blood pressure.

^*Different from PL (P < 0.01 to control for type I error);

^†different from normoxia (P < 0.01);

^‡different from Acetazolamide (P < 0.01).

Effects of acetazolamide and dexamethasone in hypoxia.

During the placebo trial, 9 h of hypoxia elevated heart rate (54%), cardiac output (38%), and pulmonary ventilation (44%), lowered Pet_CO2 (44%), Pet_CO2 (14%), arterial oxygen saturation (18%) and cerebral oxygenation (4%), and impaired cerebral autoregulation (Tables 1–3). During CO₂ rebreathing, CVCi was increased by 46% (Table 2). Six subjects (30%) were classified as AMS susceptible (mild to moderate AMS), while 14 were AMS resistant (Table 4). No differences were detected between AMS-susceptible and AMS-resistant subjects in any physiological parameters during the placebo trial. AMS susceptibility on placebo was used to classify data for subsequent statistical analysis to test whether the drugs had greater effects in AMS-susceptible than AMS-resistant subjects, thereby explaining potential prophylactic mechanisms of actions.

Table 4.

Lake Louise Questionnaire scores after 9 h of hypobaric hypoxia

	AMS Susceptible	AMS Resistant
Placebo	4.0 ± 0.6†	1.1 ± 0.7
Acetazolamide	1.3 ± 1.2*	1.1 ± 0.9
Dexamethasone	1.3 ± 0.8*	0.7 ± 0.7

Values are means ± SD; n = 20. AMS susceptible (n = 6) had Lake Louise Questionnaire ≥3 with headache on placebo; AMS resistant (n = 14) had Lake Louise Questionnaire <3 or without headache on placebo.

^*Different from Placebo (P < 0.05);

^†different from AMS resistant.

Compared with placebo, both drugs reduced AMS symptoms after 9 h of hypoxia by ∼70% (Table 4). Acetazolamide increased Pet_CO2 (8%) and arterial oxygen saturation (4%), decreased Pet_CO2 (11%) and middle cerebral artery flow velocity (11%), and improved cerebral autoregulation compared with placebo (Tables 1–3). Effects of acetazolamide were similar between AMS-susceptible and AMS-resistant subjects (i.e., there were no drug-by-AMS status interactions). No physiological differences could be detected between placebo and dexamethasone trials in hypoxia.

Analysis of best vs. worst autoregulation quartiles.

Mean ARI scores for subjects with the best cerebral autoregulation after 9 h of hypoxia during the placebo trial (4.8 ± 0.8, n = 5) were higher than those with the worst autoregulation (1.5 ± 0.8, n = 5: P < 0.01). Mean LLQ scores for subjects with the best and worst ARI scores were not different during the placebo (2.4 ± 1.8 vs. 2.4 ± 2.1, P = 0.92), acetazolamide (1.6 ± 1.1 vs. 1.2 ± 1.1, P = 0.51), or dexamethasone (0.6 ± 0.5 vs. 1.4 ± 0.9, P = 0.12) trials. Intraindividual effects of the drugs on LLQ scores did not reach significance when data were grouped according to cerebral autoregulatory ability (P > 0.10).

DISCUSSION

Our results are the first to document the cerebral hemodynamic effects of acetazolamide and dexamethasone in healthy subjects exposed to prolonged periods of hypoxia. We found that both drugs effectively reduce symptoms of AMS, but their mechanism of action appeared unrelated to measures of dynamic cerebral autoregulation and vasomotor reactivity. These findings appear to support our previous assertion that AMS does not stem from global changes in the regulation of blood flow or oxygenation, but do not rule out the possibility that the drugs act via independent mechanisms that alleviate the symptoms of AMS (22).

We specifically tested the hypothesis that acetazolamide and dexamethasone would improve cerebral autoregulation in hypoxia, thereby reducing the threat of vasogenic edema and AMS. The data demonstrated that acetazolamide reduced the cerebral autoregulation index in normoxia, but prevented further impairment in hypoxia (Fig. 1), resulting in better autoregulation scores compared with placebo after 9 h of hypoxia. However, since the effect was similar in AMS-susceptible and AMS-resistant subjects (Fig. 1) the degree of improvement could not explain the reduction in AMS symptoms with acetazolamide treatment. The lack of change in cerebral autoregulation across normoxia and hypoxia suggests that acetazolamide's effect on autoregulation is independent of changes in oxygen tension. Dexamethasone had no measureable effects on the impairment of cerebral autoregulation in hypoxia (Fig. 1), yet was highly effective at alleviating symptoms of AMS. These results argue against a common global cerebral hemodynamic pathway in AMS and provide further experimental evidence to support our previous claim that impairment of cerebral autoregulation may not be a key factor in the development of the illness (32).

Fig. 1.

Top: autoregulation index (ARI) responses from normoxic baseline to 9 h of hypoxia for all trials. On placebo, cerebral autoregulation was impaired after 9 h of hypobaric hypoxia. Acetazolamide reduced ARI scores in normoxia, but prevented further impairment in autoregulation after 9 h of hypoxia. Dexamethasone improved autoregulation in normoxia, but had no measureable effect in hypoxia. Bottom: circular markers indicate acute mountain sickness (AMS)-susceptible subjects (n = 6) identified during the placebo trial. Autoregulation was similar in AMS-susceptible and AMS-resistant subjects across all trials.

We tested an alternative hypothesis that acetazolamide would ameliorate AMS by stimulating vasodilation, thereby increasing cerebral blood flow and oxygenation in hypoxia (4). Our data provide evidence to the contrary. In hypoxia, acetazolamide reduced cerebral blood flow velocity in the middle cerebral artery and had no effect on cerebral oxygenation compared with placebo. These opposing findings may be related to dosage. While 250 mg is sufficient for AMS prophylaxis, higher doses (≥1,000 mg) may be needed to stimulate vasodilation and increase cerebral blood flow (12, 13, 15) and oxygenation (19, 33). The dosage may also have been insufficient to stimulate a measureable increase in overall pulmonary ventilation given the large variability in ventilatory responses between subjects, yet we consider the increase in Pet_O2 and decrease in Pet_CO2 evidence of mild hyperventilation. We believe this mild hyperventilation caused a CO₂-mediated reduction of cerebral blood flow velocity in hypoxia that may have countered any small vasodilatory effects from the drug, but that cerebral oxygenation was preserved by the coinciding improvement in arterial oxygen saturation.

Such improvement in arterial oxygen saturation has long been suspected to provide the therapeutic effect of acetazolamide (4). While arterial saturation increased with acetazolamide, two observations are difficult to reconcile with the conventional therapeutic explanation. First, arterial oxygen saturation values in hypoxia were similar between AMS-susceptible and AMS-resistant subjects. Although this is contrary to some reports, the correlation between saturation and AMS is generally only moderate (r = −0.50 to −0.63) (2), indicating that other factors must influence the development of the illness (24). Second, while arterial oxygenation was improved with acetazolamide, cerebral oxygenation was not. As we stated above, improved arterial oxygenation may be offset by the reduction in cerebral blood flow accompanying hyperventilation. Nonetheless, if cerebral oxygenation is unaffected by acetazolamide, it is reasonable to question the conventional wisdom and explore alternative explanations.

We evaluated the effects of dexamethasone in the same subjects, believing that this glucocorticosteroid might reveal a cerebral hemodynamic link to AMS since dexamethasone is an effective prophylactic treatment (16, 17) with known cerebral hemodynamic effects (3, 21, 35). Dexamethasone did reduce cerebral flow in middle cerebral and basilar arteries and improved cerebral autoregulation in normoxia, yet these effects were likely results of reduced Pet_CO2, presumably from a mild increase in ventilation (26). No physiological effects of dexamethasone were detected in hypoxia. Importantly, symptoms of AMS were alleviated by dexamethasone without improvements in arterial oxygen saturation or cerebral oxygenation. These findings are similar to one previous MRI study showing no cerebrovascular effects of dexamethasone in hypoxia (22), but are in contrast to other reports showing that dexamethasone blunts sympathoadrenal responses to hypoxia (18, 23). Thus the physiological mechanism by which dexamethasone either prevents AMS or masks its symptoms (22) remains unclear.

Our results do not support the hypothesis that acetazolamide and dexamethasone share a common cerebral hemodynamic pathway in the prevention of AMS, yet it is important to stress that our measurements are reflective of global changes in brain blood flow. Since blood flow is markedly heterogeneous across the brain in hypoxia (7, 5), we cannot rule out the possibility that these drugs have specific regional effects that were not measurable with current technology. It is possible that both acetazolamide and dexamethasone may protect the integrity and permeability of the blood-brain barrier in local brain regions and therefore could prevent the development of cerebral edema in areas responsible for AMS. Alternatively, it has been suggested that neural effects from increased free radical production in hypoxia are the root cause of AMS symptoms and that mild cerebral edema is merely an ancillary effect (1). If so, acetazolamide and dexamethasone may share common antioxidant properties that have not yet been described.

We acknowledge that the stringent criteria used to define AMS resulted in a relatively low sample size of AMS-susceptible subjects. While this may be seen as a limitation, we believe it was necessary to isolate the most definitive cases of AMS to maximize our chances of finding significant drug effects. By using a repeated-measures design, in which all subjects were treated with placebo, acetazolamide, and dexamethasone, statistical power was sufficient to detect major hemodynamic effects of the drugs, yet a larger sample of AMS-susceptible subjects may still be needed to detect subtle differences from AMS-resistant subjects, particularly with respect to cerebral autoregulation measurements in which day-to-day variability might obscure small drug effects (6).

In summary, we report few cerebral hemodynamic effects of acetazolamide and dexamethasone and find no evidence to suggest that AMS stems from changes in the regulation of global cerebral blood flow and oxygenation.

GRANTS

Funding was provided by National Heart, Lung, and Blood Institute Grant HL-070362, The Maren Foundation, and the Altitude Research Center.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

APPENDIX

Transfer function analysis (TFA), involving the relationship between ABP as input and CBFv as output has been the most widely used approach to quantify dynamic cerebral autoregulation (37). In general, the linear approximation has been justified by the relatively small changes in ABP and CBFv around a point of equilibrium. The cross-spectrum is defined as:

$${\text{G}}_{\text{pv}}\left(f\right)=\text{E}\left[\text{P}{\left(f\right)}^{*}\cdot \text{V}\left(f\right)\right]$$ (1)

where P(f) and V(f) are the discrete Fourier transforms of ABP and CBFv beat-to-beat time series, obtained via the FFT algorithm. The expected value of the complex product, E[P(f)*.P(f)] is obtained by smoothing the spectra with a triangular moving average window and by averaging multiple segments of data. Similarly, the autospectra of ABP is computed as:

$${\text{G}}_{\text{pp}}\left(f\right)=\text{E}\left[\text{P}{\left(f\right)}^{*}\cdot P\left(f\right)\right]$$ (2)

From the cross- and auto-spectra, the transfer function is calculated as:

$$\text{H}\left(f\right)=\frac{{\text{G}}_{\text{pv}}\left(f\right)}{{\text{G}}_{\text{pp}}\left(f\right)}$$ (3)

From the real and imaginary parts of H(f), the amplitude and phase of the frequency response are calculated as:

$$\left|\text{H}\left(f\right)\right|={\left[{\text{H}}_{\text{R}}{\left(f\right)}^2+{\text{H}}_{\text{I}}{\left(f\right)}^2\right]}^{\frac{1}{2}}$$ (4)

$$\varphi \left(f\right)={\tan }^{-1}\left[\frac{{\text{H}}_{\text{I}}\left(f\right)}{{\text{H}}_{\text{R}}\left(f\right)}\right]$$ (5)

The inverse Fourier transform of H(f) yields the CBFv impulse response as shown by Zhang et al. (37). Integration of the impulse response along time leads to the CBFv step response, which allows visualization of the speed of recovery of CBFv following a hypothetical step change in ABP. As proposed by Tiecks et al. (34), the temporal pattern of the CBFv step response can be classified with the ARI index, corresponding to 10 different template curves ranging from absence of autoregulation (ARI = 0) to best observed autoregulation (ARI = 9). The template curve with the best approximation to the CBFv step response derived by transfer function analysis is selected by least square fitting for the first 6 s of the response.