Usual clinical dose of acetazolamide does not alter cerebral blood flow velocity

Acetazolamide during acute hypoxia improves tissue oxygenation in the human brain

Low doses of the carbonic anhydrase inhibitor acetazolamide provides accelerated acclimatization to high-altitude hypoxia and prevention of cerebral and other symptoms of acute mountain sickness. We previously observed increases in cerebral O2 metabolism (CMRO2 ) during hypoxia. In this study, we investigate whether low-dose oral acetazolamide (250 mg) reduces this elevated CMRO2 and in turn might improve cerebral tissue oxygenation (PtiO2 ) during acute hypoxia. Six normal human subjects were exposed to 6 h of normobaric hypoxia with and without acetazolamide prophylaxis. We determined CMRO2 and cerebral PtiO2 from MRI measurements of cerebral blood flow (CBF) and cerebral venous O2 saturation. During normoxia, low-dose acetazolamide resulted in no significant change in CBF, CMRO2 , or PtiO2 . During hypoxia, we observed increases in CBF [48.5 (SD 12.4) (normoxia) to 65.5 (20.4) ml·100 ml(-1)·min(-1) (hypoxia), P < 0.05] and CMRO2 [1.54 (0.19) to 1.79 (0.25) μmol·ml(-1)·min(-1), P < 0.05] and a dramatic decline in PtiO2 [25.0 to 11.4 (2.7) mmHg, P < 0.05]. Acetazolamide prophylaxis mitigated these rises in CBF [53.7 (20.7) ml·100 ml(-1)·min(-1) (hypoxia + acetazolamide)] and CMRO2 [1.41 (0.09) μmol·ml(-1)·min(-1) (hypoxia + acetazolamide)] associated with acute hypoxia but also reduced O2 delivery [6.92 (1.45) (hypoxia) to 5.60 (1.14) mmol/min (hypoxia + acetazolamide), P < 0.05]. The net effect was improved cerebral tissue PtiO2 during acute hypoxia [11.4 (2.7) (hypoxia) to 16.5 (3.0) mmHg (hypoxia + acetazolamide), P < 0.05]. In addition to its renal effect, low-dose acetazolamide is effective at the capillary endothelium, and we hypothesize that local interruption in cerebral CO2 excretion accounts for the improvements in CMRO2 and ultimately in cerebral tissue oxygenation during hypoxia. This study suggests a potentially pivotal role of cerebral CO2 and pH in modulating CMRO2 and PtiO2 during acute hypoxia.

Safety of carbonic anhydrase inhibitors

INTRODUCTION

Carbonic anhydrase (CA) inhibitors have an impressive safety record despite the multiple functions that CA isozymes serve because they are not fully inhibited with most dosing. While reducing the targeted CA-dependent process sufficiently for disease control, residual activity and uncatalyzed rates in combination with compensations are adequate to avoid lethal consequences. Some drugs have in vitro selectivity differences against the 13 active isozymes, but none are convincingly selective in vivo or clinically. Efforts to synthesize selective inhibitors should result in safer drugs with fewer side effects.

AREAS COVERED

This review will focus on approved drugs with CA-inhibiting activity, whether used directly for this purpose or others. Side effects are discussed in relation to various organ systems and the disease being treated. Causes of side effects are considered, and strategies for symptom reduction are given.

EXPERT OPINION

Common side effects of paresthesias, dyspepsia, lassitude and fatigue in 30 - 40% of patients are generally tolerable or abate, but if not can be partially relieved by bicarbonate supplementation. The most important safety concerns are severe acidosis, respiratory failure and encephalopathy in patients with renal, pulmonary and hepatic disease where caution is critical, as is also the case in persons with sulfa drug allergies.

Carbonic anhydrase inhibitors and high altitude illnesses

Carbonic anhydrase (CA) inhibitors, particularly acetazolamide, have been used at high altitude for decades to prevent or reduce acute mountain sickness (AMS), a syndrome of symptomatic intolerance to altitude characterized by headache, nausea, fatigue, anorexia and poor sleep. Principally CA inhibitors act to further augment ventilation over and above that stimulated by the hypoxia of high altitude by virtue of renal and endothelial cell CA inhibition which oppose the hypocapnic alkalosis resulting from the hypoxic ventilatory response (HVR), which acts to limit the full expression of the HVR. The result is even greater arterial oxygenation than that driven by hypoxia alone and greater altitude tolerance. The severity of several additional diseases of high attitude may also be reduced by acetazolamide, including high altitude cerebral edema (HACE), high altitude pulmonary edema (HAPE) and chronic mountain sickness (CMS), both by its CA-inhibiting action as described above, but also by more recently discovered non-CA inhibiting actions, that seem almost unique to this prototypical CA inhibitor and are of most relevance to HAPE. This chapter will relate the history of CA inhibitor use at high altitude, discuss what tissues and organs containing carbonic anhydrase play a role in adaptation and maladaptation to high altitude, explore the role of the enzyme and its inhibition at those sites for the prevention and/or treatment of the four major forms of illness at high altitude.

Regional cerebral blood flow in humans at high altitude: gradual ascent and 2 wk at 5,050 m

The interindividual variation in ventilatory acclimatization to high altitude is likely reflected in variability in the cerebrovascular responses to high altitude, particularly between brain regions displaying disparate hypoxic sensitivity. We assessed regional differences in cerebral blood flow (CBF) measured with Duplex ultrasound of the left internal carotid and vertebral arteries. End-tidal Pco2, oxyhemoglobin saturation (SpO2), blood pressure, and heart rate were measured during a trekking ascent to, and during the first 2 wk at, 5,050 m. Transcranial color-coded Duplex ultrasound (TCCD) was employed to measure flow and diameter of the middle cerebral artery (MCA). Measures were collected at 344 m (TCCD-baseline), 1,338 m (CBF-baseline), 3,440 m, and 4,371 m. Following arrival to 5,050 m, regional CBF was measured every 12 h during the first 3 days, once at 5-9 days, and once at 12-16 days. Total CBF was calculated as twice the sum of internal carotid and vertebral flow and increased steadily with ascent, reaching a maximum of 842 ± 110 ml/min (+53 ± 7.6% vs. 1,338 m; mean ± SE) at ∼ 60 h after arrival at 5,050 m. These changes returned to +15 ± 12% after 12-16 days at 5,050 m and were related to changes in SpO2 (R(2) = 0.36; P < 0.0001). TCCD-measured MCA flow paralleled the temporal changes in total CBF. Dilation of the MCA was sustained on days 2 (+12.6 ± 4.6%) and 8 (+12.9 ± 2.9%) after arrival at 5,050 m. We observed no significant differences in regional CBF at any time point. In conclusion, the variability in CBF during ascent and acclimatization is related to ventilatory acclimatization, as reflected in changes in SpO2.

Effects of acetazolamide and dexamethasone on cerebral hemodynamics in hypoxia

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 VO(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.

Acetazolamide Improves Cerebral Oxygenation During Exercise at High Altitude

Vuyk, Jaap, Jan Van Den Bos, Kees Terhell, Rene De Bos, Ad Vletter, Pierre Valk, Martie Van Beuzekom, Jack Van Kleef, and Albert Dahan. Acetazolamide improves cerebral oxygenation during exercise at high altitude. High Alt. Med. Biol. 7:290-301, 2006.--Acute mountain sickness is thought to be triggered by cerebral hypoxemia and be prevented by acetazolamide (Actz). The effect of Actz on cerebral oxygenation at altitude remains unknown. In 16 members of the 2005 Dutch Cho Oyu (8201 m, Tibet) expedition, the influence of Actz and exercise (750 mg PO daily) on heart rate, peripheral and regional cerebral oxygen saturation (Sa(O(2) ) and rS(O(2) )), the Lake Louise score (LLS), and psychomotor function were studied at 0 m 14 days prior to the expedition, after arrival at 3700 m on day 3, after arrival at 5700 m on day 29, and again at 5700 m before the end of the expedition on day 51. After arrival at 3700 m, the LLS of the climbers taking Actz (n = 8) was significantly lower compared to those who did not take Actz (n = 8): 0.75 +/- 1.0 versus 2.9 +/- 2.0, p < 0.05 (ANOVA). High LLSs were associated with low rS(O(2) ) values in rest and exercise (p < 0.01 and p < 0.001). With altitude, resting Sa(O(2) ) and resting rS(O(2) ) decreased significantly (p < 0.001), irrespective of Actz use. Exercise at 3700 m and 5700 m reduced Sa(O(2) ) and rS(O(2) ) even further compared to rest (p < 0.001), although at 3700 m the rS(O(2) ) was preserved better in those who took Actz (55.3 +/- 4.3% versus 47.9 +/- 5.7%, p < 0.05). Irrespective of Actz use, with altitude, the percentage of omissions in the vigilance and tracking test increased while the climbers' scores on vigor decreased (p < 0.05). In conclusion, at altitude, exercise-induced reduction in cerebral oxygenation is less in climbers on Actz compared to climbers not taking Actz. This effect is nullified after several weeks at altitude due to acclimatization in climbers not taking Actz.

Effects of acetazolamide on ventilatory, cerebrovascular, and pulmonary vascular responses to hypoxia

RATIONALE

Acute mountain sickness (AMS) may affect individuals who (rapidly) ascend to altitudes higher than 2,000-3,000 m. A more serious consequence of rapid ascent may be high-altitude pulmonary edema, a hydrostatic edema associated with increased pulmonary capillary pressures. Acetazolamide is effective against AMS, possibly by increasing ventilation and cerebral blood flow (CBF). In animals, it inhibits hypoxic pulmonary vasoconstriction.

OBJECTIVES

We examined the influence of acetazolamide on the response to hypoxia of ventilation, CBF, and pulmonary vascular resistance (PVR).

METHODS

In this double-blind, placebo-controlled, randomized study, nine subjects ingested 250 mg acetazolamide every 8 h for 3 d. On the fourth test day, we measured the responses of ventilation, PVR, and CBF to acute isocapnic hypoxia (20 min) and sustained poikilocapnic hypoxia (4 h). Ventilation was measured with pneumotachography. Hypoxia was achieved with dynamic end-tidal forcing. The maximum pressure difference across the tricuspid valve (DeltaPmax, a good index of PVR) was measured with Doppler echocardiography. CBF was measured by transcranial Doppler ultrasound.

RESULTS

In normoxia, acetazolamide increased ventilation and reduced DeltaPmax, but did not influence CBF. The ventilatory and CBF responses to acute isocapnic hypoxia were unaltered, but the rise in DeltaPmax was reduced by 57%. The increase in DeltaPmax by sustained poikilocapnic hypoxia observed after placebo was reduced by 34% after acetazolamide, the ventilatory response was increased, but the CBF response remained unaltered.

CONCLUSIONS

Acetazolamide has complex effects on ventilation, PVR, and CBF that converge to optimize brain oxygenation and may be a valuable means to prevent/treat high-altitude pulmonary edema.

Mechanisms of action of acetazolamide in the prophylaxis and treatment of acute mountain sickness

Acetazolamide, a potent carbonic anhydrase (CA) inhibitor, is the most commonly used and best-studied agent for the amelioration of acute mountain sickness (AMS). The actual mechanisms by which acetazolamide reduces symptoms of AMS, however, remain unclear. Traditionally, acetazolamide's efficacy has been attributed to inhibition of CA in the kidneys, resulting in bicarbonaturia and metabolic acidosis. The result is offsetting hyperventilation-induced respiratory alkalosis and allowance of chemoreceptors to respond more fully to hypoxic stimuli at altitude. Studies performed on both animals and humans, however, have shown that this explanation is unsatisfactory and that the efficacy of acetazolamide in the context of AMS is likely due to a multitude of effects. This review summarizes the known systemic effects of acetazolamide and incorporates them into a model encompassing several factors that are likely to play a key role in the drug's efficacy. Such factors include not only metabolic acidosis resulting from renal CA inhibition but also improvements in ventilation from tissue respiratory acidosis, improvements in sleep quality from carotid body CA inhibition, and effects of diuresis.

The carbonic anhydrase inhibitors methazolamide and acetazolamide have different effects on the hypoxic ventilatory response in the anaesthetized cat

We compared the effects of the carbonic anhydrase inhibitors methazolamide and acetazolamide (3 mg kg(-1), i.v.) on the steady-state hypoxic ventilatory response in 10 anaesthetized cats. In five additional animals, we studied the effect of 3 and 33 mg kg(-1) methazolamide. The steady-state hypoxic ventilatory response was described by the exponential function: *Vi= G exp(-D P(O2)) + A where *Vi is the inspired ventilation, G is hypoxic sensitivity, D is the shape factor and A is hyperoxic ventilation. In the first group of 10 animals, methazolamide did not change parameters G and D, while A increased from 0.86 +/- 0.33 to 1.30 +/- 0.40 l min(-1) (mean +/- s.d., P = 0.003). However, the subsequent administration of acetazolamide reduced G by 44% (control, 1.93 +/- 1.32; acetazolamide, 1.09 +/- 0.92 l min(-1), P = 0.003), while A did not show a further change. Acetazolamide tended to reduce D (control, 0.20 +/- 0.07; acetazolamide, 0.14 +/- 0.06 kPa(-1), P = 0.023). In the second group of five animals, neither low- nor high-dose methazolamide changed parameters G, D and A. The observation that even high-dose methazolamide, causing full inhibition of carbonic anhydrase in all body tissues, did not reduce the hypoxic ventilatory response is reminiscent of previous findings by others showing no change in magnitude of the hypoxic response of the in vitro carotid body by this agent. This suggests that normal carbonic anhydrase activity is not necessary for a normal hypoxic ventilatory response to occur. The mechanism by which acetazolamide reduces the hypoxic ventilatory response needs further study.

Sleep at High Altitude

New arrivals to altitude commonly experience poor-quality sleep. These complaints are associated with increased fragmentation of sleep by frequent brief arousals, which are in turn linked to periodic breathing. Changes in sleep architecture include a shift toward lighter sleep stages, with marked decrements in slow-wave sleep and with variable decreases in rapid eye movement (REM) sleep. Respiratory periodicity at altitude reflects alternating respiratory stimulation by hypoxia and subsequent inhibition by hyperventilation-induced hypocapnia. Increased hypoxic ventilatory responsiveness and loss of regularization of breathing during sleep contribute to the occurrence of periodicity. Interventions that improve sleep quality at high altitude include acetazolamide and benzodiazepines.

Acetazolamide as a vasodilatory stimulus in cerebrovascular diseases and in conditions affecting the cerebral vasculature

Pathologic processes affecting the brain vessels may damage cerebral vasodilatory capacity. Early detection of cerebral dysfunction plays an important role in the prevention of cerebrovascular diseases. In recent decades acetazolamide (AZ) has frequently been used for this purpose. In the present work the mechanism of action and the previous studies are reviewed. The authors conclude that AZ tests are useful in cerebrovascular research. Further investigations are recommended to prove how impaired reserve capacity and reactivity influence the stroke risk in patients and whether these tests may indicate therapeutic interventions.

Acetazolamide and breathing. Does a clinical dose alter peripheral and central CO(2) sensitivity?

Improvement of blood gases with the carbonic anhydrase inhibitor acetazolamide in some patients with chronic obstructive pulmonary disease (COPD) is believed to result from an effect on the ventilatory control system. Carbonic anhydrase is ubiquitously present within the body, particularly in tissues involved in the control of breathing. Because low inhibitor concentrations are sufficient to block the enzyme in many tissues, it is of interest to document the effect of clinical doses of acetazolamide on the CO(2) sensitivities of the peripheral and central chemoreflex loops. In this study we measured the effect of chronic acetazolamide (250 mg by way of mouth, every 8 h during 3 days) on the dynamic ventilatory response to step changes in end-tidal PCO(2) in nine healthy volunteers. Data were analyzed using a two-compartment model comprising a fast peripheral and slow central compartment, enabling us to separate drug effects on the peripheral and central chemoreflex loops, respectively. Compared with placebo, acetazolamide did not change the CO(2) sensitivities and time constants of both chemoreflex loops. However, mean (+/- SD) resting ventilation increased from 12.22 +/- 2.41 to 14.01 +/- 1.85 L. min(-1), resulting in a decrease in end-tidal PCO(2) from 40.0 +/- 4.7 to 33.3 +/- 3.5 mm Hg. Base excess decreased from -0.08 +/- 1.20 to -7.48 +/- 2.07 mmol. L(-1), indicating metabolic acidosis and explaining a leftward shift of the CO(2) response curve by 7.3 mm Hg. Possible clinical implications of these results are discussed.

The effect of low-dose acetazolamide on the ventilatory CO2 response curve in the anaesthetized cat

1. The effect of 4 mg kg-1 acetazolamide (I.V.) on the slope (S) and intercept on the Pa,CO2 axis (B) of the ventilatory CO2 response curve of anaesthetized cats with intact or denervated carotid bodies was studied using the technique of dynamic end-tidal forcing. 2. This dose did not induce an arterial-to-end-tidal PCO2 (P(a-ET),CO2) gradient, indicating that erythrocytic carbonic anhydrase was not completely inhibited. Within the first 2 h after administration, this small dose caused only a slight decrease in mean standard bicarbonate of 1.8 and 1.7 mmol l-1 in intact (n = 7) and denervated animals (n = 7), respectively. Doses of acetazolamide larger than 4 mg kg-1 (up to 32 mg kg-1) caused a significant increase in the P(a-ET),CO2 gradient. 3. In carotid body-denervated cats, 4 mg kg-1 acetazolamide caused a decrease in the CO2 sensitivity of the central chemoreflex loop (Sc) from 1.52 +/- 0.42 to 0.96 +/- 0.32 l min-1 kPa-1 (mean +/- S.D.) while the intercept on the Pa,CO2 axis (B) decreased from 4.5 +/- 0.5 to 4.2 +/- 0.7 kPa. 4. In carotid body-intact animals, 4 mg kg-1 acetazolamide caused a decrease in the CO2 sensitivity of the peripheral chemoreflex loop (Sp) from 0.28 +/- 0.18 to 0.19 +/- 0.12 l min-1 kPa-1. Se and B decreased from 1.52 +/- 0.55 to 0.84 +/- 0.21 l min-1 kPa-1, and from 4.0 +/- 0.5 to 3.0 +/- 0.6 kPa, respectively, not significantly different from the changes encountered in the denervated animals. 5. It is argued that the effect of acetazolamide on the CO2 sensitivity of the peripheral chemoreflex loop in intact cats may be caused by a direct effect on the carotid bodies. Both in intact and in denervated animals the effects of the drug on Sc and B may not be due to a direct action on the central nervous system, but rather to an effect on cerebral vessels resulting in an altered relationship between brain blood flow and brain tissue PCO2.

Renal carbonic anhydrase inhibition reduces high altitude sleep periodic breathing

The efficacy of carbonic anhydrase (CA) inhibitors in amelioration of periodic breathing during sleep at high altitude is not fully understood. Although CA is present in a number of tissues, we hypothesized that selective renal CA inhibition without physiologically important inhibition of other tissue CA, may be sufficient alone by its generation of a mild metabolic acidosis to stimulate ventilation and prevent periodic breathing. We studied benzolamide (3 mg/kg), a selective inhibitor of renal CA, in 4 climbers on ventilation and ventilatory responses at sea level and on arterial O2 saturation (SaO2%) and periodic breathing during sleep at altitude. At sea level, ventilation increased and PaO2 rose accompanied by a mild metabolic acidosis. The isocapnic hypoxic ventilatory response was unchanged but the hyperoxic hypercapnic ventilatory response rose 40%. At high altitude (4400 m), daytime SaO2% improved from 81 to 85 and venous plasma HCO3- fell from 18.9 to 14.8 mM. During sleep, mean SaO2% rose from 76 to 80 and periodic breathing decreased 75%. We conclude that metabolic acidosis occurring with all CA inhibitors is one of the major stimulant actions of these drugs on ventilation while awake and during sleep at high altitude.