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.