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 (250mg) reduces this elevated CMRO2 and in turn might improve cerebral tissue oxygenation (PtiO2) during acute hypoxia. Six normal human subjects were exposed to 6-hours 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/100ml/min (hypoxia), P<0.05] and CMRO2 [1.54(0.19) to 1.79(0.25) µmol/ml/min, 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/100ml/min (hypoxia+acetazolamide)] and CMRO2 [1.41(0.09) µmol/ml/min (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.
- Cerebral oxygen metabolism
- Cerebral oxygenation
- Cerebral Blood Flow
- Magnetic Resonance Imaging
- Copyright © 2015, Journal of Applied Physiology