Effects of acute hypobaric hypoxia on regional cerebral blood flow distribution - A Single Photon Emission Computed Tomography study in humans

Single Photon Emission Computed Tomography (SPECT) and radiopharmaceutical stabilizing
agents allowed us to investigate regional cerebral blood flow (CBF) distribution in six resting
healthy subjects during acute laboratory hypobaric hypoxic conditions. In the hypobaric experiment
stabilized 99m Tc- D , L -hexamethyl-propylene amine oxime was injected 40 min after reaching
hypoxic conditions corresponding to an altitude of 5500 m above sea level. Arterial blood sample
was taken after five additional minutes. Mean arterial oxygen pressure and haemoglobin
saturation were 28 mmHg and 56%, respectively. The control experiment was performed
similarly, apart from barometric pressure and blood gas analysis. We analysed CBF distribution in
12 regions of functional interest bilaterally in frontal, parietal, temporal, occipital cortex, in the
hippocampus, in the basal ganglia and other central structures of brain. No overall effect of
hypoxia on normalized regional CBF distribution in the considered regions was found. Motor
cortex (Brodmann 4) and basal ganglia were the only regions in which hypobaric hypoxia
significantly increased relative distribution of the radiopharmaceutical [F(1,5) ˆ 18.30; P < 0.008
and F(1,5) ˆ 10.85; P < 0.022, respectively]. Despite severe hypoxia, we did not observe any
major regional CBF redistribution. We found a small relative increase in blood flow to the motor
cortex and the basal ganglia, at rest after 40 min of hypobaric hypoxia, suggesting a preferential
compensatory mechanism of these functional regions of brain.