Role of the altitude level on cerebral autoregulation in residents at high altitude

Cerebral autoregulation is impaired in Himalayan high-altitude residents who live above 4,200 m. This study was undertaken to determine the altitude at which this impairment of autoregulation occurs. A second aim of the study was to test the hypothesis that administration of oxygen can reverse this impairment in autoregulation at high altitudes. In four groups of 10 Himalayan high-altitude dwellers residing at 1,330, 2,650, 3,440, and 4,243 m, arterial oxygen saturation ( Sa(O2)), blood pressure, and middle cerebral artery blood velocity were monitored during infusion of phenylephrine to determine static cerebral autoregulation. On the basis of these measurements, the cerebral autoregulation index ( AI) was calculated. Normally, AI is between zero and 1. AI of 0 implies absent autoregulation, and AI of 1 implies intact autoregulation. At 1,330 m ( Sa(O2) = 97%), 2,650 m ( Sa(O2) = 96%), and 3,440 m ( Sa(O2) = 93%), AI values ( mean +/- SD) were, respectively, 0.63 +/- 0.27, 0.57 +/- 0.22, and 0.57 +/- 0.15. At 4,243 m ( Sa(O2) +/- 88%), AI was 0.22 +/- 0.18 ( P < 0.0005, compared with AI at the lower altitudes) and increased to 0.49 +/- 0.23 ( P = 0.008, paired t-test) when oxygen was administered ( Sa(O2) = 98%). In conclusion, high-altitude residents living at 4,243 m have almost total loss of cerebral autoregulation, which improved during oxygen administration. Those people living at 3,440 m and lower have still functioning cerebral autoregulation. This study showed that the altitude region between 3,440 and 4,243 m, marked by SaO2 in the high-altitude dwellers of 93% and 88%, is a transitional zone, above which cerebral autoregulation becomes critically impaired.