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<jats:p> 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<jats:sub>O<jats:sub>2</jats:sub></jats:sub>), 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<jats:sub>O<jats:sub>2</jats:sub></jats:sub> = 97%), 2,650 m (Sa<jats:sub>O<jats:sub>2</jats:sub></jats:sub> = 96%), and 3,440 m (Sa<jats:sub>O<jats:sub>2</jats:sub></jats:sub> = 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<jats:sub>O<jats:sub>2</jats:sub></jats:sub> = 88%), AI was 0.22 ± 0.18 ( P &lt; 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<jats:sub>O<jats:sub>2</jats:sub></jats:sub> = 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 Sa<jats:sub>O<jats:sub>2</jats:sub></jats:sub> in the high-altitude dwellers of 93% and 88%, is a transitional zone, above which cerebral autoregulation becomes critically impaired. </jats:p>

Original publication

DOI

10.1152/japplphysiol.01429.2006

Type

Journal article

Journal

Journal of Applied Physiology

Publisher

American Physiological Society

Publication Date

08/2007

Volume

103

Pages

518 - 523