Cerebrovascular responses to hypoxia and hypocapnia in Ethiopian high altitude dwellers.

Background And Purpose: Cerebrovascular responses to hypoxia and hypocapnia in Peruvian altitude dwellers are impaired. This could contribute to the high incidence of altitude-related illness in Andeans. Ethiopian high altitude dwellers may show a different pattern of adaptation to high altitude. We aimed to examine cerebral reactivity to hypoxia and hypocapnia in healthy Ethiopian high altitude dwellers. Responses were compared with our previous data from Peruvians.

Methods: We studied 9 Ethiopian men at their permanent residence of 3622 m, and one day after descent to 794 m. We continuously recorded cerebral blood flow velocity (CBFV; transcranial Doppler). End-tidal oxygen (P(ET)o(2)) was decreased from 100 mm Hg to 50 mm Hg with end-tidal carbon dioxide (P(ET)co(2)) clamped at the subject's resting level. P(ET)co(2) was then manipulated by voluntary hyper- and hypoventilation, with P(ET)o(2) clamped at 100 mm Hg (normoxia) and 50 mm Hg (hypoxia).

Results: During spontaneous breathing, P(ET)co(2) increased after descent, from 38.2+/-1.0 mm Hg to 49.8+/-0.6 mm Hg (P<0.001). There was no significant response of CBFV to hypoxia at either high (-0.19+/-3.1%) or low (1.1+/-2.9%) altitudes. Cerebrovascular reactivity to normoxic hypocapnia at high and low altitudes was 3.92+/-0.5%.mm Hg(-1) and 3.09+/-0.4%.mm Hg(-1); reactivity to hypoxic hypocapnia was 4.83+/-0.7%.mm Hg(-1) and 2.82+/-0.5%.mm Hg(-1). Responses to hypoxic hypocapnia were significantly smaller at low altitude.

Conclusions: The cerebral circulation of Ethiopian high altitude dwellers is insensitive to hypoxia, unlike Peruvian high altitude dwellers. Cerebrovascular responses to P(ET)co(2) were greater in Ethiopians than Peruvians, particularly at high altitude. This, coupled with their high P(ET)co(2) levels, would lead to high cerebral blood flows, and may be advantageous for altitude living.