Augmented hypoxic cerebral vasodilation in men during 5 days at 3,810 m altitude

The fractional increase in cerebral blood flow (CBF) velocity (V-CBF) from the control value with 5-min steps of isocapnic hypoxia and hyperoxic hypercapnia was measured by transcranial Doppler in six sea-level native men before and during a 5-day sojourn at 3,810 m altitude to determine whether cerebral vasoreactivity to low arterial O-2 saturation (Sa(o2)) gradually increased [as does the hypoxic ventilatory response (HVR)] or diminished (adapted, in concert with known slow fall of CBF) at altitude. A control resting Pco(2) value was chosen each day during preliminary hyperoxia to set ventilation at 140 ml . kg(-1). min(-1) for this and the parallel HVR study, attempting to establish control cerebrospinal fluid (CSF) and brain extracellular fluid pH values unaltered by acclimatization. The relationship of CBF to Sa(o2) was nonlinear, steepening at a lower Sa(o2). A hyperbolic equation was used to describe hypoxic cerebrovascular reactivity: fractional V-CBF = X[60/(Sa(o2) - 40) - 1], where X is the fractional increase of V-CBF at 70% X rose from 0.346 +/- 0.104 (SD) at sea level to 0.463 +/- 0.084 on altitude day 5 (P < 0.05 by paired t-test, justified by the a priori experimental plan). For comparison with CO2 sensitivity, from these X values, we estimate the rise in CBF in response to a 1% fall in Sa(o2) at 80% to be 1.30% at sea level and 1.74% after 5 days at altitude. CBF sensitivity to increased end-tidal Pco(2) rose from 4.01 +/- 0.62%/Torr at sea level to 5.12 +/- 0.79%/Torr on day 5 (P < 0.05), as expected, at the lower Pco(2) due to the logarithmic relationship of Pco(2) to CSF pH. This change was not significant after correction to log Pco(2). We conclude that the cerebral vascular response to acute isocapnic hypoxia may increase during acclimatization at high altitude. The mechanism is unknown but is presumably unrelated to the parallel carotid chemosensitization that, in these subjects, increased the HVR by 60% in the same 5-day period (from 0.91 +/- 0.38 to 1.46 +/- 0.591 . min(-1).%fall in Sao(2)(-1)).