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

Abstract

The fractional increase in cerebral blood flow (CBF) velocity (VCBF) 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 O2 saturation (SaO2) 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 PCO2 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 SaO2 was nonlinear, steepening at a lower SaO2. A hyperbolic equation was used to describe hypoxic cerebrovascular reactivity: fractional VCBF = x[60/ (SaO2-40)-1], where X is the fractional increase of VCBF 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 SaO2 at 80% to be 1.30% at sea level and 1.74% after 5 days at altitude. CBF sensitivity to increased end-tidal PCO2 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 PCO2 due to the logarithmic relationship of PCO2 to CSF pH. This change was not significant after correction to log PCO2. 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 +/- 0.59 l.min-1.% fall in SaO2-1).

Cerebrovascular Reactivity is Increased with Acclimatization to 3,454 M Altitude

“…This phenomenon has previously been suggested linked to the increased hypercapnic CVR after 2 to 4 and 16 days at high altitude. 8,10,15 In addition, the association between hypercapnic CVR and resting capillary HCO 3 − at SL, HA0, HA3, and HA22 ( Figure 5) observed in the present study, supports this notion.…”

“…3 This will be discussed in greater detail elsewhere. The consequence of high-altitude exposure on cerebrovascular function is controversial, as CVR has been reported to be increased, 8,10,14,15 unchanged, 11,13,16 or even reduced. 7,9,12 In the present study, we observed a decreased hypercapnic CVR at HA0 followed by an increased hypercapnic CVR at HA3 and HA22 compared with SL.…”

“…4,5 Cerebrovascular CO 2 reactivity (CVR), a measure of cerebrovascular function, 6 may intuitively be expected to be decreased in response to the ventilatory-induced reduction in PaCO 2 to preserve CBF. Yet, several studies have assessed CVR in response to hypoxic exposure [7][8][9][10][11][12][13][14][15][16] and the outcomes are highly controversial, ranging from increased, 8,10,14,15 unchanged 11,13,16 to reduced 7,9,12 CVR. These differences possibly derive in part from inconsistencies in study protocols and methods applied.…”

Changes in Cerebral Blood Flow During and After 48 H of Both Isocapnic and Poikilocapnic Hypoxia in Humans

“…Particular questions to which we sought answers included whether the secondary fall in CBF could be prevented by the maintenance of isocapnia and whether there was any evidence of acclimatization to the hypocapnia of the poikilocapnic protocol. There has also been a report that the sensitivity of CBF to acute changes in the level of hypoxia/hypercapnia is altered following a period of acclimatization to hypoxia (Jensen et al 1996). An additional purpose of this investigation was to examine whether we could replicate this observation, and if so, whether the results from the poikilocapnic and isocapnic protocols differed with respect to this phenomenon.…”

Blood Flow Velocity Changes in the Middle Cerebral Artery Induced by Driving Fatigue

“…It has been reported that the oxygenation delivery of brain tissue is associated with the cerebral blood flow velocities [11][12][13]. The decrease of CBFV in MCA means that the oxygenation delivery of brain tissue could come down after 1h driving.…”