Abstract
Acute hypobaric hypoxia (HH) increases cerebral blood flow (CBF) via cerebral vasodilation, thus defending cerebral O2 delivery in the face of a reduced arterial oxygen content (CaO2). However, as HH extends, CBF progressively normalizes, reaching initial values after 3-5 days. While the mechanism underlying the CBF normalization is unclear, HH-induced plasma volume contraction seems a likely candidate: On one hand, the resulting haemoconcentration restores CaO2, thus eliminating the stimulus for the initial CBF increase. On the other hand, the resulting increase in blood viscosity could reduce CBF by increasing blood flow resistance; increases in haematocrit potentially directly contributing to reductions in endothelium-dependent vasodilation. Previous data has demonstrated an inverse relationship between CBF and hemoconcentration. However, these studies have typically involved expedition-style trekking, which although provides strong ecological validity, makes it difficult to isolate the impact of hypoxia alone. Accordingly, the present study investigated whether PV contraction contributes to CBF normalization in HH, while carefully controlling for potential confounders (e.g. cold, varying altitudes, exercise, diet, fluid intake).
In a crossover design, eleven healthy males (25±4 years) completed two four-day sojourns in a hypobaric chamber, with the barometric pressure unchanged in one, and reduced to a 3500 m equivalent in the other. Global CBF was assessed by duplex ultrasound in the evening prior to chamber entry, on the first day (1h, 6h, 12h) and the morning of the last day. A further CBF measurement was performed directly after expansion of plasma volume (PVX) by infusion of a balanced crystalloid solution. Haemoglobin concentration ([Hb]) and blood viscosity at a shear rate of 225 s-1 were measured at the same time points. Four days at 3500 m reduced plasma volume from 3.24 to 2.89 l (p < 0.001). As a result, [Hb] increased from 14.1 ± 0.47 to 15.4 ± 0.49 g dl-1 (p < 0.001), whereas PVX reduced [Hb] to 14.8 ± 0.7 g dl-1 (p < 0.005). Blood viscosity increased from 3.85 ± 0.15 to 4.43 ± 0.22 cP on the first day at 3500m and remained elevated on the last day (4.91± 0.22cP: P<0.001), whereas it was decreased by PVX (4.03± 0.35cP: P<0.001). Global CBF and cerebral conductance data is currently under analysis, but will be ready for presentation at the conference. We expect the data to advance our understanding of the mechanisms regulating CBF during altitude acclimatization.