Orthostatic tolerance (OT) is a measure of a person’s ability to maintain consciousness and adequate blood pressure during gravitational stress. It is partly dependent on plasma volume (El-Sayed and Hainsworth, 1995) and on the magnitude of reflex vasoconstriction (Brown and Hainsworth, 2000). We postulated that it would also be influenced by red cell volume, and that chronically hypoxic high altitude dwellers with large red cell volumes, would also have high orthostatic tolerance.

We studied 22 male residents of Cerro de Pasco, Peru (4338m, PB 450 mmHg), including 11 with Chronic Mountain Sickness (CMS) characterised by haematocrits over 60 %. Plasma and blood volumes were determined by Evans blue dye dilution and peripheral haematocrit. Orthostatic tolerance was the time to presyncope in a test of head-up tilt (20 min), followed by tilt and lower body suction at -20, -40 and -60 mmHg for 10 min each. We recorded ECG, finger blood pressure (photoplethysmography) and forearm blood velocity (Doppler). The study was approved by the local ethics committee. Data are presented as means ± S.E.M. Statistical significance was assessed using student t test, ANOVA and Pearson correlation.

Plasma volumes in normals and CMS (38.3±1.9 and 34.8±1.6ml kg^-1) were similar to those previously reported in low altitude dwellers (42.4±3.3ml kg^-1, El-Sayed and Hainsworth, 1995). Red cell volumes were higher in CMS than normals (71.7±7.3 and 45.3 ± 2.5ml kg^-1, P < 0.01) and both were higher than in lowlanders (P < 0.05). Blood volumes also were higher in CMS than in normals (106.5±8.3 and 83.6±4.0ml kg^-1, P < 0.05) and higher in both groups than in lowlanders (P < 0.05). Orthostatic tolerance was high in both groups. All tolerated the stress for more than 40 min, compared with only 15 % of sea level dwellers (El-Bedawi and Hainsworth, 1994) and 9 of the 22 tolerated the entire procedure (50 min). Overall orthostatic tolerance was significantly correlated with red cell volume (r = 0.54; P < 0.02) and blood volume (r = 0.48; P < 0.05). The maximum change in forearm vascular resistance (pressure/velocity) was +110±20 and +88±14 % (NS) and similar to that reported in sea level dwellers (Brown and Hainsworth, 2000). All tests were repeated one day after descending to sea level. Oxygen saturation increased from 82.8±3.7 to 95.3±2.4 %, P < 0.0001. Plasma and red cell volumes, and orthostatic tolerance, however, were not significantly changed.

Although there are other differences between altitude and sea level dwellers, the high orthostatic tolerance seen in these subjects, which is not immediately altered by relief of hypoxia, is compatible with the view that red cell volume is a significant factor in orthostatic tolerance.