Fluid-electrolyte shifts and VO2max in man at simulated altitude (2,287 m)

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Abstract
Six trained distance runners were studied during an 8-day dietary control (C) period at sea level (24 m, 758 Torr), followed by 8 days at 2287 m (576 Torr) in an altitude chamber (A) and 4 days recovery (R) at sea level. The purpose was to determine whether reduction in maximal oxygen uptake (.ovrhdot.VO2 max), during early exposure to the reduction in PO2 [oxygen partial pressure] at altitude was accentuated by reduced plasma volume (PV) or to a more generalized negative water balance. For 2 mo. prior to the study the men ran 15 km/day and during the study ran 16 km/h for 1 h each day. Mean .+-. SE .ovrhdot.VO2 max was 4.75 .+-. 0.23 l/min in C, 4.05 .+-. 0.22 l/min at A (.DELTA. = -14.7%, P < 0.05) and 4.68 .+-. 0.22 l/min during R. Basal blood volume, PV and red blood cell (RBC) volume were essentially constant during C and A, and there was no change in fluid balance at A. During maximal exercise the mean calculated change (shift) of PV was -6% during C, but increased to between -11% and -15% at A and coincided with the reduction of -13% to -15% in .ovrhdot.VO2 max. On the 1st day of R (R + 1) .ovrhdot.VO2 max returned to sea-level values, but .DELTA.PV was still -15%. However, the latter returned to control levels on R + 2. The shift of plasma Na, Cl and osmotic contents followed the shift in PV, the K response was variable and total protein and Hb content shifts were relatively stable (.DELTA. = .+-. 4%). Exaggerated PV and electrolyte shifts during maximal exercise at altitude perhaps reflect alterations in cellular membrane permeability, but neither amplified PV shifts nor negative water balance contribute to the reduction in .ovrhdot.VO2 max at altitude.

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