Plasma volume after heat acclimation: Variations due to season, fitness and methods of measurement

PURPOSE: The reported magnitude of plasma volume increase (Δ%PV) following heat acclimation (HA) varies widely. Variations may result from differences in measurement techniques, season and subjects' fitness. This report compares direct and indirect measurements of Δ%PV after 10 days of HA from studies in winter (WIN, n = 8) and summer (SUM, n = 10) in men, age 21-43 yr, at two fitness levels (VO(2)max: 35 and 51 ml/min/kg). Direct measurements were made before and after HA (cycling at 30% of VO(2)max at 50 °C, for 100 min/day) by carbon monoxide (CO) rebreathing and compared with indirect estimates from changes in hematocrit, hemoglobin and plasma protein concentration. RESULTS: Overall, Δ%PV by CO was small (2.9%) and greater in SUM than WIN (5.0 vs. 0.3%). Red cell, blood and plasma volumes/kg lean body mass increased in SUM and decreased in WIN, the difference being significant, and Δ%PV by CO was similar for high and low VO(2)max. CONCLUSION: Overall, indirect estimates of Δ%PV by hemoglobin and hematocrit were similar to CO, but tended to differentiate by fitness and not season. The difference in THb increase in SUM and decrease in WIN was significant. This probably accounts for the differences from the seasonal and fitness results by the direct CO method.