Variability in human plasma volume responses during high‐altitude sojourn

When sea‐level ( SL ) residents rapidly ascend to high altitude ( HA ), plasma volume ( PV ) decreases. A quantitative model for predicting individual %∆ PV over the first 7 days at HA has recently been developed from the measurements of %∆ PV in 393 HA sojourners. We compared the measured %∆ PV with the %∆ PV predicted by the model in 17 SL natives living 21 days at HA (4300 m). Fasting hematocrit (Hct), hemoglobin (Hb) and total circulating protein ( TCP ) concentrations at SL and on days 2, 7, 13, and 19 at HA were used to calculate %∆ TCP and %∆ PV . Mean [95% CI ] measured %∆ PV on HA 2, 7, 13 and 19 was −2.5 [−8.2, 3.1], −11.0 [−16.6, −5.5], −11.7 [−15.9, −7.4], and −16.8 [−22.2, −11.3], respectively. %∆ PV and %∆ TCP were positively correlated ( P  < 0.001) at HA 2, 7, 13, and 19 ( r 2  = 0.77, 0.88, 0.78, 0.89, respectively). The model overpredicted mean [95% CI ] decrease in %∆ PV on HA 2 (−12.5 [−13.9, −11.1]) and HA 7 (−21.5 [−23.9, −19.1]), accurately predicted the mean decrease on HA 13 (−14.3, [−20.0, −8.7]), and predicted a mean increase in %∆ PV on HA 19 (12.4 [−5.0, 29.8]). On HA 2, 7, 13, and 19 only 2, 2, 6, and 1, respectively, of 17 individual measures of %∆ PV were within 95% CI for predicted %∆ PV . These observations indicate that PV responses to HA are largely oncotically mediated, vary considerably among individuals, and available quantitative models require refinement to predict %∆ PV exhibited by individual sojourners.