J.R. Steele, V.E. Brunt, B.R. Ely, L.N. Comrada, C.T. Minson, FACSM

University of Oregon, Eugene, OR

It has been well established that exercise heat acclimation (HA) expands plasma volume (PV); however, it is currently unknown if passive HA also leads to PV expansion. Short-term intervention studies have commonly estimated changes in PV based on changes in hematocrit (Hct) and hemoglobin (Hb) using the Dill & Costill equations; however, these equations do not factor in changes in red cell volume (RCV) which may lead to underestimation of PV changes across multi-day studies. PUPOSE: To determine whether passive HA results in PV expansion, and to compare changes in PV measured by the carbon monoxide (CO) rebreathing method to changes in PV estimated by the Dill & Costill equations. METHODS: Twelve young (221 yrs), sedentary subjects participated in 8wks of water immersion in either 40.5°C water (HA group, rectal temp ≥38.5°C for 60 min per session; N=4) or 36°C water (sham group; N=8). Before and after the 8wks, RCV and PV were measured by the CO rebreathing method. Hct and Hb were measured within the same session and changes in PV were estimated using Dill & Costill equations. Data are presented as mean±S.E. Changes within groups across the 8wks were compared using Student’s paired t-test. Regression analysis was used to compare changes calculated by the CO rebreathing vs. Dill & Costill equations across all subjects. RESULTS: Passive HA tended to increase PV (CO rebreathing) from 3.10.1 to 3.50.2 L (p=0.12) and RCV from 1.9 ± 0.2 to 2.0 ± 0.2 L (p=0.09), resulting in no change in Hct (42.4 ± 2.0 to 41.1 ± 0.9%, p=0.45), but a trend towards a decrease in Hb (14.5 ± 1.2 to 13.9 ± 0.9 g/dL, p=0.15). There were no consistent changes in the sham group (PV: 3.2 ± 0.2 to 3.0 ± 0.2 L, p=0.44; RCV: 1.7 ± 0.1 to 1.6 ± 0.2 L, p=0.56; Hct: 39.1 ± 1.8 to 39.9 ± 1.8%, p=0.52; Hb: 13.7 ± 0.8 to 13.9 ± 0.6 g/dL, p=0.57), although, there was variability in the PV changes in these subjects (range = +28 to -33% change). In all subjects, PV changes measured by CO rebreathing correlated well with PV changes estimated by Dill & Costill (r2 = 0.71, p<0.05); however, the Dill & Costill equations underestimated the change in PV over 8 wks (line of best fit: y = 0.20x, p<0.05 vs. line of identity). CONCLUSIONS: Although not yet significant in our preliminary subjects, passive HA tended to increase PV, similar to exercise heat acclimation. While Dill & Costill has been verified for estimating changes in PV over acute bouts of heat stress or exercise, our data indicate it greatly underestimates changes in PV across 8wks of passive HA. Methods that directly measure PV may be more appropriate for multi-day studies.

Supported by AHA Grant # 14PRE20380300 and the Eugene and Clarissa Evonuk Memorial Foundation.

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