C.E. Frisiras, M.I. Schoeberlein, C.I. Nicol, J. Chapman, K.E. Coffman

University of Puget Sound, Tacoma WA

Exercise elevates heart rate (HR) and oxygen consumption (VO2). Additionally, heat exposure causes shunting of blood toward the skin to allow for proper thermoregulation. PURPOSE: The purpose of this study was to further understand the effects of exercise in the heat on the cardiopulmonary system, using lung diffusing capacity for carbon monoxide (DLCO) as the primary measure. METHODS: Nine (4F/5M, 20-45 years) moderately active and healthy adults participated in 3 visits. Visit 1 involved demographics, health screening, and a VO2 max test on a cycle ergometer to determine maximum workload (Wmax). Visits 2 and 3 involved exercise testing in the environmental chamber in thermoneutral (TN; 22.2 ± 0.6°C and 35.3 ± 4.8% RH) or heat (HT; 39.4 ± 0.4°C and 37.6 ± 2.6% RH) conditions. Exercise testing for visits 2 and 3 began with a 30-minute resting exposure in the chamber at testing conditions. This was followed by three 15-min stages of exercise at rest, 20%, and 40% of Wmax on a cycle ergometer. Measurement­­­­s collected at each stage included DLCO, minute ventilation (VE), VO2, HR, and core (Tc) and skin temperature (Ts). Ratings of perceived exertion (RPE; 6-20) and breathlessness (DYSP; 0-10) were collected twice per stage. Statistical analysis was completed using a 2-way repeated measures ANOVA test. RESULTS: DLCO was significantly higher in HT vs TN at 40% of Wmax (53.2 ± 10.6 vs 50.0 ± 10.3 mL/min/mmHg, P=0.003). VO2 showed no differences in HT vs TN. HR was overall higher in HT vs TN (P=0.012). VE values at 0, 20, 40% of Wmax were not significantly different in HT vs TN. VO2 values at 0, 20, 40% of Wmax showed no significant differences in HT vs TN. CONCLUSION: We found a significant interaction between exercise intensity and temperature as DLCO values were higher in HT vs TN at 40% Wmax only. While HR was higher in HT vs TN, VE and VO2 were not different, suggesting heat exposure increased gas exchange surface area in the lungs, as measured by DLCO, at the same work rate. We hypothesize that this is due to increased pulmonary capillary distension and recruitment, likely related to elevated HR and higher cardiac output (Q).

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