The effect of heat on lung diffusing capacity for carbon monoxide (DLCO) during cycling exercise.

PURPOSE: This study aimed to quantify the combined effects of heat exposure and exercise of increasing intensity on pulmonary blood flow using lung diffusing capacity for carbon monoxide (DLCO) as an indirect measure. We hypothesized that, during exercise in the heat, the well-documented increase in skin blood flow for thermoregulation would lead to alterations in pulmonary blood flow and a subsequent fall in DLCO versus a thermoneutral condition. METHODS: Nine healthy subjects (4 F/5 M, 20-45 years, VO2max 46.7 ± 5.8 mL/kg/min) completed three 15-min stages including rest and during cycling at 20 and 40% of maximum workload (Wmax) in either thermoneutral (TN; 22.2 ± 0.6 °C) or heat (HT; 39.4 ± 0.4 °C) conditions. DLCO, minute ventilation (VE), oxygen consumption ([Formula: see text]), heart rate (HR), and core (TC) and skin temperature (Tsk) were measured. RESULTS: DLCO showed a significant interaction between exercise intensity and heat (P = 0.019); post hoc testing revealed that DLCO was higher at 40% of Wmax in HT vs. TN (53.2 ± 10.6 vs 50.0 ± 10.3 mL/min/mmHg, P = 0.003) only. VE and [Formula: see text] showed no difference in HT vs. TN. HR was higher in HT vs. TN (P < 0.001). TC and Tsk showed a significant interaction between temperature and intensity (P < 0.05). CONCLUSION: The unexpected increase in DLCO during exercise in HT vs. TN conditions suggests a larger lung surface area for gas exchange, perhaps due to increased pulmonary capillary recruitment and/or distension secondary to a higher cardiac output (Q) in the heat. This study furthers our understanding of how heat exposure might impact pulmonary blood flow, specifically as assessed via DLCO.