Acute Effect of Heat Exposure on Brachial Artery FMD: Impact of Modality and Measurement Timeline

Abstract

Introduction Heat exposure is receiving a lot of attention for its potential to improve endothelial function, as measured by brachial artery flow-mediated dilation (FMD). Although most intervention studies have observed an improvement in brachial artery FMD, the acute effect of heat exposure has been inconsistent. This observation likely stems from the various modalities employed (local limb heating to whole-body heating) and the timing of FMD measurement following the heating intervention (immediately after up to 60 min post-heating). Identifying a heating modality that consistently improves brachial artery FMD and establishing the timeline of the response would facilitate investigations into potential mechanisms mediating heat-induced improvements in brachial artery FMD without the need of more challenging intervention studies. The aims of this study are to 1) determine the acute effect of four heating interventions on brachial artery FMD, and 2) characterize the time-dependent changes in brachial artery FMD following each intervention. Methods On separate days, 11 young and healthy adults (3F/8M; 29±6 y; 66.5±9.4 kg; 1.7±0.1 m) were exposed to: i) 10 minutes of forearm heating using an electric blanket (EB); ii) 30 minutes of forearm heating in 40˚C water (WI); iii) 60 minutes of whole-body heating where the arm used to assess brachial artery FMD was not covered by the water perfused suit (WHB-U), or; iv) was covered for the entire 60 minutes of whole-body heating with the suit (WBH-C). During each trial, brachial artery FMD was measured prior to the heating intervention (baseline), within 5 minutes following the end of the intervention, and every 30 minutes for two hours following the intervention. Results Mean shear rate increased from baseline during the WI (+78±43 s-1, P=0.02), WBH-U (+177±99 s-1, P<0.01) and WBH-C (+140±43 s-1, P<0.01) interventions, but not during EB (+18±35 s-1, P=0.37). Brachial artery FMD was unaffected by the EB intervention (P=0.39). Conversely, there was a main effect of time for FMD during the WI (P=0.03), WBH-U (P<0.01), and WBH-C (P<0.01) interventions. Specifically, FMD was greater relative to baseline, 90 minutes following the WI intervention (+3.7±3.3%, P=0.03, increase in 8/11 participants). A lower FMD relative to baseline was observed within 5 minutes of the end of the WBH-U (-4.5±3.7%, P<0.01, decrease in 9/11 participants) and WBH-C (WBH-C: -6.1±3.5%, P<0.01, decrease in 10/11 participants) interventions. Conclusion These results suggest that 10 minutes of local heating with an electric blanket may not modulate brachial FMD, whereas whole-body heating with or without local heating of the arm causes a temporary reduction in FMD. Forearm immersion in 40°C water for 30 minutes was the only intervention that consistently improved brachial artery FMD, but only when measured 90 minutes following the intervention.