Passive Heating Impairs Cardiac Autonomic Regulation during Exercise and Post‐Exercise Recovery

Abstract

AIMThe aim of this study was to assess the effects of passive heating on exercise and post‐exercise cardiac autonomic regulation.METHODSTwelve healthy young men (25±5 yrs, 23.8±1.9 kg/m2, 47.3±7.9 ml.kg−1.min−1) randomly underwent two experimental sessions: heat stress (HEAT) and control (CON). Each session composed of: a heat stress or normothermic control intervention; 7 min of cycling at 40% HRreserve (EX1); 7 min of cycling at 60% HRreserve (EX2); and 10 min of recovery in the seated position (REC). HEAT consisted of 60 min whole‐body passive heating by perfusing hot water (48°C) in a tube‐lined suit to increase core temperature (Tc) by ~1°C. CON consisted of normothermia exposure for a similar timeframe. Heart rate (HR) and finger blood pressure were continuously assessed. HR variability (HRV) was assessed using the vagal‐related RMSSD index, baroreflex sensitivity (BRS) was assessed using the sequence technique and HR recovery (HRR) was assessed through the absolute decrease of HR at 60 (HRR60s) and 300 s (HRR300s) after exercise. Tc, skin temperature (Tsk), forearm skin blood flow (SKBF) and cutaneous vascular conductance (CVC) were also continuously measured throughout.RESULTSTc, Tsk, SKBF and CVC were significantly increased in HEAT compared with CON (Tc: 38.0±0.3 vs. 37.1±0.3 °C; Tsk: 36.8±0.6 vs. 33.2±0.8 °C; SKBF: 160±96 vs. 50±34 A.U.; CVC: 2.3±1.3 vs. 0.6±0.3 A.U/mmHg; all p<0.05). During exercise, HRV and BRS were not different between trials during EX1 and EX2. However, when comparing EX1 of CON with EX2 of HS (i.e. similar absolute workloads), HRV (0.8 ± 0.4 vs. 1.9 ± 0.7 ms; p<0.01) and BRS (0.6 vs. 0.3 vs. 1.5 ± 1.0 ms/mmHg; p<0.01) were significantly reduced in HS. During recovery, HRV (1.7 ± 0.6 vs. 2.6 ± 0.9 ms, p<0.01) and BRS (6 ± 3 vs. 4 ± 3 ms/mmHg; p<0.01) were significantly reduced in HS. Furthermore, HRR60s and HRR300s were significantly reduced in HS (HRR60s = 27±9 vs. 46±10 bpm; HRR300s = 39 ± 12 vs. 61 ± 13 bpm; p<0.01).CONCLUSIONPassive heating impairs cardiac autonomic regulation during exercise at the same absolute workload and during recovery.Support or Funding InformationFAPESP 2013/04997‐0; 2013/05519‐4; 2015/15466‐0