Evidence of disrupted human heat balance in heart failure patients exercising in the heat
Authors: Balmain Bryce , Jay Ollie , Sabapathy Surendran , Royston Danielle , Stewart Glenn , Jayasinghe Rohan , Eqbal Yusuf , Morris Norman 
Published: November 2023

IntroductionHeart failure (HF) patients appear to exhibit altered thermoregulatory responses during exercise; however, this has not been examined during exercise in the heat. Accordingly, this study compared thermoregulatory responses and human heat balance parameters of HF and healthy controls (CON) during exercise in a warm environment.MethodsEight HF (NYHA class I–II), and six CON matched for age (HF: 63±7 yr; CON: 64±7 yr; p=0.72) and body surface area (HF: 2.04±0.11 m2; CON: 1.96±0.13 m2; p=0.26) performed a 60‐min cycling test at 60% of their individual peak oxygen uptake in a 30°C environment. Oxygen uptake (VO2), core temperature (Tc), mean skin temperature (Tsk), and forearm cutaneous vascular conductance (CVC) (derived from the ratio of laser Doppler flux to mean arterial pressure) were monitored at rest and during exercise. Whole body sweat rate (WBSR) was determined from pre‐post nude body weight corrected for fluid intake. Metabolic heat production (Hprod) and evaporative heat balance requirements (Ereq), as well as dry heat loss (Hdry) and evaporative heat potential (Esk) – assuming all secreted sweat evaporated, were subsequently calculated.ResultsDuring the cycling tests, absolute Hprod (HF: 367±96 W; CON: 511±116 W) and Hprod per unit mass (HF: 4.1±0.9 W/kg; CON: 6.5±1.3 W/kg) were lower (p<0.05) for HF, and consequently Ereq (HF: 298±92 W, 3.4±0.9 W/kg; CON: 438±109 W, 5.6±1.3 W/kg) was also lower (p<0.05) for HF compared to CON. Despite a difference in metabolic heat load, Hdry was similar (p>0.05) between the two groups (HF: 69±18 W, 0.9±0.4 W/kg; CON: 72±20 W, 0.9±0.2 W/kg). In parallel, increases in Tc (HF: 0.94±0.41°C; CON: 0.97±0.29°C) and Tsk (HF: 2.48±0.88°C; CON: 3.11±1.02°C) were similar (p>0.05) for both groups. But, increases in CVC were lower (p<0.01) in HF (132±14 %CVCrest) compared to CON (553±215 %CVCrest). WBSR tended to be lower (p=0.09) in HF (0.38±0.15 L/hr) compared to CON (0.67±0.31 L/hr). Furthermore, Esk (HF: 2.9±1.0 W/kg; CON: 5.9±2.7 W/kg) was lower for HF; however, the relative difference in Esk to Hprod (HF: 70±23%; CON: 86±25%) was similar (p>0.05) between the two groups.ConclusionDespite exercising with a lower Hprod per unit mass, HF patients had a similar rise in Tc in parallel to a smaller increase in CVC compared to CON. As such, it appears that HF patients are limited in their ability to manage a thermal load and distribute internal heat content among various tissues in the body, secondary to poorer circulation to the periphery. These results suggest that from a thermoregulatory perspective, HF patients primarily exhibit blunted changes in skin blood flow during exercise in the heat. Importantly, it must be noted that both participant groups (HF vs CON) performed exercise at different rates of Hprod. Therefore, to perform an unbiased comparison of thermoregulatory responses between groups, we propose that the same Hprod per unit mass must be administered and taken into consideration.