Hysteresis in the heart rate‐core temperature relationship during acute heat stress in rats: implications for systemic hemodynamics

Abstract

Heart rate (HR) increases during hyperthermia, even in the absence of exercise and contributes to increased cardiac output necessary for dissipation of excess heat. It is unknown whether the onset and recovery from heat stress have different cardiovascular profiles. We hypothesized that hyperthermia‐induced tachycardia would exhibit hysteresis. Core temperature (Tc), HR and MAP were measured via radiotelemetry in conscious male Fischer 344 rats (n=8; 262±8 gms) during exposure to 37°C until a Tc of 41.8°C was reached and during recovery at 20°C. Tc, HR and MAP significantly increased during heat exposure (ascending phase; P<0.01 vs. control) and HR was significantly lower for a given Tc compared to recovery (descending phase; p<0.001). For example, at Tc = 39.5°C, HR was 365±27 bpm during the ascending phase and 527±50 bpm during the descending phase (P<0.001). Hysteresis during heat stress recovery may be a compensatory, baroreflex‐mediated response to maintain MAP during thermoregulatory vasodilation and decreased peripheral vascular resistance. These findings may have implications for understanding cardiovascular stress during recovery from heat exposure. Research supported by MRMC. Author views not official US Army or DoD policy.