Stroke Volume Response to Low Dose Beta Blockade during Exercise Under Normothermic and Hyperthermic Conditions

Abstract

The effect of hyperthermia during exercise on stroke volume (SV) independent of the elevation of heart rate (HR) has not been described. PURPOSE: To determine if hyperthermia independent of an elevation in HR reduces SV during exercise. METHODS: Eleven active men performed 60 min of exercise at ∼ 57% of VO2peak after receiving placebo control (PL) or a low dose (0.2mg/kg) of the b1 adrenoreceptor beta blocker (bB), atenolol. Four experimental conditions were performed; normothermia placebo (NormoPL), normothermia bB (NormobB), hyperthermia PL (HyperPL), and hyperthermia bB (HyperbB). RESULTS: Hyperthermia increased skin and core temperature by 4.3°C and 0.8°C, respectively. Beta-blockade was successful in matching HR as HR at minute 60 during NormoPL (154.6 ± 11.0) and HyperbB (153.8 ± 13.3 bpm) were similar (p = 0.82). However, SV was increased by 6 % by hyperthermia when comparing HyperbB to NormoPL (treatment x time interaction, p = 0.03). The increase in SV was associated with a higher cardiac output (CO) during hyperthermic conditions (minute 60: Hyper; 21.0 ± 2.5 vs. Normo 18.9 ± 2.6 L/min, p < 0.01). When the progressive increase in HR throughout exercise was not prevented by bB, SV was reduced by 9% and 14% for NormoPL and HyperPL, respectively (p < 0.01). Although hyperthermia increased cutaneous blood flow (CBF) and forearm blood flow (FBF) by ∼40% (p < 0.05), these increases were not temporally related to the decrease in SV. CONCLUSION: When matched for HR, hyperthermia slightly increases SV during exercise. Furthermore, the reduction in SV during exercise under both normothermic and hyperthermic conditions is due to the increase in HR.