Interaction of hyperthermia and heart rate on stroke volume during prolonged exercise

Abstract

People who become hyperthermic during exercise display large increases in heart rate (HR) and reductions in stroke volume (SV). It is not clear if the reduction in SV is due primarily to hyperthermia or if it is a secondary effect of an elevation in HR reducing ventricular filling. In the present study, the upward drift of HR during prolonged exercise was prevented by a very small dose of the β1-adrenoreceptor blocker (atenolol; βB), thus allowing SV to be compared at a given HR during normothermia and hyperthermia. Eleven men cycled for 60 min at 57% of peak O2 uptake after receiving placebo control (PL) or a low dose (0.2 mg/kg) of βB. Hyperthermia was induced by reducing heat dissipation during exercise. Four experimental conditions were studied: normothermia-PL, normothermia-βB, hyperthermia-PL, and hyperthermia-βB. Hyperthermia increased skin and core temperature by 4.3 degrees C and 0.8 degrees C (P<0.01), respectively. βB prevented HR elevation with hyperthermia: HR values were similar at minute 60 during normothermia-PL and hyperthermia-βB (155±11 and 154±13 beats/min, respectively, P=0.82). However, SV was increased by 7% during the final 20 min of exercise during hyperthermia-βB compared with normothermia-PL (treatment×time interaction, P=0.03). In conclusion, when matched for HR, mild hyperthermia increased SV during exercise. Furthermore, the reduction in SV throughout prolonged exercise under normothermic and mildly hyperthermic conditions appears to be due to the increase in HR.

Performance in the Heat—Physiological Factors of Importance for Hyperthermia‐Induced Fatigue

“…• C. The high skin blood flow reduces cardiac filling (and, therefore, stroke volume) and thus elevates heart rate for a given cardiac output (288,289,304). More extreme reductions in venous return can reduce cardiac output at higher exercise intensities (245,253).…”

Increased ventricular stiffness and decreased cardiac function in Atlantic cod (Gadus morhua) at high temperatures

“…Matikainen and Vornanen (36) reported an ϳ50% drop in force between 5 and 20°C but an ϳ50% rise in force between 20 and 35°C in isolated Crucian carp (Carassius carassius) hearts paced at 12 contractions/min. Likewise, although hyperthermia during exercise in humans is associated with an increase in heart rate and decrease in stroke volume, there is an increase in stroke volume when the hyperthermic effect on heart rate is blocked (55). Furthermore, there is evidence for increased atrial and systolic contractile function with heat stress in human hearts (4).…”

Warm Skin Alters Cardiovascular Responses to Cycling after Preheating and Precooling

“…V 0 2peak was determined at least 3 d before the initial study day, with an incremental test to volitional exhaustion on an electromagnetically braked cycle ergometer (Excaliber Sport, Lode, Groningen, The Netherlands). The protocol used to determine V 0 2peak and maximal HR consisted o f 2-min stages, with subjects reaching exhaustion between 8 and 12 min (29). Standardized motivation was given throughout the test.…”

“…Cardiac output (Qc) was determined via open-circuit acety lene washin (3,16,29). Values for Qc were calculated with cus tom software (BIPS) using a single alveolar one-compartment model (16 Systolic and diastolic blood pressure was measured every 5 min during cycling by auscultation at the brachial artery of the right arm (Tango+, SunTech Morrisville, NC), and mean arterial blood pressure (MAP) was calculated as diastolic blood pressure + 1/3 pulse pressure.…”