Aerobic fitness does not influence the biventricular response to whole body passive heat stress.

We examined biventricular function during passive heat stress in endurance trained (ET) and untrained (UT) men to evaluate whether aerobic fitness alters the volumetric response. Body temperature was elevated ~0.8°C above baseline in 20 healthy men (10 ET, 64.4 ± 3.0 ml·kg(-1)·min(-1); and 10 UT, 46.3 ± 6.2 ml·kg(-1)·min(-1)) by circulating warm water (50°C) throughout a tube-lined suit. Cardiac magnetic resonance imaging was used to measure biventricular volumes, function, filling velocities, volumetric flow rates, and left ventricular (LV) twist and circumferential strain at baseline (BL) and after 45 min of heat stress. In both groups, passive heat stress reduced biventricular end-diastolic (ET, -19.5 ± 24.0 ml; UT, -25.1 ± 23.8 ml) and end-systolic (ET, -15.9 ± 8.8 ml; UT, -17.6 ± 7.9 ml) volumes and left atrial volume (ET, -19.2 ± 11.6 ml; UT, -15.0 ± 12.7 ml) and significantly increased heart rate (ET, 29.3 ± 9.0 beats/min; UT, 31.7 ± 10.4 beats/min) and cardiac output (ET, 3.8 ± 2.2 l/min; UT, 3.2 ± 1.4 l/min) similarly, while biventricular stroke volume was unchanged. There were no between-group differences in any parameter. Heat stress increased (P < 0.05), as a percentage of baseline values, biventricular ejection fraction (ET, 3.4 ± 5.3%; UT, 4.4 ± 3.7%), annular systolic tissue velocities (ET, 32.5 ± 34.9%; UT, 44.0 ± 38.1%), and peak LV twist (ET, 51.6 ± 59.7%; UT, 59.7 ± 54.2%) and untwisting rates (ET, 45.5 ± 42.3%; UT, 51.8 ± 55.0%) similarly in both groups. Early LV diastolic tissue and blood velocities, volumetric flow rates, and strain rates (diastole) were unchanged with heat stress in both groups. The present findings indicate that aerobic fitness does not influence the biventricular response to passive heat stress.