Effects of graded heat stress on global left ventricular function and twist mechanics at rest and during exercise in healthy humans

Increased left ventricular (LV) twist and untwisting (LV twist mechanics) contribute to the maintenance of stroke volume during passive heat stress. However, it remains unknown whether changes in LV twist mechanics are related to the magnitude of heat stress and whether performing exercise during heat stress alters this response. We examined global LV function and LV twist mechanics in 10 healthy men at baseline and three progressive levels of heat stress, at rest and during knee-extensor exercise. At rest, heat stress increased cardiac output and reduced end-diastolic volume and end-systolic volume, whilst stroke volume and mean arterial pressure (MAP) were maintained. Left ventricular twist and untwisting velocity also increased from baseline to severe heat stress (from 10.6 +/- 3.3 to 15.1 +/- 5.2 deg and from -123 +/- 55 to -210 +/- 49 deg s-1, respectively, both P < 0.01) and correlated significantly with body temperature, heart rate and LV volumes (P < 0.05). Similar to resting conditions, progressive heat stress during exercise increased cardiac output and reduced end-diastolic volume and end-systolic volume with a maintained stroke volume. However, MAP declined (P < 0.01) and there was no significant change in LV twist and untwisting velocity, resulting in non-significant relationships between twist mechanics and systemic responses. In conclusion, LV twist mechanics increase proportionally with the magnitude of heat stress at rest. However, there is no increase in LV twist and untwisting velocity from control exercise to severe heat stress during exercise despite a significant increase in body temperatures and cardiac output. We, therefore, suggest that the maintenance of stroke volume in the combined conditions of heat stress and small muscle mass exercise may be further facilitated by other peripheral factors, such as the continuous decline in MAP.