| Passive heat stress is a severe cardiovascular challenge, producing large increases in skin blood flow, cardiac output and sympathetic nerve activity, while significantly reducing central blood volume and cardiac filling pressures. Seminal research decades ago found that, despite reduced filling pressures, stroke volume is maintained, and even slightly elevated during passive heat stress. The mechanism for this response remains to be elucidated, and is the focus of this thesis. A series of three studies were therefore designed, using cardiac magnetic resonance imaging and two-dimensional echocardiography, to study the systolic and diastolic volumetric and functional response to passive heat stress. To broaden our understanding, the study population varied from untrained to endurance trained (Chapter 3), and venous return was further manipulated via passive tilt table positioning (Chapter 4). Results suggest that: preload is significantly reduced during passive heat stress, as demonstrated by marked reductions in end-diastolic left ventricular (LV) and right ventricular (RV) volumes, and end-systolic left atrial volume; global early diastolic function is maintained during heat stress despite significant reductions in filling pressures, believed to be associated with increased LV twist and untwisting rate and reduced LV end-systolic volume (i.e. LV diastolic suction); myocardial contractility is augmented during passive heat stress, as demonstrated by a preload-independent increase in annular systolic tissue velocities, ejection fraction, and LV end-systolic single point elastance; and, afterload is significantly reduced during passive heat stress, as demonstrated by a significant reduction in LV end-systolic wall stress, and reduced systemic vascular conductance. The results of this thesis research lead to the conclusion, that the seemingly paradoxical maintenance of stroke volume in the face of reduced filling pressures is multifactoral, related to an increase in myocardial contractility, improved diastolic function, and reduced afterload. |
