Left ventricular systolic and diastolic function during tilt table positioning and passive heat

32 The ventricular response to passive heat stress has predominately been studied in the 33 supine position. It is presently unclear how acute changes in venous return influence ventricular 34 function during heat stress. To address this question, left ventricular (LV) systolic and diastolic 35 function were studied in seventeen healthy men (24.3 + 4.0 yrs; mean + SD), using two36 dimensional transthoracic echocardiography with Doppler ultrasound, during tilt table 37 positioning (supine, 30° head-up tilt, HUT; and 30° head-down tilt, HDT), under normothermic 38 and passive heat stress (core temperature 0.8 + 0.1°C above baseline) conditions. The supine heat 39 stress LV volumetric and functional response was consistent with previous reports. Combining 40 HUT with heat stress reduced end-diastolic (25.2 + 4.1%) and end-systolic (65.4 + 10.5%) 41 volume from baseline, while heart rate (37.7 + 2.0%), ejection fraction (9.4 + 2.4%), and LV 42 elastance (37.7 + 3.6%) increased, and stroke volume (-28.6 + 9.4%) and early diastolic inflow (43 17.5 + 6.5%) and annular tissue (-35.6 + 7.0%) velocities were reduced. Combining HDT with 44 heat stress restored end-diastolic volume, while LV elastance (16.8 + 3.2%), ejection fraction 45 (7.2 + 2.1%), and systolic annular tissue velocities (22.4+ 5.0%) remained elevated above 46 baseline, and end-systolic volume was reduced (-15.3 + 3.9%). Stroke volume, and the early and 47 late diastolic inflow and annular tissue velocities were unchanged from baseline. This 48 investigation extends previous work by demonstrating increased LV systolic function with heat 49 stress, under varied levels of venous return, and highlights the preload dependency of early 50 diastolic function during passive heat stress. 51