Left ventricular performance in normal subjects: A comparison of the responses to exercise in the upright and supine positions

Abstract

Left ventricular (LV) performance at rest and during multilevel exercise in the supine and upright positions was studied in seven normal subjects with equilibrium radionuclide ventriculography. The mean left ventricular end-diastolic volume (LVEDV) during supine rest was 107±10 ml (±SEM) and 85±6 ml (p<0.02) in the upright position; the mean resting left ventricular end-systolic volumes (LVESV) were not different in the upright and supine positions. The LV ejection fraction (LVEF) tended to be slightly higher in the supine (76±2%) than in the upright position (72±4%). The resting heart rate was 89±5 beats/min upright, compared with 71±6 beats/min supine (p<0.05). Multilevel exercise testing was carried out at a low work load of 300 kpm/min, an intermediate work load of 600-750 kpm/min and a peak work load of 1092±66 kpm/min supine and 946±146 kpm/min upright (p<0.05). With peak exercise, supine LVEDV increased significantly, to 135±13 ml (27%), but LVESV did not change. LVEF increased from 76±2% to 84±2% (p<0.05). With upright exercise, LVEDV increased 39% above the resting level, to 116±8 ml (p<0.02), but remained lower than the supine LVEDVs at intermediate (p<0.05) and peak work loads. LVESV decreased significantly by 41%, to 19 ± 3ml, and was significantly smaller than the corresponding supine volume at intermediate and peak exercise (p<0.05). LVEF increased from 72±4% to 91±2% (p<0.05), which was significantly higher than peak supine LVEF (p<0.05). Heart rates at rest and during exercise were higher in the upright position (p<0.05), but arterial pressures and double products did not differ significantly. Measurements of LV volumes at rest and during exercise in both the supine and upright positions by dynamic radionuclide scintigraphy suggest that stroke volume during exercise is maintained by a combination of the Frank-Starling mechanism and an enhanced contractile state.