Left ventricular (LV) performance at rest and during multilevel exercise in the supine and up-right positions was studied in seven normal subjects with equilibrium radionuclide ventriculography. The mean left ventricular end-diastolic volume (LVEDV) during supine rest was 107 4 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 diSferent 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 hpart rate was 89+ 5 beats/min up-right, 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 of1092 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 i 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±3 ml, 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 maintainedby a combination of the Frank-Starling mechanism and an enhanced contractile state.

THE EFFECTS OF EXERCISE on left ventricular (LV) function in man have been investigated exten-sively by various techniques. Hemodynamic studies have been supplemented by data on dynamic changes in LV dimensions. Measurements have been obtained by several different methods, but there is disagreement regarding the interactive effects of posture and exercise on LV volumes and performance. Changes in posture at rest are associated with significant changes inLV filling and stroke volume. A transition from the supine to the upright position produces a decrease inLV end-diastolic pressure1-3 and volume4-6 and in stroke volume.'-4 6-10 The results of previous studies of the alterations in LV enddiastolic volume during exercise in the supine position have varied. 5, 6, 11-15 There is general agreement that end-systolic volumeis smaller during exercise than at rest; 6 11, 12, 14, 15 most investigators3'6, 12, 13 have reported an exercise-induced increase in stroke volume, although others have not. 1 4These data are generally consistent with an enhanced contractile state