Effect of Large Variations in Preload

The normal human left ventricular response to large variations in preload was studied in 12 young men. M-mode echocardiograms were recorded at supine rest and compared with studies obtained during head-down tilt (HDT) at 50 and during progressive lower body negative pressure (LBNP) to -40 mm Hg. During HDT, end-diastolic volume (EDV) increased 23% (p < 0.001), stroke volume (SV) increased 35% (p < 0.001) and ejection fraction (EF) increased 10% (p < 0.05). Heart rate (HR) decreased by 5 beats/min (p < 0.025). During LBNP, EDV decreased 28% (p < 0.001), end-systolic volume (ESV) decreased 21% (p < 0.001) and SV decreased 33% (p < 0.001). LBNP was accompanied by a minor increase in HR (9 beats/min,p < 0.001) and a small increase in systolic blood pressure (11 mm Hg, p < 0.01). Comparisons between measurements obtained during HDT and LBNP showed significant differences in EDV, ESV, SV, EF, HR and diastolic blood pressure. The mean velocity of circumferential fiber shortening was unchanged. The combined data from the control studies and the interventions defined the normal left ventricular function curve as an exponential function where SV = 0.36 EDVY'' ml (r = 0.93, p < 0.001), which was not significantly different from the linear regression SV = 0.6 EDV + 0.57 (r = 0.92, p < 0.001). Our data indicate that the mean velocity of circumferential fiber shortening is an index of contractile state that is independent of preload, whereas other echocardiographic measurements are significantly altered by large changes in preload.