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SUMMARY 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.THE RESPONSE of the normal human left ventricle to changes in preload has been studied by various methods.1 12 However, the range of preload variation has been limited, and the results have often been difficult to interpret due to simultaneous changes in arterial blood pressure and heart rate. Previous studies in our laboratory showed that large variations in preload with minimal effects on heart rate or arterial blood pressure can be introduced noninvasively by head-down tilt and lower body negative pressure (LBNP).12, 13 The primary purpose of the study was to define echocardiographically the extreme ranges of the normal relationship between left ventricular end-diastolic dimensions and stroke volume (i.e., a classic Frank-Starling curve). We also studied the effect of wide variations in preload on commonly used indexes of myocardial contractile state. Materials and MethodsTwelve normal male volunteers, ages 22-31 years, were studied. Details of the protocol were explained, and each subject gave informed written consent. The study protocol was reviewed and approved by the sity of Texas Health Science Center at Dallas. The subjects underwent physical examination and were in excellent health. No medications were being taken at the time of the studies. Echocardiograms were obtained using a Unirad 100 series ultrasonoscope, model 902, with a 2.25-MHz, 13-mm-diameter transducer collimated to 7.5 cm, with a repetition rate of 1000 Hz. M-mode tracings and an ECG were recorded on a Honeywell 1856A s...

Ten men repeatedly performed leg exercise (100-150 W) for 7 min with 30-min recovery periods interspersed. Both legs were made ischemic by total occlusion (OCCL), first for 3 min immediately after exercise and second for 30 s before exercise ended and 3 min into recovery. In addition legs were occluded for 3 min at rest (seated). OCCL at rest increased mean arterial pressure (MAP) by 9 Torr but did not affect cardiac output (CO) or heart rate (HR). OCCL at the end of exercise significantly raised MAP and HR above control values during 3-min recovery but CO was unaffected. OCCL 30 s before the end of exercise further increased MAP and HR significantly during recovery; MAP, CO, and HR were significantly increased above control values (CO by 2.1 1-min-1) during the 3rd min of recovery. We conclude that a strong reflex from ischemic legs maintains normal or elevated CO during leg OCCL. Thus CO was too high relative to total vascular conductance so that MAP was elevated.

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