Load, Work, and Velocity of Muscle Shortening of the Left Ventricle in Normal and Abnormal Human Hearts*

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Abstract
Methods for calculating the forces opposing left ventricular ejection were studies in 6 conscious dogs. It was concluded from these experiments that force-time relationships during left ventricular systole could be studied with reasonable accuracy without knowing the instantaneous aortic flow rate if the end-diastolic volume, the intraventricular pressure, and the stroke volume are measured and a constant rate of fiber shortening is assumed. Although fiber shortening rate was not constant, the errors introduced by assuming a constant rate were found to be small. Evidence for the applicability of these data to normal and abnormal human hearts is discussed. Based on these findings, instantaneous force and the integrated mean force opposing left ventricular ejection were calculated in 21 patients, using left ventricular volume and pressure measurements. The effects of the variations in force during contraction on the external work performed were considered. The data were consistent with the load varying in such a manner as to permit the normal left ventricle to perform nearly maximal external work during contraction. This was not the case in patients in heart failure. In these the contractions tended to be isotonic. In 8 patients with normal left ventricles, the mean force acting tangentially to the circumference varied between 1.59 and 2.28 (mean 1.93) kg/sq m of body surface area. Patients with mitral stenosis had small left ventricular volumes, and the forces opposing ejection were less. Large forces were found in a group of patients with abnormal left ventricles. External work and power were related to the forces opposed. Values obtained in the normal patients and in those with mitral stenosis tended to fall within the same range. In the patients with abnormal left ventricles the values tended to be reduced. The implications of these findings are discussed.