Objectives. We sought to evaluate in the young heart the primary assumptions on which the current use of the mean “velocity of fiber shortening corrected for heart rate” as a noninvasive index of contractility are based.Background. End-systolic wall stress-velocity of fiber shortening relation has been applied as a single-beat, load-independent index of contractility in children. This use is based on poorly validated assumptions of linearity, parallel shifts with changing contractile state and inotropic sensitivity of the end-systolic wall stress-velocity of fiber shortening relation.Methods. In eight anesthetized young piglets, 5F micromanometric catheters were placed in the ascending aorta and balloon occlusion catheters in the descending aorta. End-systolic wall stress and velocity of fiber shortening were calculated from aortic pressure and M-mode echocardiography under six conditions: in three contractile states 1) baseline, 2) increased contractility during dobutamine infusion (10 μg/kg per min), and 3) decreased contractility after propranolol injection (1 mg/kg), each at two afterload states (normal and increased load by partial aortic occlusion).Results. Dobutamine increased and propranolol decreased afterloadd-matched velocity of fiber shortening corrected for heart rate significantly to 140% aid 77% of baseline, respectively. However, the slope of end-systolic wall stress-velocity of fiber shortening was (251% of baseline) during dobutamine infusion, which also significantly decreased wall stress, and was much less (27% of baseline) after propranolol injection, which increased wall stress.Conclusions. The velocity of fiber shortening corrected for heart rate did change predictably with changes in contractility and as such can be ued noninvasively in the temporal evaluation of individual patients undergoing therapeutic interventions or to define the natural history of a disease process. However, the relation on which it is based is not defined by parallel straight lines across contractile states, so that abnormal single measurements may reflect only the nonlinearity of the relation rather than in contractility. Thus, we recommend that the end-systolic wall stress-velocity of fiber shortening relation should not be used as a single-beat index of contractility.
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