Series elasticity in the intact heart. Evidence for the application of the Hill model for muscle to the intact left ventricle.

Several recent studies have shown that the contractile state of the intact heart may be described by the instantaneous relationships between contractile element velocity, length and stress. However, there is little direct evidence that the intact heart can be described by a model containing at least a contractile element and a series elastic component (SEC). In isolated muscle the series elastic component can be analyzed by determining the length changes following quick releases to known loads during contraction. The characteristics of the effective SEC of the intact left ventricle (LV) were determined by a quick release method in 8 dogs in which the left ventricle contracted isovolumically against a balloon inserted via the mitral annulus. During active contraction, sequential withdrawals of 0.5 to 7.0 ml of fluid were performed rapidly (5-18 msec) by an electrically-timed, springloaded syringe. From these releases the calculated maximum extension of the SEC averaged 4.03 +/- 0.27% of LV circumference at LV pressures averaging 79/6 mmHg (systolic/end-diastolic; range 66-107/0-15 mmHg.) The reductions of the left ventricular midwall circumference were plotted against the corresponding changes in mean wall stress. The load extension curve determined in this manner was unchanged by varying the time of release and by norepinephrine infusion. These data support a model for the intact LV that contains an undamped SEC, the characteristics of which resemble those of isolated cardiac muscle.