Instantaneous force-velocity-length relations in isolated dog heart

The mechanics of left ventricular contraction were investigated in terms of instantaneous mural force, mid-wall circumferential fiber shortening velocity (dL/dt), and fiber length (L) in seven servo-regulated hearts. The steady-state response of series of variably preloaded or afterloaded allasotonic contractions were utilized. Norepinephrine (0.55-1.38 mug/min) or propranolol (0.07-0.14 mg/min) were given to alter inotropic background. For any given contractile state and initial L, maximum dL/dt was not attained instantly but after a finite time of ejection (39 ms +/- 0.2 SE; range, 24-60) and from a lesser length (92% +/- 0.5 SE; range, 83-99) than present at end diastole. Beyond this initial period instantaneous dL/dt was dependent on both instantaneous force and length while independent of time after contraction and initial L. Instantaneous dL/dt also varied with contractile state, e.g., dL/dt was less after propranolol and greater after norepinephrine. Peak dL/dt for all conditions was a function of the extent of shortening (r = 0.90). Thus, the trajectory describing instantaneous force, velocity, and length provides a useful description of both the mechanical behavior and contractile state of the ventricular myocardium.