June 1, 1965Open Access
Myocardial Force-Velocity Relations Studied in Intact Unanesthetized Man*
Structured PICO
Does altering afterload or inotropic state change the myocardial force-velocity relationship in intact unanesthetized humans?
P
Population
13 conscious human subjects (7 men, 6 women), aged 16 to 55 years, who had previously undergone corrective cardiac operations (ASD closure, VSD closure, pulmonary valvulotomy, mitral commissurotomy, or valve replacement) during which silver-tantalum markers were sutured to their ventricles.
I
Intervention
Various physiological and pharmacological interventions including intravenous infusions of methoxamine, isoproterenol (average 2.5 µg/min), or norepinephrine (average 25 µg/min); impeding venous return via a balloon inflated in the inferior vena cava; and altering heart rate via electrical stimulation.
C
Comparator
Baseline control measurements obtained prior to each specific intervention.
O
Outcome
Instantaneous myocardial force-velocity relations (velocity of shortening and intraventricular pressure measured at the isolength point).surrogate
This classic physiological study demonstrated that the intact human ventricle adheres to the fundamental inverse relationship between force and velocity of shortening, and that positive inotropic agents shift this relationship to characterize the contractile state of the myocardium.
Abstract
In 1938, A. V. Hill demonstrated that the inverse relationship between the force generated and the velocity of shortening constitutes one of the most fundamental mechanical properties of skeletal muscle (1). More recently it has been shown that a similar reciprocal relationship be- tween force and velocity also exists in the isolated cat papillary muscle (2-5). Moreover, at any given muscle length, various positive inotropic interventions, such as augmentation of heart rate (2, 3), administration of norepinephrine (3, 4), Ca++ (3), or digitalis (6), shift the force-velocity relation so that the velocity of shortening is greater at any given load and the maximal velocity of the unloaded muscle is increased. This type of shift in the force-velocity relation is interpreted to reflect an augmentation of the contractile state of the myocardium. The ability to alter the re- lationship between the force generated and the velocity of shortening in heart muscle is in con- trast with the relative stability of the forcevelocity relation in skeletal muscle.