Influence of acute myocardial depression on left ventricular stiffness and its elastic and viscous components.

Abstract

A B S T R A C T The influence of acute myocardial depression on ventricular stiffness and on its elastic and viscous components was studied in 19 dogs. After the animals were placed on cardiopulmonary bypass, stiffness was measured by sinusoidally injecting volume changes of 0.5 ml (AV) at 22 Hz into paced, isovolumically contracting left ventricles and determining the amplitude (AP) of the sinusoidal pressure response. Stiffness was linearly related to pressure (P) throughout the cardiac cycle, so that AP/AV = aP + A, where a and i are constants. Myocardial depression was induced in one of three different ways: by coronary artery ligation, by administration of propranolol (Inderal), or by administration of pentobarbital. All three interventions caused significant increases in the slope, a, of the stiffness-pressure relationship, while the intercept, A, remained unchanged. Release of the coronary occlusion or administration of acetylstrophanthidin partially reversed depression and the change in a. Approximation of the mechanical nature of the left ventricle in terms of a linear second-order mechanical system permitted the division of stiffness into its elastic and viscous components. Like total stiffness, both the elastic and the viscous components were linearly related to ventricular pressure. Elastic stiffness was not changed, but the slope of the line relating viscous stiffness to pressure was significantly increased during ischemic depression, Dr. Wildenthal holds a Research Career Development Award (HL 70125)

Prolonged contraction duration in aged myocardium.

“…But such a muscle would generate less active tension and active tension does not decerase with age. In addition, it has been demonstrated that although stiffness increases during hypoxia it decreases towards baseline without an overshoot during reoxygenation (25,26). Therefore, the overshoot prolongation of CD during reoxygenation cannot be explained by a change in stiffness properties.…”

Demonstration of free radical generation in the "stunned" myocardium in the conscious dog and identification of major differences between conscious and open-chest dogs.

“…To our knowledge, generation of free radicals in postischemic myocardium in conscious animals has never been demonstrated. Several investigators (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) have pointed out that observations in open-chest models are confounded by the effects of anesthesia, surgical trauma, abnormal hemodynamic conditions, excessive levels of circulating catecholamines, and other factors. Certain conclusions derived from open-chest preparations have subsequently been found not to be applicable to conscious animals (9,10,15,17,18,20).…”

Factors which affect the diastolic pressure-volume curve.

“…However, many investigators have examined the effects of hypoxia or anoxia on the resting properties of isolated papillary muscle In addition to these studies of isolated muscle, experiments on dogs do not support the view that heart muscle reversibly stiffens during transient ischemia. Templeton et al 55 produced regional ischemia for 1 hour in isovol- umically contracting dog hearts by ligating the proximal left anterior descending coronary artery and found that ischemia did not affect the ventricle's passive elasticity. Palacious et al 5(i reached the same conclusion using a different preparation.…”