Muscle fiber length: a determinant of left ventricular contraction pattern

Abstract

The contractile pattern of the left ventricle during isovolumic contraction was studied by means of mercury-in-rubber length gauges sewed to the epicardium. All dogs were studied during thoracotomy, some having intact circulations and some being on complete cardio-pulmonary bypass with a fluid-filled ballon in the ventricle. The contractile pattern was found to be quite variable from animal to animal, but in every animal it was greatly influenced by ventricular volume. Activation time at various sites on the epicardium was much less influenced by volume changes as long as the ventricle was not overstretched. Beta adrenergic blockade did not negate the effect of volume on the contraction pattern but did modify the pattern at all volumes. From these data it is concluded that at any volume ventricular muscle fibers are not of uniform length and do not lie at the same point on their length-tension curves. This is a primary determinant of the contraction pattern. Also implied in this statement is a nonuniformity of contractility. These conclusions must be considered in assessing function of the left ventricle.