An Official Journal of the American Heart Association Effects of Alterations in Aortic Impedance on the Performance of the Ventricles

• The relationship between the external work done in an isotonic muscular contraction and the load has been extensively investigated for isolated skeletal muscle and, more recently, in isolated strips of heart muscle.1- These studies have shown that, for contractions from a constant initial length and tension, the relationships between load and external work or power describe a parabola which is rather more symmetrical for cardiac than for skeletal muscle. Comparable studies relating external work to load have been done on various isola-ted heart preparations.3"5 In none of these in-vestigations so far has it been possible to de-fine the point or points on the parabola at which the ventricle normally functions. General conclusions in all these studies have been similar. Nevertheless the applicability of these data to the normal heart remains un-certain. In studies with isolated hearts or in open chest dogs under general anesthesia, ven-tricular volumes, stroke volumes, and pres-sures may be abnormal absolutely and rela-tive to each other;6> 7 environmental factors like temperature, electrolyte concentrations, and hormonal supply may differ from normal; and the important controls exerted by th