Systolic time intervals: assessment by isolated cardiac muscle studies

To document the independent effects of acute changes in preload, afterload and inotropic state on the systolic time intervals, 10 isolated rat left ventricular muscle preparations were studied. Experiments were performed using physiologically sequenced contractions that simulate the loading conditions of the intact left ventricle. The preshortening period was measured from the time of the electrical stimulus to the onset of muscle shortening, and the isotonic contraction time was measured as the duration of shortening. These variables are analogous to the preejection period and the left ventricular ejection time in the intact heart. It was found that an isolated increase in preload shortened the preshortening period and prolonged the isotonic contraction time, whereas an increase in afterload prolonged the former and shortened the latter. Isoproterenol shortened both the preshortening period and the isotonic contraction time, while an increase in calcium shortened the preshortening period and lengthened the isotonic contraction time. All changes were significant (p less than 0.01) by analysis of variance. Thus, the similar dependence of preshortening period, isotonic contraction time and clinical systolic time intervals on changes in preload, afterload and inotropic state supports the derivation of systolic time intervals from fundamental principles of myocardial mechanics. These data provide an improved basis for the rational interpretation of systolic time intervals in patients with and without heart disease.