The ratio of maximum rate of isometric tension development to integrated isometric tension (dp/dt/IIT) serves as a quantitative, length-independent index of myocardial contractility.
The mechanics of isometric contraction in the heart were studied in both isolated papillary muscle and in an innervated, isovolumetric heart in situ. The maximum rate of development of isometric tension (dp/dt) and the integrated isometric tension (IIT), were studied on a beat-to-beat basis. The results indicate: 1) At a given heart rate, both the IIT as well as dp/dt vary directionally as changes in end diastolic volume and fiber length. During isometric contraction, the ratio dp/dt/IIT is independent of changes in fiber length, and remains a constant for any given state of contractility. 2) At a constant heart rate, and at constant volume, cardiac sympathetic nerve stimulation markedly increases dp/dt and IIT. The ratio dp/dt/IIT is elevated to a new constant which is also independent of fiber length. 3) Increasing heart rate increases the ratio dp/dt/IIT. This increase is, however, small compared to the increase in this ratio produced by sympathetic stimulation at the same rate. 4) The effects of heart rate and sympathetic stimulation are additive. 5) Differences have been noted between the manner in which norepinephrine, calcium, and acetyl strophanthidin alter the mechanics of the isovolumetric ventricle. This evaluation of the myocardial contraction is also uninfluenced by the adequacy of ventricular relaxation. 6) Comparison of the isovolumetric ventricle with isolated papillary muscle yielded directly analogous results, indicating that the properties observed were intrinsic to cardiac muscle and are not dependent upon structural arrangements in the intact heart. It is suggested that this ratio dp/dt/IIT is a quantitative measure of the contractility of the myocardium.
Siegel et al. (Sat,) studied this question.