Background: The isovolumic non-ejecting left ventricle (LV) is functionally analogous to the isometric myocardium and the curve of the time course of left ventricular isovolumic pressure, P(t), resembles the curve of the time course of myocardial isometric force, F(t). In this study therefore, we first tested the hypothesis that the same general empirical model that fit F(t) and L(t), the isotonic shortening curve, will also fit P(t) by evaluating the model P(t) = P0 + C(t/A)B exp[1-(t/A)]B in 8 Suga-Sagawa isolated cross-circulated canine left ventricles. P0 is the end-diastolic pressure; A, B, and C are parameters. We went further to compare the time course of P(t) to those of F(t) and L(t) exploiting their common basic analytical models not only to characterize similarities but also to highlight differences between them.
Methods: To obtain curves of P(t), we inserted a latex balloon in the LV and injected water into it and recorded the isovolumic pressure curve, P(t), in each of 8 ventricles. Eight in situ right ventricular papillary muscles of the isolated canine heart were also studied with a servo system that clamped length or force to produce F(t) or L(t). Each curve, P(t), F(t), or L(t) was fit with the above model by Marquardt's algorithm and A, B, and C determined.
Results: The model fit the curves closely, the coefficient of determination being 0.995 +/- 0.003 for P(t); 0.994 +/- 0.003 for F(t), and 0.990 +/- 0.004 for L(t). The common model allowed direct quantitative comparison of the time course of either myocardial isometric or isotonic dynamics with the time course of left ventricular isovolumic dynamics by comparing the ventricular parameters A and B, which determine details of time course, with their corresponding myocardial counterparts. Under basal (i.e. control) conditions, A and B averaged 0.194 +/- 0.015 and 2.09 +/- 0.065 respectively for P(t). Mean A for isotonic shortening was 0.219 +/- 0.022 (p < 0.02 compared to mean A for P(t)). Mean B for F(t) was 1.87 +/- 0.100 (p < 0.001 compared to mean B for P(t)). The values of A for P(t) and F(t) were not different and the values of B for P(t) and L(t) were closely similar.
Conclusion: The same empirical model that fits F(t) and L(t) also fits P(t). Comparison of myocardial to ventricular curves showed that during contraction, P(t) corresponded closely to F(t) and L(t). However during relaxation, P(t) led both F(t) and L(t) both of which pursued a similar time course. This suggests that myocardial contraction may be isometric during LV isovolumic contraction and it may be valid to extrapolate information about myocardial isometric contraction directly to left ventricular isovolumic contraction and vice versa. On the other hand during left ventricular isovolumic relaxation, myocardial dynamics may well involve some length changes. It is difficult, however, to reconcile such presumed length changes to the putative isometricity of myocardial function during contraction.