Stiffness and relaxation components of the exponential and logistic time constants may be used to derive a load-independent index of isovolumic pressure decay

In current practice, empirical parameters such as the monoexponential time constant τ or the logistic model time constant τ_L are used to quantitate isovolumic relaxation. Previous work indicates that τ and τ_L are load dependent. A load-independent index of isovolumic pressure decline (LIIIVPD) does not exist. In this study, we derive and validate a LIIIVPD. Recently, we have derived and validated a kinematic model of isovolumic pressure decay (IVPD), where IVPD is accurately predicted by the solution to an equation of motion parameterized by stiffness (E_k), relaxation (τ_c), and pressure asymptote (P∞) parameters. In this study, we use this kinematic model to predict, derive, and validate the load-independent index M_LIIIVPD. We predict that the plot of lumped recoil effects [E_k·(P_max* - P∞)] versus resistance effects [τ_c · (dP/dt_min)], defined by a set of load-varying IVPD contours, where P_max* is maximum pressure and dP/dt_min is the minimum first derivative of pressure, yields a linear relation with a constant (i.e., load independent) slope M _LIIIVPD. To validate the load independence, we analyzed an average of 107 IVPD contours in 25 subjects (2,669 beats total) undergoing diagnostic catheterization. For the group as a whole, we found the E_k· (P_max* - P∞) versus τ_c · (dP/dt_min) relation to be highly linear, with the average slope M_LIIIVPD = 1.107 ± 0.044 and the average r² = 0.993 ± 0.006. For all subjects, M_LIIIVPD was found to be linearly correlated to the subject averaged τ (r² = 0.65), τ_L(r² = 0.50), and dP/dt_min (r² = 0.63), as well as to ejection fraction (r² = 0.52). We conclude that M_LIIIVPD is a LIIIVPD because it is load independent and correlates with conventional IVPD parameters. Further validation of M _LIIIVPD in selected pathophysiological settings is warranted.