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

摘要

In current practice, empirical parameters such as the monoexponential time constant τ or the logistic model time constant τL are used to quantitate isovolumic relaxation. Previous work indicates that τ and τL 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 (Ek), relaxation (τc), and pressure asymptote (P∞) parameters. In this study, we use this kinematic model to predict, derive, and validate the load-independent index MLIIIVPD. We predict that the plot of lumped recoil effects [Ek·(P*max − P∞)] versus resistance effects [τc·(dP/dtmin)], defined by a set of load-varying IVPD contours, where P*max is maximum pressure and dP/dtmin is the minimum first derivative of pressure, yields a linear relation with a constant (i.e., load independent) slope MLIIIVPD. 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 Ek·(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.