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

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Abstract

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·(Pmax* - P∞)] versus resistance effects [τc · (dP/dtmin)], defined by a set of load-varying IVPD contours, where Pmax* 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 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 Ek· (Pmax* - P∞) versus τc · (dP/dtmin) relation to be highly linear, with the average slope MLIIIVPD = 1.107 ± 0.044 and the average r2 = 0.993 ± 0.006. For all subjects, MLIIIVPD was found to be linearly correlated to the subject averaged τ (r2 = 0.65), τL(r2 = 0.50), and dP/dtmin (r2 = 0.63), as well as to ejection fraction (r2 = 0.52). We conclude that MLIIIVPD 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.

Original languageEnglish
Pages (from-to)H2551-H2559
JournalAmerican Journal of Physiology - Heart and Circulatory Physiology
Volume295
Issue number6
DOIs
StatePublished - Dec 2008

Keywords

  • Hemodynamics
  • Isovolumic relaxation
  • Mathematical modeling

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