In current practice, empirical parameters such as the monoexponential time-constant or the logistic model time-constant _L, are used to quantitate isovolumic relaxation. Previous work indicates that and _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 parametrized by stiffness (E_k), relaxation (_c), and pressure asymptote (P) parameters. In this manuscript, 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)] vs. resistance effects [_c·(dP/dt_MIN)] defined by a set of load-varying IVPD contours yields a linear relation with a constant (i.e. load-independent) slope M_LIIIVPD. To validate load-independence we analyzed an average of 107 IVPD contours in 25 subjects (2669 beats total), undergoing diagnostic catheterization. For the group as a whole we found the E_k·(P^*_MAX-P) vs. _c·(dP/dt_MIN) relation to be highly linear, with average slope M_LIIIVPD=1.107±0.044 and average r²=0.993±0.006. For all subjects, M_LIIIVPD was found to be linearly correlated to subject averaged (r²= 0.65), _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 pathophysiologic settings is warranted.