The slope and diameter axis intercept (D0) of the linear indexes end-systolic pressure-diameter relation (ESPDR), maximum of the first derivative of left ventricular pressure (dP/dtmax)-end-diastolic diameter relation (dP/dtmax-Ded), and dimensional preload-recruitable stroke work relation (PRDSW) are used to describe left ventricular performance. We tested the hypothesis that nonlinearity in these indexes would preclude accurate estimation of slope and D0. In nine pentobarbital-anesthetized dogs, right heart bypass was used to obtain a wide range of pressure-minor axis diameter (sonomicrometry) points from which the three indexes were derived. For ESPDR and dP/dtmax-Ded, a nonlinear fit (y = ax2 + bx + c) approximated the data better than a linear fit, with significant nonlinearity toward the diameter axis (a = -10.28 +/- 3.42 and -111.2 +/- 26.2, respectively, P less than 0.05). Although linear D0 was significantly less than nonlinear D0, this difference was overcome by the diameter intercept at a midrange value of end-systolic pressure or dP/dtmax. PRDSW demonstrated no significant nonlinearity (a = -4.40 +/- 3.53, P = 0.86) but extrapolation to D0 demonstrated linear and nonlinear differences. We conclude that 1) ESPDR and dP/dtmax-Ded demonstrate significant nonlinearity, while PRDSW is well-approximated by a linear fit over a large range of data points; and 2) extrapolation of D0 is inaccurate in all three indexes, while a midrange intercept is independent of the model used to fit the data. Left ventricular performance may be more accurately described by linear slope and midrange diameter intercept over comparable ranges of data.