Effects of arterial input impedance on mean ventricular pressure-flow relation

The mean left ventricular pressure-flow relationship [P̄(v)-F̄(v)], determined under a constant preload and variable peripheral resistance, has been proposed as a quantitative representation of ventricular pump function. We determined the P̄(v)-F̄(v) relation in seven isolated cross-perfused canine hearts by varying resistance of a simulated arterial load in five steps from 6.0 to 0.375 mm Hg·s·ml^-1 while keeping end-diastolic volume, inotropic state, compliance, and characteristic impedance at various constant values. All of the 27 P̄(v)-F̄(v) relations thus determined were moderately nonlinear. Varying end-diastolic volume at three levels shifted the relation curve in an approximately parallel fashion (P < 0.0001). At three levels of inotropic state (mean LVP of isovolumic contractions 34.3 ± 8.2, 48.0 ± 6.3, and 59.2 ± 9.6 mm Hg), the P̄(v)-F̄(v) relation shifted with predominantly a slope change (P < 0.0001). Changing compliance at three levels (0.2, 0.4, and 0.8 ml/mm Hg) caused a statistically significant but quantitatively small crossover of the P̄(v)-F̄(v) curves (P < 0.001). Changing characteristic impedance to 0.1, 0.2, and 0.4 mm Hg·s·ml^-1 caused a highly significant (P < 0.0001) divergence of P̄(v)-F̄(v) relation over the high F̄(v) range. We conclude that this sensitivity of the P̄(v)-F̄(v) relation to characteristic impedance limits its use as a contractility index.