Time to dP/dt max , a preload-independent index of contractility: open-chest dog study

A mathematical model of left ventricular pressure (LVP) during isovolumic contraction in the time domain shows the following predictions: 1) td, the time from onset of contraction to dP/dtmax and (dP/dt)/P, reflect only the time-dependent aspects of contraction, and are independent of preload; 2) dP/dtmax depends on both preload and the time-dependent aspects of contraction. To test preload independence we reduced filling volume (FV) by the method of ventricular volume clamps with a remote-controlled mitral valve in 7 anesthetized open-chest dogs. A decrease in FV of 80±15% produced a 29±12% (p<0.001) decrease in LVP, 34±13% (p<.001) decrease in dP/dtmax, 13±4% (p<0.001) decrease in t-dP/dtneg, and no change in td(−3±5%, NS). The heart rate (HR) dependence on td was assessed in other 5 anesthetized open-chest dogs. HR was changed with atrial pacing (50–240 bpm). td was linearly and inversely related to HR in each dog, and at each HR: dobutamine lowered and propranolol elevated this relation when compared to control (p<.001, both). Since dP/dtmax occurs usually before the opening of the aortic valve, td is, thus, also afterload-independent. Conclusion: This study supports the theoretical predictions that td is independent of preload and that it can serve, at any given HR, as a reliable index of contractility, provided that dP/dtmax occurs before the opening of the aortic valve.