Determinants of instantaneous pressure in canine left ventricle. Time and volume specification.

Instantaneous pressure (P) in the canine left ventricle is determined predominantly by the simultaneous volume (V) of the ventricular lumen; that is, the instantaneous P-V relation is reasonably well approximated by a regression line of isochronous pressures from different beats on the simultaneous volume values.Strictly speaking, however, the P-V data points from differently loaded contractions slightly scatter around the regression line, indicating that V is not the sole determinant of P. To identify the secondary determinants of P at a given V, we forced the ventricle to have a fixed V at a specified time in the cardiac cycle while changing the end-diastolic volume and the velocity of ejection over a wide range under a constant inotropic background.We found that, at any specified time and V, P decreased with increases in the simultaneous velocity of ejection (-dV/dt, ml/ sec), the peak velocity of ejection (peak -dV/dt, ml/sec), and the volume ejected by the specified time (/ -dV, ml).Multiple regression analysis of the data yielded a formula: %P = 100 -0.14 (-dV/dt) -0.07 (peak -dV/dt) -0.54 (J -dV), in which %P denotes P of an ejecting beat in percent of the isovolumic P at the same time and V.All those partial regression coefficients are statistically significant (P < 0.005).The results indicate that the instantaneous ventricular pressure can be reduced from isovolumic pressure by 10-20% if the magnitude of those ejection parameters are near physiological maxima.