Electromechanical delay in the intact dog heart

A computer method was developed for the determination of electromechanical delay defined as the time between the onset of Q-wave and the onset of the left ventricular systolic pressure rise. It was validated for heart catheterization studies on 56 intact anaesthetized beagle dogs in 86 sessions. The mean basal value of the electromechanical delay was 22 ± 4 msec. Heart rate, contractility, preload and afterload were changed by atrial pacing and by infusions of calcium chloride, isoproterenol, propranolol, dextran and phenylephrine. Increase of heart rate by pacing from the spontaneous rate of 90 per min to 240 per min prolonged the electromechanical delay from 21 ± 5 to 33 ± 14 msec ( P < 0.001). Otherwise the duration of electromechanical delay changed independently of the heart rate. If it changed, the direction of the change followed that of the pre-ejection period. Its proportion of the pre-ejection period varied from 26 to 52%. The electromechanical delay shortened when a positive inotropic effect was noticed or the presystolic fibre length increased.