Inotropic effect of post-stimulation potentiation in man: An echocardiographic study

To determine the magnitude of enhancement of the inotropic state in the intact human left ventricle, echocardiographic studies were performed in 11 subjects during pacing-induced changes in heart rate. Incremental atrial or ventricular pacing (80 to 160 beats/min) was performed for 1 minute at each rate, and echocardiographic variables of left ventricular function and indexes of left ventricular thickening were assessed for the post-stimulation beats and compared with the values obtained during sinus rhythm before pacing. For the first post-stimulation beat at maximal paced rates (146 ± 3 beats/min, mean ± standard error of mean), the ejection fraction (+23 percent P < 0.001), percent fractional shortening (+45 percent, P < 0.001) and mean velocity of circumferential fiber shortening (+45 percent, P < 0.001) increased significantly, whereas the end-systolic dimension (-22.4 percent, P < 0.001) and volume (- 52 percent, P < 0.001) decreased significantly. The end-diastolic dimension (+1.42 percent) and volume (+4.8 percent) did not change significantly. Similarly, septal thickening (+21.89 percent, P < 0.005), posterior wall thickening (+23.4 percent, P < 0.001), left ventricular thickening (+21.91 percent, P < 0.001) and the mean rate of left ventricular thickening (+33 percent, P < 0.001) increased significantly. Similar significant changes, although of a lesser magnitude, occurred at lower paced rates; the magnitude of these changes was proportional to the pacing rate. Decay in potentiation occurred within four to five beats after pacing was stopped at any given rate. Thus, in the intact human heart (1) the positive inotropic effects of stimulation on the left ventricle are reflected as post-stimulation potentiation, (2) potentiation is related to the pacing rate, and (3) decay in potentiation occurs rapidly when pacing is stopped.