HEART-RATE AND FORCE-FREQUENCY EFFECTS ON DIASTOLIC FUNCTION OF THE LEFT-VENTRICLE IN EXERCISING DOGS

Background Previous studies from our laboratory have shown pronounced augmentation of the force-frequency relation on myocardial contraction during exercise, but the influence of this effect on diastole has not been investigated. Methods and Results Accordingly, the effect of cha nging heart rate on left ventricular (LV) relaxation, filling dynamics , and pressure-volume relations during exercise was studied in eight c onscious dogs. The exercise heart rate was slowed from 208+/-21 (SD) t o 163+/-11 beats per minute by injection of a specific sinus node inhi bitor (UL-FS 49, or zatebradine, 0.6 mg/kg) during continuous exercise . Heart rate was then abruptly restored to the predrug level by atrial pacing during continued exercise. LV volume was calculated by use of implanted ultrasonic crystals, and LV pressure was determined with an implanted micromanometer. Comparing conditions after pacing back to a heart rate of 210 beats per minute with those obtained when the heart rate was slowed by atrial pacing, LV dP/dt(max) was increased by 27% a t the higher rate (P<.01), despite a marked decrease in LV end-diastol ic pressure (24+/-4 versus 10+/-5 mm Hg, P<.01), and the time constant of isovolumic LV pressure decay (tau) was significantly shortened (19 +/-5 versus 14+/-4 milliseconds, P<.01). The peak rapid filling rate i n early diastole (PFR) was not significantly changed by increasing the heart rate, since it was maintained at the slower rate. During exerci se, at the slowed heart rate the early portion of the diastolic pressu re-volume curve was significantly shifted upward and to the right comp ared with that at the physiological heart rate, but the late portion o f the curve was unchanged. Conclusions These data indicate that the ne gative inotropic effect of the force-frequency relation when heart rat e was slowed during exercise caused pronounced impairment of LV relaxa tion and early-filling dynamics. Conversely, an important component of the pronounced improvement of diastolic ventricular function during n ormal exercise was shown to result from exercise-induced enhancement o f the positive inotropic effects of the force-frequency relation on my ocardial contraction and relaxation.