TheForce-Interval Relationship oftheLeftVentricle

SUMMARY Theforce-interval relationship the dependence ofcardiac contractility on the rate andpat-tern ofstimulation has been shown to be independent of preload, but sensitive to the inotropic and disease state ofthe heart. Theforce-interval relationship was evaluated fortheleft ventricles of42 patients, ages 1.5- 20 years, during cardiac catheterization using a micromanometer pressure transducer-tipped catheter and an atrial pacing electrode catheter. Theleft ventricular (LV) minor axis was monitored echocardiographically,andthe end-diastolic dimension (EDD) and posterior wall thickness (PW) were measured. Hearts were paced at a variety ofbasic cycleintervals, to (1/heartrate), andtwotest stimuli were introducedatvarioustimesdur- ing a pause in the regular stimulation (test intervals t, andt2 were measuredrelativetothelast regularsystole before the pause). For t, < to andt2 2 tl + to Pma ofthesecond test systole was greaterthan Pma forthelast regularsystolebeforethe pause (equivalent