Effects of right ventricular pacing rate on the pattern of left ventricular contraction and filing as assessed by radionuclide ventriculography.

Changes in the pattern of the global left ventricular (LV) time-activity curve with increasing pacing rates were evaluated using radionuclide ventriculography in 15 patients with chronically implanted right ventricular VVI pacemakers. The variables obtained were correlated with the conventional systolic time intervals recorded immediately after radionuclide ventriculography in 8 of the 15 patients. Increasing the pacing rate from 50 to 110 beats per minute (bpm) altered the pattern and timing of LV contraction and filling dramatically. It produced progressive delay and reduction of LV contraction, thus shortening the LV diastolic filling time, and sometimes resulting in LV filing even during early systole. Despite these profound alterations, relative cardiac output increased progressively, peaking at a pacing rate of 110 bpm in most patients. In paced patients, filling parameters and diastolic timing intervals require careful attention, since they may be difficult to determine from the altered LV time-activity curve at higher pacing rates.