Stroke volume and the three phase cardiac output rate relationship with ventricular pacing

Knowledge of how stroke volume (SV), and hence cardiac output (CO), changes with ventricular pacing rate (R) constitutes a key aspect of sensor driven, variable rate pacemakers. It has been established that the relationship between CO and pacing rate exhibits three phases for rest and constant exercise. At low rates (phase 1), CO increases with increasing R; with additional rate increase (phase 2), CO either remains constant or increases slightly; and above some critical rate, CO decreases (phase 3). However, the nature of the relationship between SV and pacing rate has not been as clearly described. Therefore, the objectives of this study were (1) to describe and document the relationship between SV and R, and (2) to demonstrate the consequence of this relationship in terms of the three phase CO versus R relationship. In six anesthetized dogs, right ventricular SV was determined from pulmonary artery blood flow measured using an electromagnetic flow meter, and the right ventricle was paced over a range of rates. In general, SV decreased with increasing R, although the exact nature of the relationship varied from animal to animal. The results demonstrate that it is the manner in which SV decreases with increasing R that determines the three phase relationship between CO and R. The relationships described in this study have important implications for choosing pacing rates for patients receiving sensor driven, variable rate pacemakers.