Effect of pacemaker sites on contractile forces of the local myocardium and blood flow in the major branches of the left coronary artery in anesthetized open-chest dogs.

Decreased cardiac performance during ventricular tachycardia or ventricular pacing has been ascribed to the lack of synchrony due to altered ventricular contraction. However, the direct effect of the pacemaking site on blood flow in the major coronary arterial branches and on regional contractile forces has not been adequately examined. In this investigation, twelve dogs were used to assess coronary flow in the left anterior descending (LAD) and circumflex coronary arteries (LCx) and the regional myocardial function in response to ectopic electrical pacing. The sites were the outflow tract of the right ventricle (RV pacing), the anterior left ventricular wall supplied by the LAD (LAD pacing), and the lateral wall perfused by the LCx (LCx pacing). Strain gauge arches were transmurally fixed to myocardial segments of the anterior and lateral walls of the left ventricle. The mechanical forces generated were measured while simultaneously recording blood flow in the LAD and LCx. RV pacing diminished developed-tension similarly in both areas. LAD flow was further decreased during anterior pacing as was the developed-tension in the LAD supplied area. A similar relationship was observed between LCx flow and developed-tension in the LCx area during lateral pacing. Compared to RV pacing, anterior pacing did not affect LCx flow and local myocardial force in the lateral wall of the left ventricle, and lateral pacing did not cause further changes in LAD flow and anterior myocardial function. The decrements in blood flow were closely related to decreased myocardial function (p less than 0.01). These findings indicate that ventricular pacing decreases ventricular performance with attenuated regional myocardial function at the pacing area, in addition to causing altered ventricular contraction sequence.