The effects of atrial pacing on the synergy and hemodynamics of the orthotopically transplanted canine heart.

Instantaneous left ventricular dynamics were measured from cineradiographs utilizing implanted tantalum coils as myocardial site markers in the orthotopically transplanted canine heart. Four parameters of myocardial synergy were obtained: mean shortening, anisotropy of contraction, mean time to half shortening, and asynchrony of contraction. Estimates of end diastolic volume, end systolic volume, stroke volume, ejection fraction, and cardiac output were obtained. Heart rate was altered by pacing the left atrium of the donor heart. Of the nine parameters, only cardiac output was significantly changed by alterations in heart rate. The invariance of stroke volume with rate resulted in a proportional increase in cardiac output with cardioacceleration.