Left ventricular asynergy during intracoronary isoproterenol infusion in dogs

Myocardial contractions were examined in the left ventricle of anesthetized, open-chest dogs during infusion of a beta-adrenergic agent, isoproterenol (0.1-0.5 micrograms/min) into a shunt line to the left anterior descending coronary artery. Myocardial chord lengths were continuously monitored by pairs of ultrasonic elements inserted into the isoproterenol-infused and control regions. Heart rate remained constant, but isoproterenol altered contraction patterns in both control and infused regions. Contraction in the infused region started before ejection and stretched the control myocardium in early systole. Because of early relaxation, however, the infusion region was stretched at the end of ejection and in early diastole, while the control myocardium continued to shorten. Thus, isoproterenol infusion to a part of the left ventricle induces asynergic muscle contractions and despite localized inotropic stimulation stroke volume may not be significantly increased.