Systolic ventricular interactions may be partially responsible for right ventricular failure that sometimes occurs during clinical use of prosthetic left ventricular assist devices. In this hypothesis, it is proposed that the left ventricular assist device reduces left ventricular pressure and its contribution to right ventricular performance, thus impairing right ventricular output. On the other hand, these effects may be small compared with other causes of right ventricular failure such as ischemia. To test the systolic interaction hypothesis in the normal and ischemic right ventricle, we used a left ventricular assist device to pressure unload the left ventricle of anesthetized pigs, and we compared its effect on right heart function before and after 2 minutes of acute right coronary artery occlusion as a model of right heart failure. Pigs were instrumented for measurements of septal to left ventricular and right ventricular free wall dimensions with ultrasonic crystals, ventricular chamber pressures, and cardiac output with a pulmonary artery blood flow probe. Without right ventricular ischemia, the left ventricular assist device produced an 80% +/- 6% reduction in left ventricular pressure-time integral while maintaining aortic pressure. This resulted in a leftward septal shift with an 11.6% +/- 1.8% decrease in left ventricular septal-to-free wall dimension and a 12.5% +/- 2.4% increase in right ventricular septal-to-free wall dimension, with no changes in right ventricular cardiac output or stroke work. In contrast, right coronary artery occlusion alone produced right heart failure, with a 50% +/- 6% reduction in right ventricular global stroke work and 26% +/- 6% and 27% +/- 3% reductions in cardiac output and right ventricular peak systolic pressure, respectively. This right heart failure persisted during left ventricular unloading with the left ventricular assist device, which resulted in further leftward septal shifting and unchanged but still depressed stroke work and flow output. These findings support the hypothesis that a preexisting pathologic condition is the dominant factor in determining right ventricular function during prosthetic left ventricular support and that direct anatomic interactions play a minor role.