Studies from our laboratory have demonstrated the presence of a vagally mediated reflex in acute cardiac tamponade. To determine the effect of thisreflex on cardiac performance, we continuously recorded hemodynamic parameters during induction of tamponade in 16 closed-chest mongrel dogs anesthetized with alpha chloralose. The studies were done 1 to 7 days following the insertion of a catheter in the pericardial sac. With increasing pericardial volume, there was a progressive deterioration of cardiac performance, the most marked changes occuring in the late phases of cardiac tamponade...