Pressure surrounding the heart (PSH) rises with maneuvers that increase pleural pressure. This may decrease left ventricular (LV) oxygen demand by reducing LV afterload. However, positive PSH may also directly impede coronary flow. To study the effects of positive PSH on coronary perfusion, PSH was increased in 10-mmHg increments from 0 to 60 mmHg in an isolated canine heart-lung preparation with constant venous return, arterial pressure, and lung volume. Increased PSH caused a rapid significant (P less than 0.001) fall in left atrial transmural pressure (PLATM) of up to 1.28 +/- 0.31 mmHg. With constant venous return and lung volume, this was interpreted to reflect decreased LV afterload. However, at levels of PSH greater than 30 mmHg, initial decreases in PLATM were followed by sustained increases, suggesting that there was a deterioration in cardiac function despite the lower level of afterload. Increased PSH was also associated with decreases in circumflex coronary artery flow [flow (ml/min) = 52.4 - 0.4PSH, P less than 0.01]. Moreover, when the circumflex coronary artery was maximally dilated with adenosine, the effects of PSH were amplified [flow (ml/min) = 137.9 - 1.78PSH, P less than 0.001], indicating that positive PSH mechanically impeded coronary flow. When PSH was raised to 60 mmHg for 90 s, the aortic-coronary sinus lactate concentration difference fell from 0.71 +/- 0.09 to 0.10 +/- 0.21 mM (mean +/- SE, P less than 0.001, n = 8), suggesting myocardial ischemia. We conclude that positive PSH directly decreases myocardial perfusion. This may lead to ischemic cardiac dysfunction, especially in patients with low arterial pressure or coronary artery disease.