Jon-Emile S. Kenny MD [@heart_lung] Background If venous pressure were always the downstream pressure of an artery, then when flow is zero, the arterial pressure should equal the venous pressure. For example, if one were to measure the coronary artery pressure and coronary sinus pressure at zero flow, their pressures should be equal. Yet, this has been done; when flow is zero in the heart’s vascular tree, an instantaneous measurement of arterial and venous pressure