We now proceed to consider one of the basic elements of most of the proposed coronary venous interventions, namely the relatively brief and continuous, or else more persistent obstruction (or occlusion) of the coronary veins, most often the coronary sinus or the great cardiac vein. As already indicated, the ostium of the coronary sinus drains a major portion of the left ventricular myocardial blood. This drainage occurs primarily in systole, largely by virtue of cardiac contraction, essentially propelling into the coronary veins the myocardial blood volume accumulated during a prior diastolic arterial input. By virtue of very significant shunting, coronary vein occlusion actually entails lower occluded pressures than would otherwise be expected. Among other anatomic specifics to be recalled are the frequent communications between the coronary sinus vessel and the anterior cardiac veins which drain the right coronary artery perfused territory, and also the Thebesian channels which, in circumstances of major coronary vein occlusion, can be mobilized to provide substantial venous drainage directly into the cardiac chambers. Species differences need to be considered, and in a recent study in sheep (extensive Thebesian system) elevated coronary sinus pressure had little effect [1], as compared to significant changes found in dogs. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.