THE COMMON aims of the various methods1-3 of assisted circulation now under laboratory or clinical assessment are temporary relief of the heart from a greater or lesser portion of its mechanical work while improving the perfusion of the rest of the body. Proof that the heart has been helped by such intervention is not always easily achieved. The evaluation of myocardial oxygen consumption as a direct function of the difference between the oxygen content of aortic and coronary sinus blood is a classical method which has enjoyed wide application in the experimental and diagnostic cardiovascular laboratory.4,5 However, other investigators have noted that oxygenated blood appears in the coronary sinus under certain circumstances. In 1947, Eckenhoff and coworkers4 observed that in animals in "circulatory collapse" coronary venous blood contained more oxygen than the mixed venous blood collected in the right ventricle. Catchpole and associates6 demonstrated striking increase