Catheterization of the heart, originally performed by Forssmann,1 its physiologic application perfected by Klein,2 and utilized as an important diagnostic method by Cournand and Ranges,3 Bing,4 and Dexter,5 has made possible a new era in the diagnosis and surgical alleviation of a variety of cardiovascular lesions, both congenital and acquired. The usual complications of this procedure consist of auricular and ventricular extrasystoles or more sustained arrhythmias, and their frequency of occurrence is in direct proportion to the amount of probing carried out in manipulating the catheter and to the excitability of the myocardium. Other complications include conduction defects, syncopal episodes, air embolism, venospasm, pyrogenic reactions, and trauma to the endocardium. The latter, a frequent complication in animals,6 is rare in man. Since the procedure entails some risk to the patient, a committee was established by the Scientific Council of the American Heart Association 7 in order to evaluate the risk