SINCE Clark and Bargeron** introduced the hydrogen-electrode principle for detection of intracardiac shunts, this method has found widespread application.We have recently utilized a 100-cm. single-lumen No.6 Cournand-type cardiac catheter with three radiopaque platinum electrodes (Fig. 1 and 2) to facilitate the determination of the site of a left-to-right shunt. The distal electrode is 1 mm. from the tip of the catheter; the second electrode is 6 cm. proximal, and the third electrode is 16 cm. proximal to the tip. These electrodes are connected by insulated platinum wire woven into the lining of the catheter to three 1-cm. copper sleeves . . .