Some Clinical Aspects of Isotope Circulation Studies

Rapidly advancing technics in the surgical repair of congenital cardiac lesions have made precise preoperative diagnosis a matter of great importance. The site of the defect and its hemodynamic behavior must be determined with accuracy, since operative approaches and bypass procedures vary widely with lesions in different locations. Diagnosis depends on close cooperation between the clinician, radiologist, and physiologist. Each of these must have information from the others in order to evaluate his findings correctly and completely, and each must have some general knowledge of all three fields. Newer knowledge of the normal variations and pathologic lesions provided by the surgeon and pathologist, combined with advances in radiologic technics, have greatly increased the importance of the radiologist as a member of the diagnostic team. The present authors have developed a method for aid in diagnosis which is in the realm of the radiologist, employing radioisotopes as a physiologic tool for the detection and location of intra- and extracardiac shunts (1, 2). Left-to-right shunts cau usually be detected by use of the cardiac catheter and determinaticn of the oxygen content of blood samples withdrawn from the various chambers and vessels. Variations in pressure will reveal stenotic or obstructed sites. Right-to-left shunts are, however, considerably more difficult to pinpoint. Dye dilution with the use of Evans blue, or more recently Fox green, is a reliable and precise procedure. For most accurate results, however, arterial puncture and the placing of an indwelling arterial catheter are required (3). This procedure is fairly easy to accomplish in the adult, but is difficult in the infant and young child, and at the University of Minnesota Hospitals a large percentage of the patients seen for catheterization are under five years of age. Also, since our catheterizations are done under local anesthesia, we are interested in producing the least trauma to the patient and avoiding the changes in circulatory dynamics that occur secondary to struggling and crying. An additional drawback to dye dilution methods consists of the need for highly trained personnel to insure accurate results. Selective angiocardiography, with injection at the time of catheterization, is being used with greater frequency as rapid film-changing devices and cine-equipment become more reliable. Small shunts, however, may be missed due to rapid dilution of the opaque material by non-opacified blood as it traverses the shunt. The volume required for good visualization frequently may be such that unusual pressure is produced in the injected chamber, and a misleading right-to-left shunt may be produced where one does not actually exist in the physiologic state. In addition, although significant advances have been made in increasing the safety of contrast media, there is still some inherent danger in the injection of large amounts of iodinated compounds.