A simple and accurate test for left-to-right cardiac shunts.

Abstract

One of the drawbacks of conventional oxygen determination as used in right heart catheterization is lack of sensitivity and accuracy. Small left-to-right shunts are missed, and in many cases the values obtained can be interpreted only in the light of the clinical findings. We have developed a new and practical test which we believe to be safe, simple, and more accurate than any other method, including conventional dye dilution. Basic Principle: A small amount of radioactive methyl iodide is introduced into the left side of the heart by inhalation, i.e., lung, pulmonary veins, left atrium, left ventricle, and aorta (1). In the presence of a left-to-right shunt, radioactive blood is sampled almost immediately after inhalation (one to three seconds) through a catheter placed in the right side of the heart. The amount of radioactivity in the blood samples is very great, because part of the highly radioactive “bolus” enters the right heart in relatively high concentration. In the absence of a shunt there is a gradual slow increase of activity commencing after seven to twelve seconds through normal venous return (Fig. 1). The activity in the venous blood is relatively low, because the radioactive methyl iodide has been diluted in many liters of blood, and part has diffused into extravascular spaces. This diffusion has been observed by other investigators using this substance for inhalation radiocardiography (2). Localization of Shunt: A catheter is placed in the pulmonary artery, and the first inhalation is made. If early blood samples are inactive, a shunt is excluded and no further samples are necessary. If there is evidence of early appearance of activity, two additional inhalations are made with the catheter in the right ventricle and then in the right atrium, in order to localize the site of the shunt. Method: We have developed a very simple method of producing radioactive methyl iodide which will enable any hospital to have its own “plant.”4 Ten millicuries of sodium iodide and 20 c.c. of distilled water are introduced through a rubber stopper into an empty 100-c.c. saline bottle. One drop of stable methyl iodide is added. The next day radioactive gas which has an activity of 25 microcuries per cubic centimeter can be withdrawn into stainless steel tubing for injection. A rubber or plastic catheter is introduced into the patient's posterior nasopharynx and the head is placed under a plastic hood, which is being continuously evacuated by a suction pump. The air is drawn through an activated carbon filter which traps the exhaled and unabsorbed methyl iodide. A special stainless steel tubing in which the radioactive methyl iodide is stored is connected to the nasal catheter, and 5 to 10 microcuries are injected at the beginning of inspiration. Simultaneously, interrupted blood samples are taken through the cardiac catheter with a specially constructed sampler at a rate of one sample every two seconds (Fig. 2).