Use of krypton85 for the measurement of cardiac output by the single-injection indicator-dilution technique.

W HEN radioactive krypton (Kr 85), a beta-emitting radioisotope, is introduced into the circulation, it does not act as a simple intravaseular indicator. As this substance, in solution in the blood stream, traverses the systemic capillary bed, it diffuses freely into the interstitial and intra-cellular spaces. When blood containing Kr 85 passes through the pulmonary capillary bed, approximately 95 per cent of this isotope crosses the alveolar-capillary membrane, and this fraction is eliminated from the circulation. 1 These properties of Kr 85 , as well as the ease with which its concentration can be determined in blood and in air, have made it useful for the measurement of cerebral blood flow, 2 for the characterization of circulatory shunts, 3-4 and in the study of intrapulmonary ventilation-perfusion relationships. 5 It has been demonstrated in previous investigations in this laboratory that the fraction of systemic venous blood which bypasses the lungs in patients with right-to-left cardiac shunts can be measured by means of the Stew-art-Hamilton formula by determining the radioactivity in systemic arterial blood following the injection of Kr 85 into a peripheral vein or the right side of the heart. 4 These observations suggested the use of a similar approach for the measurement of the cardiac output. The present communication details the experimental validation and the initial clinical applications of this technique. Theory When Kr 85 is injected into the left side of the heart and blood is sampled from a systemic artery, or when it is injected into a peripheral vein and blood is sampled from the pulmonary artery, only the primary time-concentration curve is obtained. Recirculation, for all practical purposes, is eliminated by the diffusion of the Kr 85 into the extra-vascular space and its loss from the pulmonary capillary bed. Determination of the radioactivity in a single blood sample, withdrawn at a constant rate, permits calculation of the average concentration of Kr 85 during the dilution curve. Since neither the appearance nor the disappearance times are known exactly, a sampling period which is certain to bracket the entire primary curve must be selected. If the amount of indicator injected, its mean concentration during the sampling period, and the exact duration of this period are all known, the Stewart-Hamilton formula may be applied for the measurement of ventricular output. Example: (1) Radioactivity injected: 1 X 10 B counts per minute (c.p.m.)/ml., 3 ml. (2) Mean concentration during sampling period: …