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Contribution of Recirculation and Fat Diffusion to Myocardial Washout Curves Obtained by External Counting in Man: Stochastic Versus Monoexponential Analysis

133Xenon and 125I-iodoantipyrine (IAP) solutions were simultaneously injected into the left coronary artery in four normal patients and four patients with coronary artery obstructions. Recirculation was computed by convolution of the fractional decrease of the curve following inlet injection over a second curve obtained following right atrial injection. Uncorrected xenon and IAP curves were not statistically different in spite of the lower myocardium-blood partition coefficient and the recirculation of xenon. Corrected IAP curves could be followed down to 1.7%, whereas xenon curves could be followed only to 7.5%, indicating significant xenon diffusion holdup in fat. Corrected IAP curves deviated from a monoexponential course below 5% of peak in normal patients and at about 10% in patients with coronary artery obstructions. Stochastic analysis gave flow values 22% lower and 45% higher, respectively, than those computed from monoexponential extrapolation of corrected IAP and uncorrected xenon curves. With the double-injection technique, IAP washout can be calculated down to about 1% of peak, accounting for the distribution of 99% of the flow through the tissue to which the indicator is delivered. Monoexponential analysis of corrected IAP curves overestimates flow, because it cannot fit the initial delay component and the final reduction of slope. Xenon washout does not allow quantification of myocardial flow in man.