Quantitation of Left-to-right Cardiac Shunts with Radionuclide Angiography

Abstract

A quantitative radionuclide angiogram technique using an intravenous injection of technetium-99m (Tc-99m) pertechnetate is described for quantitation of left-to-right shunts in the central circulation. Data analysis assumes exponential indicator clearance from normal cardiac chambers by dilution. A late prolongation of tracer disappearance compared to the initial clearance rate indicates an abnormally early return of indicator to the cardiac chamber. Patients with left-to-right shunts demonstrate prolonged clearance of radioactivity from all cardiac chambers distal to the site of shunt. The magnitude of curve distortion is quantitatively related to the size of shunt, and counts recorded from the right lung are used for shunt quantitation. Quadratic regression analysis of data from 21 patients with left-to-right shunts and eight normal subjects demonstrated a correlation coefficient of 0.951 between measurements of left-to-right shunts by the radionuclide and by the Fick method. Observations on 14 patients with aortic or mitral valve disease revealed this method of shunt evaluation to be of limited value in the presence of severe cardiac valvular disease. However, the method permits detection of left-to-right shunts that are too small to be diagnosed by the Fick method and accurately quantitates shunts which are not at either extreme in size and flow. This technique appears to be potentially useful for the initial diagnosis and serial evaluation of patients with these disorders.