The spread to appearance time ratio in the estimation of left-to-right shunts

An evaluation of the Spread/AT ratio from earpiece dye-dilution curves in patients with left-to-right shunts is presented, and the simplest method of calculating the size of a shunt from one pulmonary artery curve, regardless of cardiac output, has been found. A significant correlation between this ratio and the Fick per cent left-to-right shunt has been demonstrated in 34 patients who had shunts over 20 per cent of total pulmonary flow. The inverse relationship between these two measurements, spread and appearance time, believed to be caused by the mixing or turbulence created by the shunt itself is discussed. The fact that a reasonable estimate of the size of the shunt, regardless of cardiac output, may be made quickly from one pulmonary artery earpiece curve emphasizes the practical significance of this simple method.