Lung curve indicating a left-to-right shunt in an infant with a large heart

Abstract

A 1.5-mo-old boy had episodes of choking accompanied by becoming blue. There was a loud holosystolic murmer heard over the entire precordium. The chest film, Fig. 1, showed a large heart with increased pulmonary vascularity. A shunt study was performed by injecting a 2-mCi bolus of 99mTcDTPA via the left external jugular vein. Time activity histograms at halfsecond intervals, normalized to maximum counts for each histogram, are shown in Fig. 2 for the superior vena cava and left lung. In addition, the first portion of the original lung curve (B) has been extrapolated using a least squares fit of a gamma variate function (C). 1'2 The difference between this idealized curve and the original lung curve is shown in Fig. 2D. Finally, the gamma variate fit of this difference histogram is shown in Fig. 2E. The area under curve C represents the total circulation through the lung, Qp. The area under curve C (pulmonary flow) minus the area under curve E, the shunt flow, represents the systemic blood flow, Qs. The ratio of these areas (Area C) / (Area C Area E) gives the Qp:Qs ratio, which in this case was 1.9:1. It is essential for validity of the technique that a good bolus be achieved. This is shown in the superior vena cava curve in Fig. 2A. The causes for left-to-right shunt in an infant with a large heart are listed below. 3 5 This gamut roughly corresponds to the listing in Edwards et al. 5 of the groupings, Increased pulmonary arterial vasculature; cyanosis absent; (A) extracardiac left-to-right shunts, and (B) intracardiac left-to-right shunts. In this particular case, a brain scan performed because of a bruit heard in the head (Fig. 3) serves to remind us that uncommon extracardiac left-to-right shunts can mimic the more common entities responsible for this finding. 6 The scan shows a left-sided arteriovenous malformation, a portion of which drains to the deep system and causes aneurysmal dilatation of the vein of Galen. This aneurysmal dilatation of the vein of Galen should be separated from a primary aneurysm of the vein of Galen. The very large heart seen in a newborn infant should raise the possibility of a systemic arteriovenous shunt. The shunting in this case occurs with the circulation in fetal condition in utero without waiting for a fall in pulmonary vascular resistance. Thus, the heart has ample time to enlarge and fail by the time of birth.