Purpose or Case Report: Infants with one functional cardiac ventricle undergo operations that yield unobstructed systemic outflow and a controlled source of pulmonary blood flow, often starting with the Norwood operation with Blalock-Taussig (BT) shunt. Infants with shunt dependent pulmonary blood flow are at risk for developing proximal pulmonary artery (PA) stenoses, which may result in morbidity and mortality. Echocardiography (TTE) is the primary means of surveillance for PA narrowing, but is limited by acoustic windows, operator experience and patient cooperation. Computed tomography (CT) offers high spatial resolution, rapid acquisition and relatively low radiation exposure. To date, there have been no studies comparing TTE and CT-derived measurements of the proximal PA anatomy in this high-risk population.
Methods & Materials: A retrospective chart review identified infants with shunt dependent pulmonary blood flow that had both TTE and CT performed less than 14 days apart between 1/1/2010 and 9/15/2015. TTE images were reviewed to measure proximal right (RPA) and left PA (LPA) diameters. CT measurements of the proximal RPA and LPA were made using multi-planar reconstruction. Statistics included calculation of PA Z-scores, linear and intra-class correlation (ICC) coefficients and Bland-Altman plots.
Results: Twenty two pairs of studies were analyzed. Sixty percent had Hypoplastic Left Heart Syndrome and 60% had a BT shunt. Analysis of all proximal PA data had moderate linear correlation and agreement (R = 0.55, p = 0.0003, ICC = 0.65, Fig. 1A), though the RPA showed both stronger correlation and agreement (R = 0.72, p = 0.0005, ICC = 0.73, Fig. 2A) than the LPA (R = 0.41, p = 0.07, ICC = 0.60, Fig. 2B). CT detected missed PA stenoses (Z-score < -2) in 4 cases on the left compared to 1 on the right. There was significant variation but no systematic bias over the range of PA diameters by Bland-Altman analysis (Fig. 1B). The median radiation dose was 0.3 mGy (CTDI vol).
Conclusions: In this study of infants with shunt dependent pulmonary blood flow, TTE measurements of proximal PA diameter had only moderate correlation and agreement with CT measurements. RPA diameter was more accurately determined by TTE than LPA diameter using CT as the gold standard, with missed LPA stenosis identified more commonly by CT. In this population at high-risk for circulatory failure due to PA obstruction, a rapid, low-radiation CT may be warranted routinely to prevent missed PA stenosis.