Pulmonary vein flow pattern in children with bidirectional cavopulmonary connection or Fontan circuit

Background Typical flow velocity profiles in the extraparenchymal pulmonary veins (PVs) demonstrate two major antegrade flow waves: a biphasic systolic wave (S), with S1 and S2 peaks and a monophasic early diastolic wave (D). Flow reversal during atrial systole (A) is common. There is agreement that the forward diastolic PV flow wave is caused by left ventricular relaxation with opening of the mitral valve. The origin of the PV systolic wave, however, remains a topic of debate. Some studies have suggested that the S wave is created by the relaxation of the left atrium and descent of the mitral valve plane. These studies have concluded that forces generated by the right ventricle (RV) have no effect on the S wave. Others suggest that the forward propagation of the right ventricular systolic pressure pulse is the major contributor to the S wave. Objective To determine whether any part of the systolic wave of PV flow is dependent on forces created by the right ventricle. Materials and methods We assessed PV flow pattern, as obtained by cardiac MRI in 12 cases (39 pulmonary veins) with RV-independent pulmonary circulation (bidirectional cavopulmonary connection or Fontan circulation). Phasecontrast imaging of the PVs was performed on a 1.5-T MR scanner with velocity encoding set at 120 cm/s. We compared these flow patterns with those of a control group of ten children (15 pulmonary veins) who had RV-dependent pulmonary circulation and underwent CMR for other indications. Results In all PVs of children with RV-independent pulmonary circulation the flow curves showed a single systolic peak in early systole (S1) with the S2 peak consistently absent. PV flow pattern in the control group consistently showed distinct early and late systolic peaks. Conclusion This study supports the concept that S2 is caused by forward propagation of the right ventricular systolic pressure pulse. It also demonstrates that the S1 is independent of the right ventricle.