Understanding the mechanism for branch pulmonary artery stenosis after the arterial switch operation for transposition of the great arteries.

Aims: The pathophysiology of branch pulmonary artery (PA) stenosis after the arterial switch operation, most commonly on the left, is incompletely understood. This study examines factors associated with left PA (LPA) obstruction.

Methods And Results: Cardiac magnetic resonance (CMR) imaging studies performed in patients after arterial switch operation (ASO) were retrospectively analysed. Blood flow was measured in both branch PAs and neo-pulmonary root position in relation to the aorta was expressed as an angle, relative to a line connecting the sternum and the spine. Sixty-six patients were included for analysis. Seventy per cent (n = 46) had balanced pulmonary blood flow, 28% (n = 18) had decreased flow to the left, and 2% (n = 2) had decreased flow to the right lung. LPA area indexed to body surface area (BSA) was smaller than RPA area (62 ± 37 vs. 120 ± 64 mm/m, P < 0.0001). Patients with reduced LPA flow were more likely to have required pulmonary arterioplasty at the time of ASO (17 vs. 2%, P = 0.04) and had a larger aortic root diameter (25 ± 7 vs. 22 ± 5 mm/m, P = 0.01). Greater rightward orientation of the neo-pulmonary root correlated inversely with LPA cross-sectional area (r = -0.39, P = 0.001) but not with LPA flow. Aortic root diameter correlated inversely with LPA flow (r = -0.43, P = 0.0004) but not with LPA cross-sectional area (P = 0.32). Patients with a rightward neo-pulmonary root and/or a dilated aortic root in the upper quartile range had a smaller LPA area (53 vs. 73 mm/m, P = 0.04) and less pulmonary blood flow (41 vs. 46%, P = 0.02) compared with patients without those risk factors.

Conclusions: Neo-pulmonary to neo-aortic geometry as well as post-operative compression of the LPA by an enlarged aorta impact LPA size and perfusion of the left lung.