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

<span class="paragraphSection"><div class="boxTitle">Aims</div>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.<div class="boxTitle">Methods and results</div>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 (<span style="font-style:italic;">n</span> = 46) had balanced pulmonary blood flow, 28% (<span style="font-style:italic;">n</span> = 18) had decreased flow to the left, and 2% (<span style="font-style:italic;">n</span> = 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 mm²/m², <span style="font-style:italic;">P</span> < 0.0001). Patients with reduced LPA flow were more likely to have required pulmonary arterioplasty at the time of ASO (17 vs. 2%, <span style="font-style:italic;">P</span> = 0.04) and had a larger aortic root diameter (25 ± 7 vs. 22 ± 5 mm²/m², <span style="font-style:italic;">P</span> = 0.01). Greater rightward orientation of the neo-pulmonary root correlated inversely with LPA cross-sectional area (<span style="font-style:italic;">r</span> = −0.39, <span style="font-style:italic;">P</span> = 0.001) but not with LPA flow. Aortic root diameter correlated inversely with LPA flow (<span style="font-style:italic;">r</span> = −0.43, <span style="font-style:italic;">P</span> = 0.0004) but not with LPA cross-sectional area (<span style="font-style:italic;">P</span> = 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 mm²/m², <span style="font-style:italic;">P</span> = 0.04) and less pulmonary blood flow (41 vs. 46%, <span style="font-style:italic;">P</span> = 0.02) compared with patients without those risk factors.<div class="boxTitle">Conclusions</div>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.</span>

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