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Systemic to pulmonary artery collaterals (SPCs) are ubiquitous in single ventricle physiology, being found in nearly two-thirds of patients after a bidirectional Glenn operation and in approximately half of patients after a Fontan operation.1 They arise most commonly from the subclavian artery or one of its branches and supply systemic arterial blood to the distal pulmonary vasculature. The risk factors and pathogenic mechanisms that lead to the formation of SPCs remain poorly characterized, but chronic hypoxaemia, diminished pulmonary blood flow, surgical scarring, branch pulmonary artery stenosis, inflammation and abnormal flow patterns in the pulmonary arteries have all been hypothesised to contribute to their formation, although these hypotheses remain largely untested.
In theory, SPCs may have beneficial effects in single ventricle physiology. By providing additional pulmonary blood flow, SPCs may promote pulmonary artery growth and improve systemic oxygen saturation. They may also prevent the formation of pulmonary arteriovenous malformations by supplying ‘hepatic factor’. However, more attention is usually focused on the potential harmful consequences of SPC flow including ventricular dilation, recirculation of oxygenated blood to the lungs resulting in ineffective pulmonary blood flow, energy losses related to competing sources of pulmonary blood flow and, rarely, erosion into a bronchus resulting in life-threatening haemoptysis. These physiological abnormalities have been hypothesised to contribute to adverse clinical outcomes. Excessive SPC flow is thought to contribute to increased pleural drainage after a Fontan operation, which can prolong recovery in the postoperative period. In the longer term, persistent SPC flow has been implicated in the pathogenesis of ventricular dysfunction, heart failure and protein-losing enteropathy. However, most of these hypotheses have not been tested and the few data that exist are contradictory. For example, two similar studies that sought to investigate if increased SPC flow was associated …