Systemic-to-pulmonary collateral flow is a well-recognised phenomenon in patients with single ventricle physiology, but remains difficult to quantify. The aim was to compare the reported formula’s that have been used for calculation of systemic-to-pulmonary-collateral flow to assess their consistency and to quantify systemic-to-pulmonary collateral flow in patients with a Glenn and/or Fontan circulation using four-dimensional flow MRI (4D flow MR).

Retrospective case–control study of Glenn and Fontan patients who had a 4D flow MR study. Flows were measured at the ascending aorta, left and right pulmonary arteries, left and right pulmonary veins, and both caval veins. Systemic-to-pulmonary collateral flow was calculated using two formulas: 1) pulmonary veins – pulmonary arteries and 2) ascending aorta – caval veins. Anatomical identification of collaterals was performed using the 4D MR image set.

Fourteen patients (n = 11 Fontan, n = 3 Glenn) were included (age 26 [22–30] years). Systemic-to-pulmonary collateral flow was significantly higher in the patients than the controls (n = 10, age 31.2 [15.1–38.4] years) with both formulas: 0.28 [0.09–0.5] versus 0.04 [−0.66–0.21] l/min/m2 (p = 0.036, formula 1) and 0.67 [0.24–0.88] versus -0.07 [−0.16–0.08] l/min/m2 (p < 0.001, formula 2). In patients, systemic-to-pulmonary collateral flow differed significantly between formulas 1 and 2 (13% versus 26% of aortic flow, p = 0.038). In seven patients, veno-venous collaterals were detected and no aortopulmonary collaterals were visualised.

4D flow MR is able to detect increased systemic-to-pulmonary collateral flow and visualise collaterals vessels in Glenn and Fontan patients. However, the amount of systemic-to-pulmonary collateral flow varies with the formula employed. Therefore, further research is necessary before it could be applied in clinical care.