Noninvasive Assessment of Pulmonary Artery Flow and Resistance by Cardiac Magnetic Resonance in Congenital Heart Diseases With Unrestricted Left-to-Right Shunt

Objectives: To determine whether noninvasive assessment of pulmonary artery flow (Qp) by cardiac magnetic resonance (CMR) would predict pulmonary vascular resistance (PVR) in patients with congenital heart disease characterized by an unrestricted left-to-right shunt. Background: Patients with an unrestricted left-to-right shunt who are at risk of obstructive pulmonary vascular disease require PVR evaluation preoperatively. CMR cardiac catheter (XMR) combines noninvasive measurement of Qp by phase contrast imaging with invasive pressure measurement to accurately determine the PVR. Methods: Patients referred for clinical assessment of the PVR were included. The XMR was used to determine the PVR. The noninvasive parameters, Qp and left-to-right shunt (Qp/Qs), were compared with the PVR using univariate regression models. Results: The XMR was undertaken in 26 patients (median age 0.87 years)-ventricular septal defect 46.2%, atrioventricular septal defect 42.3%. Mean aortic flow was 2.24 ± 0.59 l/min/m², and mean Qp was 6.25 ± 2.78 l/min/m². Mean Qp/Qs was 2.77 ± 1.02. Mean pulmonary artery pressure was 34.8 ± 10.9 mm Hg. Mean/median PVR was 5.5/3.0 Woods Units (WU)/m² (range 1.7 to 31.4 WU/m²). The PVR was related to both Qp and Qp/Qs in an inverse exponential fashion by the univariate regression equations PVR = exp(2.53 - 0.20[Qp]) and PVR = exp(2.75 - 0.52[Qp/Qs]). Receiver-operator characteristic (ROC) analysis was used to determine cutoff values for Qp and Qp/Qs above which the PVR could be regarded as clinically acceptable. A Qp of ≥6.05 l/min/m² predicted a PVR of ≤3.5 WU/m² with sensitivity 72%, specificity 100%, and area under the ROC curve 0.90 (p = 0.002). A Qp/Qs of ≥2.5/1 predicted a PVR of ≤3.5 WU/m² with sensitivity 83%, specificity 100%, and area under the curve ROC 0.94 (p < 0.001). Conclusions: Measurement of Qp or left-to-right shunt noninvasively by CMR has potential to predict the PVR in patients with an unrestricted left-to-right shunt and could potentially determine operability without having to undertake invasive testing.