The Pulmonary Circulation in Congenital Heart Disease

Abstract

The right-to-left shunting in patients with a large ventricular septal defect (VSD) is caused by increased pulmonary vascular resistance (PVR) secondary to pulmonary vascular disease. The pulmonary circulation hugely impacts the clinical phenotype of many congenital heart lesions. For example, PVR is a major determinant of pulmonary blood flow (PBF; hence left ventricular output and therefore the likelihood of congestive heart failure) in infants with a large interventricular or great vessel communication (e.g., VSD; patent ductus arteriosus). In babies with a single cardiac ventricle, the magnitude of PVR often is a major determinant of both ventricular volume load and systemic oxygen delivery. On the other hand, over the longer run, many cardiac malformations can cause PVR to increase, often resulting in serious disability for the patient so afflicted. This is especially true for single-ventricle patients outside the neonatal period, for whom operative palliation with a cavopulmonary circulation (e.g., surgically directing the systemic venous return directly to the lungs, without an inter­mediary pumping chamber) requires a healthy (i.e., low PVR) pulmonary circulation. But patients with two ventricles can also suffer disability and early demise from increased PVR related to a cardiac defect. So, depending upon the circumstances, with heart malformations the pulmonary vascular bed may prove problematic owing to too little (actually, normal) or too much PVR. The primary focus of this chapter is on the latter, increased PVR secondary to increased pulmonary artery (PA) pressure and/or flow caused by cardiac structural malfor­mations. We use pulmonary vascular disease (PVD) to denote this increased PVR, which is accounted for by both pathological remodeling of the pulmonary circulation and active vasoconstriction of small PAs, in varying relative degrees.