The Role of the Pulmonary Vascular Bed in Congenital Heart Disease

In the normal human being systemic blood pressure is maintained at a high level by means of the high resistance offered to the flow of blood by the smaller systemic arteries and arterioles. In contrast, pressure in the pulmonary circulation is maintained at a low level because of the relatively large size of the pulmonary vessels. In the fetal and newborn period pulmonary arteries resemble systemic arteries in that they have a small lumen and thick media. In this period pulmonary pressure and resistance are high. As these vessels gradually thin out and enlarge, resistance to pulmonary blood flow falls and consequently pulmonary blood pressure falls. There is evidence to suggest that the fetal state of thick-walled, thin-lumened pulmonary arteries is retained in instances where the two circulations are joined, such as in patients with a single ventricle, large ventricular defect, aortic septal defect or large patent ductus arteriosus. As the patient grows, resistance to pulmonary blood flow may: (1) decrease. If the small pulmonary arteries develop normally and become thin-walled and large-lumened, pulmonary resistance will fall. More and more blood will be shunted into the lungs and the patient will develop the signs and symptoms of cardiac decompensation. (2) remain the same. If the small pulmonary arteries retain their fetal characteristics, pulmonary resistance will remain elevated. A balance between systemic and pulmonary blood flow will be maintained compatible with a relatively normal life. (3) increase. If, because of secondary intimal changes, the pulmonary arteries become thicker-walled and smaller-lumened, pulmonary resistance will increase. Progressively less blood will be shunted to the lungs and finally the shunt will reverse and become predominantly venous arterial. The patient will develop the signs and symptoms of pulmonary stenosis with increasing dyspnea and cyanosis. Data from two groups of patients are presented to illustrate the importance of the pulmonary vascular bed. In the first group, early and severe symptoms of cardiac failure necessitated closure of a large patent ductus arteriosus early in life. The ductus was large, comparable to the size of the aorta, pressures from the aorta and pulmonary artery were similar and a study of the lung biopsies demonstrated that the lumens of the small pulmonary arteries were decreased in size and the media were abnormally thick. In each instance closure of the ductus resulted in a cure. In the second group the two circulations were joined by either a large ventricular defect or a single ventricle. Three patients were in cardiac failure due to excessive pulmonary blood flow. Pulmonary blood flow was decreased by the creation of pulmonary stenosis, the main pulmonary artery being narrowed 60 to 80%. The postoperative course in these three patients illustrates the value of the creation of pulmonary stenosis in instances where the two circulations are joined by a large communication between the ventricles or great vessels.