Transposition of the great vessels. Pathophysiologic considerations based upon a study of the lungs.

A microscopic study of the lungs of 106 patients with transposition of the great vessels has revealed early and severe hypertensive alterations in the pulmonary arteries. These striking changes, associated in almost all instances with normal pulmonary veins, indicate a state of high resistance of the arterial bed. Pulmonary arterial damage was, in general, most advanced in the patients with a large ventricular septal defect, but in spite of this these were the patients who lived longest. It seemed obvious, therefore, that their relatively favorable course was due to a greater beneficial than damaging effect of the ventricular septal defect. Consideration of these findings in the light of known physiologic data has led to a clarification of the physiologic variations of the transposition circulation. Using the recognized physiologic definitions of flows and shunts the transposition circulation is characterized by a diminished effective flow and a bidirectional but predominantly left-to-right shunt. The effect of a large intracardiac communication is to increase the effective flow and to lessen the shunts. With increasing size of the intracardiac communication the spectrum is completed when the circulatory characteristics of a single ventricle are reached in which the anatomic position of the great vessels becomes physiologically insignificant. The effect of a common systolic ejectile force in patients with a large ventricular septal defect will be similar in the transposition circulation and in the normal circulation. Three physiologic states are possible which depend upon the relative magnitude of the systemic and pulmonary resistances. The early appearance and malignant nature of the hypertensive alterations in the pulmonary arteries of patients with transposition of the great vessels suggest that the impact upon the lungs of high blood flow and pressure is aggravated by arterial vasoconstriction. This may be initiated by anoxia and a lowered blood pH. Elevation of pulmonary venous pressure and the increased viscosity of the blood due to polycythemia may add to the damage. Prominence of the bronchial arteries noted in the lung sections suggests that an expanded bronchial circulation augments the effective flow in these patients. If the bronchial venous drainage leads to systemic veins, the bronchial circulation could act as an accessory pulmonary circulation and could improve the effective flow without having to cross the already overloaded intracardiac communications. The anatomic findings and physiologic considerations reported in this study reveal several aspects of the early and severely damaging cardiopulmonary effect of transposition of the great vessels and indicate an urgent necessity of surgical therapy in early life.