Observations on the behavior of the right ventricular outflow tract, with reference to its developmental origins

Abstract

The behavior of the mammalian right ventricular outflow tract has been considered, in accordance with its classic position in developmental anatomy, as a derivative of the bulbus cordis, a division of the primitive vertebrate cardiac tube. The outflow tract was studied by high speed cinematography and by strain gauge arches sutured to appropriate segments of the right ventricle in dogs. Cinematographic observations were also made on the bulbus cordis at the epitome of its development in gill-breathers, as represented by a typical elasmobranch; it was similarly observed in its modified form in the turtle, where it has been adapted and modified to the requirements of the newly evolved pulmonary circulation. The observations on the outflow tract in these studies indicate that it is a functionally distinct portion of the right ventricle. It expands in early systole concordantly with the pulmonary artery. Its contraction is delayed both in the cinefilms and in curves from strain gauge arches which measure the onset of tension in the segments of myocardium to which they are sutured. Relaxation and diastolic filling begin in the inflow tract at a time when outflow tract tonus remains at least partially sustained. Cinefilm observations on the bulbus cordis in both the shark and turtle are described. The roles of this structure in the single gill circulation of the shark and of its modified homologue in the incomplete double circulation of the turtle are interpreted. The study suggests that the mammalian right ventricular outflow tract may retain and utilize the inheritance from its parent structure. This is particularly evident in congenital heart disease in which abnormal pressure and flow loads may exist and, in essence, certain of the conditions of a more primitive circulatory organization are present. The manner in which the outflow tract responds to these atavistic circumstances is illustrated by selective examples from cases of human disease. Both the experimental and the clinical data support the view of Sir Arthur Keith that, regardless of the anatomic form it may take and regardless of the stage of vertebrate development, the functional integrity of the bulbus cordis remains, in its essentials, unchanged.