Titus, Jack L.

At the Mayo Clinic, four patients were seen in whom a ventricular septal defect (VSD) of the atrioventricular (A-V) canal type was associated with an angle of nearly 90° between the intact atrial septum and the ventricular septum. The right atrium overrode the ventricular septum and communicated with both ventricles. A complex, straddling right A-V valve had some attachments in the left ventricle and resembled the common A-V valve of persistent common A-V canal. In three of the patients, a congenital mitral valve deformity also existed. Mild cyanosis probably is not a contraindication to surgical repair in such patients because right atrial blood may enter the left ventricle directly.

The morphology of the sinus and atrioventricular (A-V) nodes of human hearts was studied and connecting pathways between these nodes were sought. The cells of both nodes differed from ordinary atrial and ventricular myocardium in respect to size, shape, number, and arrangement of cytoplasmic organelles. Using the criteria of fiber-to-fiber connection, shown by means of serial histological sectioning, this study confirmed previous reports of three pathways between the sinus and A-V nodes in the human heart. It was not required that these pathways be composed of classic Purkinje-type fibers, though many of the fibers did possess some of the features of Purkinje fibers. While recent evidence reveals that specialized atrial pathways do demonstrate increased conduction velocity, their prime functional significance may be in the facilitation of orderly atrial depolarization, the maintenance of sinus node control of ventricular depolarization under various physiological conditions, and the provision of an orderly "input" into the A-V node, a tissue generally acknowledged to have a low margin of safety.

The first 50 cases at the Mayo Clinic of insertion of an aortic valvular homograft after excision of the aortic valve are reported. The hospital mortality rate was 6%. An aortic diastolic murmur was noted in 17% of the surviving patients just prior to their dismissal from the hospital. In none was there evidence of hemodynamically significant aortic valvular incompetence. At follow-up examination 3 to 12 months postoperatively, an aortic diastolic murmur was audible in 44% of the traced patients. Only one patient had hemodynamically significant aortic valvular incompetence. The patients were not given anticoagulant therapy, and there were no thromboembolic complications.

The atrioventricular conduction system was studied by a technique of selected serial histological sections in examples of tetralogy of Fallot and ventricular septal defect to correlate the anatomic findings observed with the electrophysiological findings as manifested by the vectorcardiographic pattern. Eight hearts were studied; two examples of each anomaly had electrocardiographic and VCG patterns in the frontal plane similar to A-V canal defects and two examples of each had ECG and VCG patterns usually associated with these lesions. Of the four examples of tetralogy, the two examples with VCG patterns similar to the A-V canal pattern differed from the other examples in that the distance between the A-V node and the origin of left bundle branching was shorter and the fibers destined to become the right bundle branch took an elongated course. The VSD examples offered less striking differences. A short distance from the A-V node to the onset of left bundle branching was seen in both examples with VCG patterns similar to the A-V canal pattern, but one example of VSD with a representative VCG pattern had a similar anatomic configuration. When measurement ratios were compared, the right bundle took its origin at a relatively greater distance from the A-V node in the examples with VCG patterns similar to the A-V canal pattern. The anatomic configuration of those examples of tetralogy and VSD with VCG patterns similar to the A-V canal pattern was such that relatively early conduction to the left bundle branch system could occur.

The major parts of the atrioventricular conduction system of the human heart were traced in 21 instances of ventricular septal defect: 19 were examples of variously located uncomplicated ventricular septal defects and two of the tetralogy of Fallot. In the presence of a ventricular septal defect, the conduction system was found to have a normal course, except when the ventricular septal defect lay in a position normally occupied by the conduction system. In each specimen with a defect posterior and inferior to the crista supraventricularis, the conduction system occupied a position posterior and inferior to the defect. In no instance did the conduction system occupy a position superior to a defect of this type. Defects located in the posterobasal portion of the muscular part of the ventricular septum sometimes were posterior to the main parts of the conduction system, so that the conduction tissue was related to the anterior edge of the defect. No example of a defect lying superior to the crista supraventricularis was studied. In our two examples of tetralogy of Fallot, the position of the conduction system was essentially similar to that occurring in the usual variety of ventricular septal defect, that is, posterior and inferior to the crista supraventricularis.

Among 60 necropsy specimens of isolated ventricular septal defects and more than 300 cases of ventricular septal defect observed at operation, 15 cases demonstrating unusual anatomic positions of the defect were found. The ventricular defect differed from the ordinary ventricular septal defect in that it usually occupied the position of the ventricular component of the defect in persistent common atrioventricular canal. For this reason it was named ventricular septal defect of the persistent common atrioventricular canal type. Deformities of one or both atrioventricular valves were common (nine of 15 cases). No atrial septal defects of the ostium primum type were present. Anatomic studies of the conduction tissue revealed that this tissue skirted the posterior and inferior aspects of the ventricular septal defect and that, as in persistent common atrioventricular canal, the course taken by the conduction tissue was unusually long, as a result of the peculiar posteroinferior position of the lower edge of the defect. The electrocardiographic features were striking. In all cases the mean electrical axis of the QRS lay above the isoelectric point; the vector loop obtained in the frontal plane from the scalar electrocardiogram was directed counterclockwise, and its main mass was above the zero line. In addition, in all cases there were signs of right ventricular overload, and in some cases of left ventricular overload as well. Electrocardiographic findings of this pattern have been thought by some authors to be diagnostic of persistent common atrioventricular canal, but we observed that they also occurred in each of the cases of isolated ventricular septal defect of the variety described herein. We recognize that in the usual variety of ventricular septal defect this electrocardiographic pattern occurs, but it does so uncommonly. We have not studied its exact incidence. The anatomy, hemodynamics, and surgical considerations are different in cases with this defect from those with persistent common atrioventricular canal. The surgical risk in these cases has been higher than that in the usual type of ventricular septal defect. In the discussion of the electrophysiologic theories that seek to explain the unusual electrocardiographic patterns in this group of cases, a new theory is offered, based on studies of the conduction system. In our opinion, the different orientation of the advancing fronts of depolarization is the result of congenital displacement of the bundle of His in its relation to the ventricular septal defect.