Titus, Jack L.

Some, but not all, cardiac arrhythmias are related to pathologic lesions of the cardiac conduction system. Common atrial dysrhythmias and first-degree atrioventricular (A-V) block rarely are explained on the basis of anatomic lesions in specific sites of the conduction system or its blood supply. Second-degree A-V block of Mobitz type II, which may be a precursor of complete (third-degree) heart block, commonly is associated with fibrotic lesions of uncertain etiology in the branching part of the bundle of His or the bundle branches. Ischemic lesions are found less often, and other pathologic processes rarely are present. Chronic complete heart block most often results from nonspecific, fibrotic interruption of the distal bundle of His, or of the first parts of the bundle branches after their origins. Ischemic lesions are uncommonly the cause of chronic block. High-grade A-V block complicating acute myocardial infarction may be associated with infarction of the A-V conduction system, but often morphologic evidence of ischemia cannot be identified. Congenital variants in anatomy of the conduction system are responsible for some relatively uncommon arrhythmias.

One hundred sixty-nine patients underwent replacement of the aortic valve with an aortic valve homograft; of these patients 17 had concomitant commissurotomy or annuloplasty for an associated mitral valve lesion. Homografts were prepared by sterilization with beta-propiolactone solution and stored in Hanks' solution, or were prepared by sterilization with irradiation and stored at -70 C. Operative mortality was 5.3%. Thirty (19%) of the 160 surviving patients had an aortic diastolic murmur at the time of hospital dismissal, and five of these patients had hemodynamically significant homograft incompetence. For 156 patients, follow-up was longer than 6 months; among the 147 patients who survived operation, there were 12 late deaths. Eighty-four per cent (132) of these 156 patients were alive and well 6 to 54 months after operation. Six patients have required reoperation for valve incompetence. An aortic diastolic murmur was present in 43% of patients. Five patients had significant valve incompetence; four of these patients are markedly improved over their preoperative status. Thromboembolism has been a rare complication despite the fact that anticoagulant therapy was not used. Late calcification of the homograft valve has been noted in only one patient.

Serial histologic sections of the atrioventricular (A-V) conduction system were studied in four hearts with persistent common A-V canal, and the anatomic findings were related to the abnormal ECG patterns. Two hearts were examples of the partial form and two of the complete form of A-V canal. The major anatomic abnormalities were: (1) postero-inferior displacement of the A-V node; (2) relatively short distances between the A-V node and the origin of the left bundle branching; (3) marked posteroinferior displacement of the left bundle branching system; and (4) relative hypoplasia of the anterior (superior) left bundle branches. The anatomic configuration of the A-V conduction system in A-V canal could result in relatively early impulse propagation to the posterior aspect of the ventricular septum as suggested by reported electrophysiologica studies. The anatomic findings are considered to be related closely to the known ECG patterns of A-V canal.

The morphology of the atrioventricular (A-V) conduction system was studied by serial histologic section in two cases of tricuspid atresia type IB (without transposition of the great arteries, with subpulmonary stenosis). The A-V node was adjacent to the central fibrous body, and its location was indicated by a "dimple" in the floor of the right atrium. The left bundle branches originated unusually close to the nodal-bundle junction. The right bundle branch was markedly elongated in its course to the right septal endocardium. It was situated along the inferior aspect of the channel-like ventricular septal defect as it traversed the septum. The early origin of the left bundle branches and the markedly elongated course of the right bundle branch were such that, theoretically, abnormal spread of the activation wave might result.

Obstructive pulmonary vascular disease (PVD) was evaluated in lung sections taken at autopsy in 53 patients with complete transposition of the great arteries. Twentythree had intact ventricular septa and 30 had ventricular septal defects. Children of similar age and associated defects had widely differing degrees of PVD and presented a spectrum of disease. In a small number of patients, the presence of pulmonic stenosis (congenital or acquired by pulmonary artery banding in infancy), a small ventricular septal defect, or a large atrial septal defect was not protective against the development of severe PVD. The findings of this study demonstrate the need for thorough hemodynamic evaluation of patients with complete transposition of the great arteries prior to repair by present technics. In addition, operation should probably be performed as soon as technically feasible.

Fourteen hearts with tricuspid atresia were reviewed at autopsy. The study included examples of normally related great arteries and of transposition of the great arteries; instances of pulmonary stenosis were present in both groups. In these examples of tricuspid atresia the size of the right ventricle depended on the size of the ventricular septal defect. The ventricular septal defect and the right ventricle usually were smaller in proportion to the size of the heart in specimens with normally related great arteries. In the absence of transposition of the great arteries, the ventricular septal defect was found to be the site of obstruction to pulmonary flow, and its size in relation to the size of the aorta determined the size of the pulmonary artery. Morphologic abnormalities exist in the region in which the proximal portions of the major atrioventricular conduction system (A-V node, bundle of His, origin of bundle branches) normally are located; thus, anatomic abnormalities in the course of the atrioventricular conduction tissue could be expected in tricuspid atresia. Based on histologic evidence, severe elevation of pulmonary vascular resistance, in general, does not occur in infancy in tricuspid atresia.