De subitaneis mortibus. XV. Hereditary stenosis of the His bundle in Pug dogs.

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Abstract
Syncope and sudden death occurs in certain purebred Pug dogs which have been found to have intermittent sinus pauses and paroxysmal second degree heart block on electrocardiographic (ECG) study. We have established a colony of such dogs to study this problem and here report the results of histological examination of the cardiac conduction system in twenty-one of them. These include a dam which may be considered the proband, three of her offspring (two littermates) and three fetal pups in an unborn litter of one of these; three of the four adult dogs died suddenly and unexpectedly. Two groups of puppies descendants of these lines were also studied after they all died within three days of birth. The first group of puppies (8) did not have suitable ECG studies but the second group (6) did and showed ECG changes similar to those in the adult dogs. In all 21 dogs the sinus node was anatomically normal, but in every one there was significant stenosis of the midportion of the His bundle. There was no abnormality of the atrioventricular (A-V) node or of the bundle branches, and the cardiac valves and coronary arteries were normal. Two pups had ventricular septal defects and one of these had an atrial defect as well. All the dogs were of the normal fawn color, had a normal physical appearance and no visible evidence of associated extracardiac abnormalities. The adult dogs were able to hear normally but the puppies died too early to test their hearing. The paroxysmal heart block may in part be attributable to the abnormality in the His bundle, but other factors possibly leading to heart block and long sinus pauses are discussed. Stenosis of the midportion of the His bundle appears to be a heritable trait in these purebred Pug dogs.