Focal lesions in the canine bundle of His. Their effect on ventricular excitation.

The alterations in ventricular excitation resulting from production of discrete punctate electrocautery lesions in the canine bundle of His were studied. Electrocardiographs changes were correlated with lesions identified anatomically by microscopic examination of the atrioventricular bundle. Although partial or complete atrioventricular block developed from lesions at several levels of the atrioventricular bundle, changes in the form of the QRS complex appeared only when lesions involved the distal portion of the bundle. Such QRS changes were recorded in 21 of 32 instances when the lesion involved the branching portion of the common bundle but in none of 6 instances when the lesion involved only the nonbranching portion of the common bundle. This study, then, provides evidence for functionally distinct longitudinal fascicles only within the distal portion of the common bundle. The syncytial character of the terminal fascicular (Purkinje) network may minimize the electrocardiographic evidence for such functional specificity within the common bundle.