A 3-month-old infant is diagnosed with a membranous ventricular septal defect. A cardiac operation is performed, and the septal defect is patched inferior to the noncoronary cusp of the aorta. Two days postoperatively the infant develops severe arrhythmias affecting both ventricles. Which part of the conduction tissue was most likely injured during the procedure?

The bundle of His is a collection of specialized cardiac muscle cells that carry electrical activity to the right and left bundle branches. Because both ventricles are affected, this is the logical site of injury, for this bundle leads to the bundle branches supplying both ventricles. An injury either to the right or left bundle branch would affect only one ventricle. Terminal Purkinje fibers transmit the electrical activity to the greater sections of the ventricles, yet dysfunction in the terminal part of the conduction system would affect only a small section of one ventricle, not both. The atrioventricular node is a group of specialized cardiac muscle cells that serve to decrease the rate of conduction to the ventricles and is located in the region deep to the septal wall of the right atrium. The posterior internodal pathway is in the roof of the right atrium and is not involved here.