His bundle block.

In patients with narrow QRS complexes, atrioventricular block is usually localized to the AV node, and it is frequently benign. The more distal intra-His bundle blocks are rare, potentially malignant, and difficult to recognize. This case report illustrates how His bundle block can be diagnosed by analyzing the effects of certain maneuvers on the heart rate. Paradoxically, exercise and atropine tend to provoke His bundle block, whereas carotid massage may release it. Implantation of a permanent pacemaker is generally indicated.