Intraoperative diagnosis of rate-dependent bundle branch block

Rate-dependent left bundle branch block (LBBB) occasionally occurs during anaesthesia when the heart rate exceeds a critical value. While it is usually a benign disorder, it may mask the electrocardiographic manifestations of myocardial ischaemia and the ST-T wave pattern associated with LBBB may be mistaken for those of ischaemia. This case report presents two cases in which rate-dependent LBBB was clearly documented during the perioperative period. It demonstrates the use of pharmacologic agents (e.g., atropine and neostigmine) and physiologic manipulations (e.g., carotid sinus massage) to alter the heart rate and confirm the diagnosis of benign rate-dependent LBBB in the operating room. These interventions should be used with caution in patients who have hypertension, angina, cerebrovascular, or AV node disease or in the setting of myocardial ischaemia or severe bundle branch disease.