Mamiya K, Aono J, Manabe M Department of Anesthesiology, Kochi Medical School, Japan. mamiyak@med.kochi-ms.ac.jp. Can J Anaesth 1999 Mar;46(3):265-7. PURPOSE: To describe a case of asymptomatic first degree atrioventricular block with a bifascicular block that progressed to complete atrioventricular block during anesthesia. This potentially fatal block was successfully treated with transesophageal ventricular pacing. CLINICAL FEATURES: A 67-yr-old man was scheduled for microvascular decompression of the right trigeminal nerve under general anesthesia. His preoperative ECG showed first degree atrioventricular block with complete right bundle branch block and left anterior hemiblock, but he had experienced no cardiovascular symptoms. Anesthesia was induced with sevoflurane 5%, and maintained with isoflurane 1.5-2% in oxygen. Fifteen minutes later in the left lateral decubitus position, the systolic arterial blood pressure suddenly decreased from 80 mmHg to 0 mmHg. Then, the ECG abruptly changed from sinus rhythm to complete atrioventricular block. The heart was unresponsive to drug therapy such as atropine 1.3 mg and isoproterenol 0.5 mg, or transcutaneous pacing but transesophageal pacing was successful. CONCLUSION: Asymptomatic first degree atrioventricular block with bifascicular block advanced to complete atrioventricular block during anesthesia. The block was successfully managed with transesophageal pacing.