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Atrioventricular conduction defects in patients presenting with syncope and normal PR interval.

Ten patients presenting with syncope and a normal PR interval in the standard electrocardiogram were investigated by bundle of His recordings. There was no previous evidence of impaired AV conduction in any of these patients. According to their QRS pattern in the I2 lead electrocardiogram the patients were found to belong to one of three groups: (i) right bundle-branch block with left anterior hemiblock, (2) left bundle-branch block, and (3) normal QRS complex. In all three groups there was prolongation of the HV time indicating a first degree block distal to the His bundle. A bilateral bundle-branch block or a lesion in the main stem of the His bundle was suggested as the underlying cause of the clinical and His bundle electrogram findings. Demand cardiac pacemakers were implanted in 9 patients. Symptoms of disturbances in consciousness were abolished after cardiac pacing. The value of His bundle recording in the diagnosis of these patients was stressed, and the indications of cardiac pacing were discussed.