Escape-capture bigeminy in rate dependent (phase 3) block of atrioventricular conduction.

A specific, hitherto unpublished mechanism of escape-capture bigeminy based on rate dependent (phase 3) block of atrioventricular conduction is described. Escape-capture bigeminy is a bigeminal rhythm in which each escape beat is followed by a conducted beat. Bradley and Marriott (1958) were the first to describe this interesting electrocardiographic pattern in a case of sinoatrial block. Schamroth and Dubb (1965) reported cases of escape-capture bigeminy resulting from sinoatrial block, atrio-ventricular block, and a second degree Wenckebach type atrioventricular block ending in an atrial echo beat. A fourth mechanism was published by Dubb and Schamroth (1975) in which escape-capture bigeminy was caused by blocked atrial extrasystoles. We present a case of escape-capture bigeminy caused by a specific, hitherto unpublished mechanism. Case report A 73-year-old man with coronary heart disease was admitted with a history of retrosternal chest pain. No myocardial infarction could be proved. He received no cardioactive medication. Electrocardiogram on admission showed sinus arrhythmia with left bundle-branch block and atrioventricular dissociation caused by advanced atrioventricular block. For a short period constant ratio of block with bigeminy could be seen after which the advanced atrioventricular block with varying ratio of block returned. One week after admission the atrioventricular conduction disturbance disappeared. Fig. 1 shows sinus arrhythmia, second degree atrioventricular block, with varying ratio of block. Most of the sinus impulses are not transmitted to the ventricles. An escape rhythm is interrupted by sinus beats transmitted with normal or prolonged PR intervals. If a P wave follows the escape QRS complex closely, the next sinus impulse is conducted to the ventricles (5th, 12th, and 17th P waves). If the P wave falls just before or is concealed within the QRS complex of the escape beat (3rd, 8th, 10th, and 20th P waves), the following P is not conducted to the ventricles. Fig. 2 shows an escape-capture bigeminy pattern. At an altered sinus and escape frequency the preceding P wave of the conducted sinus impulse follows closely (3rd and 6th P waves) or is concealed within (9th and 12th P waves) the QRS complex of the escape. Discussion Frequency dependency of the impulse conducting structures of the heart is a proved electrophysio-logical phenomenon (Mendez, Gruhzit, and Moe, 1956). The functional behaviour of the atrioven-tricular junction and His-Purkinje system both in the antegrade and retrograde direction has been extensively studied in clinical electrophysiology (Akhtar et al., 1975). Our case is a specific manifestation of …