[Coexistence of left branch block and ectopic rhythm simulating alternating block]

During the acute phase of diaphragmatic myocardial infarction with septal extension, the ECG of a patient with a chronic left bundle branch block changed in a period of seconds from complete left bundle branch block to incomplete right bundle branch block then to narrow QRS complexes followed by incomplete and then complete left bundle branch block: the same QRS changes then occurred in reverse order; the atrial rhythm was absolutely stable during the recording. These appearances are explained by fusion of sinus and of an ectopic rhythm arising distal to the zone of block, the rate of which (sometimes faster and sometimes slower than the sinus rhythm) could have been influenced by an electrotonic effect after retrograde activation of the right bundle and concealed conduction in the left bundle. Appearances of bundle branch block may be recorded when the ventricle is partially activated from the point of breakthrough of the blocked branch.