Intermittent trifascicular block. Different mechanisms of conduction disturbances in the bundle branches

Analysis of the electrocardiograms of a patient with acute anterior wall myocardial infarction revealed the development of fixed left anterior hemiblock and tachycardia-dependent intermittent conduction in the left posterior branch and right bundle branch systems, with a higher critical rate for conduction in the former system. The electrocardiograms provided a classic illustration of this type of trifascicular block. Both grade I and type I (Mobitz) blocks of atrioventricular (A-V) conduction are attributed to conduction disturbance in the left posterior branch system. The distinction made between the functional nature of type I (Mobitz) block and the organic nature of type II block is criticized especially in the presence of a bundle branch block pattern. Failure of conduction of 3 consecutive impulses in the left posterior branch system, shown in some tracings, is attributed to 3 different electrophysiologic mechanisms: (1) conduction through an incompletely repolarized posterior branch due to pathologic prolongation of its functional refractory period (phase 3 block); (2) enhanced phase 4 depolarization (phase 4 block); and (3) retrograde activation of the posterior branch by an impulse traveling down the right bundle. Regular alternation of the second and third mechanisms occasionally gave rise to long periods of consistent block in the posterior branch. Other tracings demonstrated that successful conduction in the posterior branch could perpetuate transmission failure in the right bundle through retrograde activation of the right bundle branch system. The value of deductive analysis in unraveling complex disturbances of the cardiac rhythm is illustrated. © 1973.