Rate-Dependent Bundle Branch Block: Not Only Phase 3 Exists

Rate-dependent bundle branch blocks are often related with increased heart rate(tachycardia-dependent or phase-3 bundle branch block). Less often, they arecaused by heart rate reduction (bradycardia-dependent or phase-4 bundle branchblock). Phase-3 block is the most common type of paroxysmal intraventricularconduction disturbances documented in several publications. However, few articlesdescribe phase-4 bundle branch block, and both mechanisms rarely coexist in thesame patient. We report the case of a patient admitted with acute myocardialinfarction, presenting with both tachycardia- and bradycardia-dependentparoxysmal intraventricular conduction disturbances.