2:1 bundle branch block1: Classification with special reference to the critical heart rate

A case of 2:1 bundle branch block is described. All 138 consecutive ventricular complexes recorded in the routine twelve lead electrocardiogram were of this pattern. Only twelve other acceptable cases of 2:1 bundle branch block were found in a review of the literature. A synopsis of these is given. The mechanism of production of 2:1 bundle branch block is discussed, especially in regard to critical rate phenomena, theories of the site of delay in bundle branch block and especially in regard to the great rarity of 2:1 bundle branch block. The cases of 2:1 bundle branch block are classified into three groups on the basis of specified changes in cardiac rate.