Incomplete Bundle Branch Block.

Lewis (1925) stated that delay in the passage of the impulse down a bundle branch could alter materially the form of the electrocardiogram. Wilson (1940) defined incomplete bundle branch block as due to a conduction defect that merely retards the passage of the impulse through one of the main bundle branches, and stated that it gives rise to electrocardiograms which are " transitional, both as regards the length of the QRS interval and the form of the ventricular deflections between those that represent complete bundle branch block and those of normal outline." Partial bundle branch block is used to describe curves that contain both bundle branch block and normal complexes. There may be a rhythmic sequence, as in 2: 1 bundle branch block, or occasional normal QRS deflections may occur in a record predominantly composed of bundle branch block complexes. In common with the New York Heart Association's nomenclature, Wilson (1944) regards 0-12 sec. as the minimum QRS duration ordinarily compatible with the diagnosis of conmplete bundle branch block. In this country, however, a QRS that exceeds 0-10 sec. has generally been considered as indicating complete bundle branch block providing other criteria are present (Lewis, 1925; and Hunter, Papp, and Parkinson, 1940). This small divergence may be due to a different technique in measurement. In the cases to be described the QRS has been measured straight across at the widest point in any lead. In no case has it measured more than 0 10 sec., when incomplete bundle branch block has been judged to be present.