Bundle-branch block dependent upon critical rate is a form of phasic aberrant ventricular conduction (Schamroth and Chesler, 1963). In this arrhythmia, bundle-branch block appears when the rate increases and disappears when it slows. The basis of this arrhythmia, as with all other forms of phasic aberrant ventricular conduction, is the presence of unequal refractory periods of the bundle branches. With relatively slow heart rates (illustrated as RI to R2 in Fig. 1), consecutive impulses find both bundle branches fully recovered and normal intraventricular conduction ensues. With an increase in heart rate (illustrated as RI to R3 in Fig. 1), consecutive impulses find one bundle branch-usually the, right-refractory and the other fully recovered; conduction to the ventricles thus proceeds through one bundle branch only, resulting in an aberrant or bundlebranch block pattern. Examples of this arrhythmia have been reported by Vesell (1941), Shearn and Rytand (1953), and Gardberg and Rosen (1958): in all these cases the transition from the bundle-branch block pattern to normal intraventricular conduction and vice-versa was sudden. We report another example of this arrhythmia where the aberrant pattern was in the form of left bundle-branch block and the transition from normal to aberrant conduction was gradual; this presented a unique opportunity to prove the clinical existence of incomplete left bundle-branch block and to study the various grades of incomplete left bundle-branch block clinically-a procedure that has hitherto been possible only in the experimental animal.
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