Interesting Electrocardiograms RATE-DEPENDENT BUNDLE BRANCH BLOCK

These electrocardiograms were taken on a 36 yearold man, with no known cardiac abnormality. The twelve-lead tracing, which shows sinus rhythm at rates between 53 and 60/min., has abnormal T waves in Vl V4. A s{rip, using lead V6, shows the interesting phenomenon of rate-dependent left bundle branch block (LBBB). Please. note in this strip that the first four sinus beats have normal QRS-T waves. The head rates of these beats are of special interest as the sinus rate starting at 64/min. gradually increases, from 67 to 71/min. At the fifth beat (marked with arrow) the heart rate has reached 75/min. and LBBB appears. The rest of the beats, with rates of 77/min., also show complete LBBB. When heart rate dropped below 75/ rain. normal (narrow) QRS complexes were immediately seen. This sequence was seen repeatedly, with the crucial rate being 74-75/min. each time.