Eectrophysiological observations in pateints with rate dependent bundle branch block.

Electrophysiological studies were conducted in 15 patients with tachycardic rate dependent bundle branch block (RDBBB): ten with left, and five with right. No bradycardic RDBBB was observed, despite occurrence of cycle lengths (CL) longer than 1200 msec in over half the patients studied. Onset of RDBBB was abrupt in 13 patients, and gradual in two. In three patients, the CL allowing reversion to normal conduction (once RDBBB was initiated) was 50, 55, and 190 msec longer, respectively, than the CL inducing RDBBB. Bundle brance refractory periods (RP) were measured with atrial extrastimulus technique in five patients. All RP (except one) were prolonged at all tested CL when compared to patients without conduction disease. The expected decrease in RP with shortening of CL did not occur in four of the five patients. The electrophysiological abnormality in patients with DBBB thus appeared to be an increase in refractoriness in the affected bundle branch, along with a loss of the normal decrease in refractoriness with decrease in CL. Similar findings might be demonstrable in patients with early bundle branch disease.