OVERDRIVE PROLONGATION OF REFRACTORINESS AND FATIGUE IN THE EARLY STAGES OF HUMAN BUNDLE-BRANCH DISEASE

Objectives. The aim of this study was to assess the response of refrac toriness in normal and diseased human bundle branches to changes in cy cle length, as web as during a long period of continuous overdrive pac ing. Background. The anterograde refractory period of the bundle branc hes in patients with functional bundle branch block shortens as the ra te is increased. The rate dependent response of refractoriness in dise ased bundle branches is quite different. However, this difference has not been precisely delineated, and its physiologic meaning is uncertai n. Methods. Refractoriness of the bundle branches was measured by the extrastimulus technique in 16 patients with tachycardia- dependent bun dle branch block and 10 patients with functional bundle branch block, both after basic trains of 8 atrial-paced impulses at different cycle lengths and during a 10 min period of continuous overdrive pacing. Res ults. The baseline refractory period in the bundle branches of patient s with functional bundle branch block measured 430 +/- 32 ms (mean +/- SD) and shortened to 368 +/- 30 ms at the shortest cycle length. The maximal effect was reached within the 1st min of overdrive pacing. The baseline refractory period of the bundle branches was significantly l onger in patients with tachycardia dependent bundle branch block (611 +/- 184 ms) and demonstrated a cumulative overdrive prolongation in 15 (83%) of 18 studies with typical manifestations of fatigue. In two ot her studies, this occurred only after ajmaline administration. Conclus ions. A rate- and time-dependent prolongation of refractoriness freque ntly occurs in diseased human bundle branches. When absent, this respo nse may be induced under the effects of sodium channel blockers. This would suggest that an abnormality in the recovery from inactivation of the sodium channel might underlie the early stages of bundle branch d isease.