THE USE OF THE BLOCK CYCLE LENGTH AS A SAFE AND EFFICIENT MEANS OF INTERRUPTING SUSTAINED VENTRICULAR-TACHYCARDIA AND ITS PHARMACOLOGICAL MODIFICATION

In nine patients who had inducible monomorphic sustained ventricular t achycardia (VT), rapid pacing was performed in 11 episodes of morpholo gically distinct VT at progressively shorter cycle lengths and VT was interrupted at a critical cycle length. The VT interrupting critical c ycle length was defined as the block cycle length (BCL) and the effect of Class I antiarrhythmic drugs were examined. Both the VT cycle leng th (VTCL) and the BCL were prolonged after administration of either dr ug. The overall mean ratio of the BCL to the VTCL was unchanged after procainamide administration, but increased after the use of mexiletine . The ratio, however, varied in individual VTs and the BCL after treat ment with Class I antiarrhythmic drugs could not be predicted from the ratio baseline value, although the ratio was always > 60% and the haz ard of VT acceleration might be avoided if the BCL is used.