LETTERS TO THE EDITOR

We read with great interest the article by L. Kritharides and J. Vohra on "Late development of conduction block over the Mahaim fibers after electrical atrioventricular junction ablation for Mahaim fiber tachycardia" (PACE, 1992; 15:256-261). However, their Figure 2 may be interpretable in a different way. In their interpretation, an RV extrastimulus advanced QRS by only 10 msec but prolonged VA conduction by 35 msec (60 msec —» 95 msec). If this were true, atrial cycle length encompassing the RV extrastimulus should have been lengthened from 300 msec to 325 msec by 25 msec (35 msec 10 msec = 25 msec). But the atrial cycle length on HRA and PCS encompassing the RV extrastimulus is 300 msec (according to our measurement), indicating that the RV extrastimulus most probably did not penetrate the tachycardia circuit. If one were to assume that the RV extrastimulus delayed the activation of the low septal right atrium on HBE alone by 25 msec, shortening by 25 msec in intraatrial conduction time between the low septal right atrium to HRA and PCS would have to occur, which would be unlikely. It seems to us that exact measurement of VA conduction time on HBE in Figure 2 is very difficult in consideration of the very sinall atrial deflection on HBE during sinus beat (last beat in Fig. 2).