Effect of activation sequence on ventricular refractoriness as determined by extrastimuli

Study objective – The aim was to determine by the extrastimulus method the effect on the right and left ventricular effective refractory periods of moving the site of the pacing train away from the site of the extrastimulus. Design – The ventricular effective refractory period was measured at the right and left ventricular apices using pacing trains at two heart rates, delivered to the ipsilateral ventricle, the contralateral ventricle, and the right atrium. Subjects – Seven patients (six male), mean age 52 years (range 26-75 years), with either documented (six) or suspected (one) ventricular tachycardia were studied. Four had ischaemic heart disease and the remaining three had morphologically normal hearts. Measurements and main results – The pacing train and extrastimulus delivered in the right ventricle produced the shortest effective refractory period at both heart rates: 220.8(SD 19) ms and 207.9(16) ms respectively. As the pacing train was moved to the right atrium, the effective refractory period lengthened to 246.4(22) ms and 219.3(20) ms at the two heart rates. There was further lengthening as the site of the pacing train was moved to the left ventricle, to 269.2(20) ms and 240.7(35) ms respectively. The same pattern was observed in the left ventricular effective refractory periods as the pacing train was moved from left ventricle to right ventricle and to right atrium. Conclusions – The ventricular effective refractory period lengthens as the site of the pacing train is moved away from the site of the extrastimulus. This may be explained by the effects of the distribution of the pacing energy within the myocardium and by intercellular electrotonic interactions. This has important clinical implications for the arrhythmogenic mechanisms of ventricular tachyarrhythmias.