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SUMMARY Small, high-frequency electrocardiographic signals were recorded from the body surface in 39 patients with and 27 patients without ventricular tachycardia (VT). All patients were in normal sinus rhythm, had a previous myocardial infarction, were The control group consisted of 27 patients without VT who had no clinical history of complex ventricular arrhythmias. A 24-hour ECG, performed within 2 days of study, showed less than 200 premature ventricular complexes (PVCs) per day and the absence of multiform PVCs, couplets or VT. Ventriculography was performed in 19 patients and showed a ventricular aneurysm in four.The VT group included 39 patients who had had repeated episodes of symptomatic VT. All VT patients were studied in the clinical electrophysiologic laboratory with previously described techniques'7 and had sustained (> 1 minute) VT inducible by one to three VPCs. All patients with VT had ventriculography.There was no difference in location of infarcts between groups; 14 patients in the control group and 15 patients in the VT group had anterior MI and the remainder had inferior MI. Eighteen of the 39 VT patients and four of the 27 control patients had ventricular aneurysms (p = 0.06). Seventeen control patients and 10 VT patients had infarctions within 3 months (p = 0.003). Thirteen control patients and no VT patients were taking propranolol (p < 0.0001). The incidence of left anterior hemiblock was similar in both groups (two in the control group and five in the VT group) (p = 0.39). Surface RecordingBipolar X, Y, Z leads were used.

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The mechanism of cycle length oscillation and its role in spontaneous termination of reentry was studied in an in vitro preparation of canine atrial tissue surrounding the tricuspid orifice. Reentry occurred around a fixed path with incomplete recovery of excitability. Among 18 experiments, there was complete concordance between the occurrence of spontaneous cycle length oscillation and spontaneous terminations; both were observed in 10 experiments and neither in the other eight (p<0.001). Local changes in conduction during oscillations resulted from the dependence of both conduction velocity and action potential duration on the preceding local diastolic interval. Interval-dependent changes in action potential duration contributed to the oscillation by altering the next diastolic interval. Because of changes in action potential duration, changes in cycle length were poorly correlated with changes in diastolic interval and, therefore, with local conduction velocity. Complex oscillations resulted from variations in conduction time at multiple sites in the circuit. Oscillations caused most spontaneous terminations. The critical event was an exceptionally long diastolic interval preceding the next-to-last cycle that accelerated local conduction (which tended to shorten the last cycle) and prolonged action potential duration and refractoriness at the site of block. Ninety-two of 99 recordings of spontaneous termination showed evidence of oscillation of conduction and refractoriness causing block. (Circulation 1988;78:1277-1287

Sufficient data are available to recommend that the high-resolution or signal-averaged electrocardiogram can be used in patients recovering from myocardial infarction without bundle branch block to help to determine their risk for developing sustained ventricular tachyarrhythmias. However, no data are available regarding the extent to which pharmacologic or non-pharmacologic interventions in patients with late potentials have an impact on the incidence of sudden cardiac death. Therefore, controlled, prospective studies are required before this issue can be definitely answered. As refinements in techniques evolve, it is anticipated that the clinical value of high-resolution or signal-averaged electrocardiography will continue to increase in the future.