Cycle length-dependent effects on normal and abnormal intraventricular electrograms: Effect of procainamide

The effect of procainamide (mean concentration 9.1 ± 2.0 μg/ml) on cycle length-dependent changes in electrographic characteristics was determined in 10 patients with prior myocardial infarction. Intracardiac bipolar electrograms were recorded from an abnormal left ventricular site in the distribution of prior (>6 month) myocardial infarction and from a normal right ventricular site. Pacing was performed for 15 beats from the right ventricular apex at cycle lengths of 600 (or 500), 400 and 300 ms, In the control state, the QRS width, the normal electrogram and in 9 of the 10 patients the abnormal electrogram did not change with decreasing cycle lengths After procainamide the mean QRS width increased from 203 ± 32 to 240 ± 50 ms (+18%, p < 0.01) at a paced cycle length of 600 (or 500) ms, from 198 ±34 to 245 ± 59 ms (+24%, p < 0.01) at a paced cycle length of 400 ms and from 197 ± 36 to 258 ± 67 ms (+31%, p < 0.01) at a paced cycle length of 300 ms. Corresponding increases in electrogram duration at the normal site were 31 ± 14 to 36 ± 14ms (+16%, p < 0.01), 33 ±13 to 39 ± 19 ms (+18%, p < 0.02) and 33 ± 15 to 43 ± 20 ms (+30%, p < 0.01), respectively, and at the abnormal site were 70 ± 11 to 93 ± 19 ms (+33%, p < 0.01), 74 ± 10 to 102 ± 20 ms (+38%, p < 0.01) and 73 ± 12 ms to 122 ± 32 ms (+67%, p < 0.01), respectively. Electrogram amplitudes did not change significantly after procainamide administration. After procainamide, QRS width and abnormal electrogram duration increased significantly with pacing at short versus long cycle lengths. Three patients developed intermittent loss of a late component of the abnormal electrogram during pacing at the shorter paced cycle length that was associated with the development of ventricular tachycardia in two of the patients. Comparison of the percent change in local electrogram duration after procainamide in those patients who did not demonstrate intermittent loss of (be late component of the abnormal electrogram showed a more pronounced prolongation of the abnormal electrogram at the shorter paced cycle length (p = 0.05). In summary, procainamide appears to have a greater cycle length-dependent effect on electrogram duration in chronically infarcted tissue. This selective effect might be an important mechanism for the anti- or proarrhythmic action of procalnamide.