Effects of infarction, procainamide, coupling interval, and cycle length on refractoriness of extrastimuli

The effects of prematurity, cycle length, site of stimulation, and procainamide on ventricular refractoriness following an extrastimulus (S2) were assessed in 10 dogs with and 10 dogs without infarction. Extrastimuli were introduced at preselected coupling intervals (S1-S2) from normal right and left ventricular sites and from left ventricular sites of infarction during drive cycle lengths (S1-S1) of 350 and 250 ms. At each S1-S2 interval, the effective refractory period of S2 was determined by introducing a second extrastimulus (S3). At all stimulation sites, cycle lengths, and before and during infusion of procainamide (mean concn 18.6 +/- 3.5 micrograms/ml), shortening (greater than 10 ms change) in refractoriness was most marked over a narrow range of closely coupled S1-S2 intervals. Regardless of stimulation site, the effective refractory period of S2 was less during a cycle length of 250 ms compared with a cycle length of 350 ms. In dogs without infarction, the effective refractory periods of S2 from left ventricular sites tended to be longer than from right ventricular sites, particularly during procainamide administration. The refractory period of S2 at sites of infarction did not differ consistently from those at normal sites. Finally, at all stimulation sites and cycle lengths, procainamide prolonged refractoriness of S2 at each S1-S2 interval and blunted the total shortening in refractoriness in response to S2.