Summation and inhibition by ultrarapid train pacing in the human ventricle.

Trains of ultrarapid stimuli that begin late in the refractory period have been reported both to produce early single captures to terminate tachyarrhythmias and to inhibit the response to subsequent threshold stimuli. To determine which characteristics of trains facilitate capture and which enhance inhibition, we compared the right ventricular strength interval relationship for single extrastimuli (S2) with that for 100 Hz trains with a duration of 100 msec in 29 patients. Pulse frequency was varied in 12 patients (50, 100, and 200 Hz) and train duration (50, 100, and 150 msec) was varied in 11 patients; the effect of procainamide (10.1 +/- 2.3 micrograms/ml) was assessed in 10 patients. Relative to S2, 100 Hz trains with a duration of 100 msec prolonged the effective refractory period (ERP) at low current strength (inhibition), but shortened the ERP at high-current strength (summation): at 0.5 mA, the train ERP was 47 +/- 6 (SEM) msec longer than the S2 ERP (p less than .001); at 16 mA it was 12 +/- 1 msec shorter (p less than .001). Trains prolonged the functional refractory period (FRP) slightly at low currents (13 +/- 3 msec, p = .001 at .05 mA), but did not shorten FRP significantly at high currents (2 +/- 2 msec, p = NS at 16 mA) because of increased stimulus-response latency. Inhibition increased with increasing pulse frequency (p less than .001), increasing train duration (p less than .001), and procainamide (p less than .01). Summation increased with increasing pulse frequency (p less than .001), but not increasing train duration or procainamide, suggesting that inhibition and summation depend on different electrophysiologic mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)