Summation and inhibition by ultrarapid train pulses in dogs: effects of frequency and duration of trains, lidocaine, and beta blockade.

To test whether summation and inhibition of excitation by ultrarapid train pulses were modified by frequency and duration of trains, using lidocaine and beta blockade, the strength-interval relationship was determined with a single extrastimulus and train pulses (100 and 200 Hz) in 28 anesthetized, open-chest dogs. The effective refractory period (ERP) determined at greater than or equal to 5 mA was shorter for trains than for single stimulus (summation). Summation determined at 15 mA increased progressively as the duration of trains was increased to 50 msec after which it reached a plateau. ERP determined at less than or equal to 2 mA was longer with train pulses than with single stimulus (inhibition). Inhibition for 200 Hz trains increased linearly with train duration up to 200 msec, while for 100 Hz trains it became constant when the duration was greater than or equal to 80 msec. Lidocaine (plasma concentration: 2.6 +/- 1.0 micrograms/mL) did not affect inhibition and summation in 11 dogs. Propranolol (1 mg/kg, iv) increased summation at 15 mA (n = 7, 30 +/- 10 vs 46 +/- 11 msec with 100 Hz trains, P less than 0.01), but did not affect inhibition (37 +/- 26 vs 29 +/- 17 msec, NS). In conclusion, (1) effect of frequency of train pulses was more on inhibition than on summation; (2) lidocaine did not affect inhibition and summation; and (3) propranolol increased summation but not inhibition.