Summation of excitation with a single conditioning stimulus in the canine heart

A subthreshold stimulus was reported to become effective in producing a propagated ventricular response when preceded by another subthreshold stimulus or stimuli, a phenomenon that is known as summation of excitation but has not been studied systematically. Effect on summation of current intensity and coupling interval of a conditioning stimulus (Sc, protocol #1) and that of stimulation site of Sc (protocol #2) were determined in anesthetized, open-chest dogs. Effective refractory period (ERP) was determined with an extrastimulus (S2) at a ventricular pacing cycle length (S1S1) of 500 msec. Pulse width of S1, S2 and Sc was 2 msec. For pacing protocol #1 (12 dogs), current intensity of S1 and S2 was equal (twice diastolic threshold, 2, 5, or 15 mA), and that of Sc was equal to or lower than that of S1. S1S2 interval was fixed 1 msec shorter than ERP, and Sc was delivered 5 msec prior to S2. The S1Sc interval was shortened in 2 msec steps. Summation was present if Sc plus S2 evoked a propagated response. Prevalence of summation increased along with an increase in current intensity of Sc (P less than 0.01). For pacing protocol #2 (7 dogs), current intensity of S1, S2 and Sc was equal (twice diastolic threshold, 5 or 10 mA). As the distance between site of Sc and that of S2 increased, prevalence of summation decreased (P less than 0.01). Summation did not occur when Sc was delivered at a site only 5 mm away from the site of S2. In conclusion, summation of excitation with a single conditioning stimulus was both time and strength dependent, and limited in development spatially.