Effects of procainamide on the excitable gap composition in a canine model of atrial flutter

The effects of increasing concentrations of procainamide on the composition of the excitable gap were determined in a canine model of atrial flutter. Using the model of a Y-shaped lesion in the right atrium, reentry around the tricuspid valve was induced by burst pacing in 10 open-chest chloralose-anesthetized dogs. Diastole was scanned with a single premature stimulus and the relationship between the coupling interval of the premature beat and the return cycle length (CL) determined a reset-response curve that described the excitable gap. This was repeated up to the maximum flutter CL while infusing procainamide (30 mg/kg) over 1 h. Procainamide progressively prolonged the flutter CL from 131 +/- 21 (+/-SD) to 188 +/- 46 ms (p < 0.01) and the effective refractory period from 96 +/- 19 to 149 +/- 47 ms (p < 0.01). At peak plasma levels of 77 +/- 33 mumol/L the drug terminated flutter only in two dogs. Neither the duration (35 +/- 10 to 39 +/- 13 ms) nor the composition of the excitable gap changed on drug. A fully excitable portion (7 +/- 3 ms or 20 +/- 11% of the excitable gap) persisted on procainamide (7 +/- 3 ms or 19 +/- 9% of the excitable gap). It was concluded that procainamide prolongs the atrial flutter CL and the effective refractory period but does not change either the duration or composition of the excitable gap even at plasma concentrations that significantly exceed those recommended in man.