EFFECTS OF PROCAINAMIDE AND PROPAFENONE ON THE COMPOSITION OF THE EXCITABLE GAP IN CANINE ATRIAL REENTRY TACHYCARDIA

The effects of procainamide and propafenone on the composition of the excitable gap (EG) were studied in a canine model of atrial flutter (A FI) around the tricuspid valve. In 14 open-chest, chloralose-anestheti zed dogs, a Y-shaped incision was made in the intercaval area extendin g to the right atrial appendage. Atrial effective refractory period (E RP) was measured at constant stimulation cycle lengths (CLs) (200 and 300 msec) at each of five recording sites around the tricuspid valve. The EG as defined by the reset-response curve was determined by introd ucing premature stimuli during AFI induced by burstpacing. Seven dogs each received procainamide or propafenone as a bolus followed by infus ion. At constant plasma levels, both drugs increased ERP at constant p aced CL and prolonged the reentry CL. In the absence of drug, reset-re sponse curves were mixed, demonstrating an EG composed of both partial ly (increasing portion) and fully (Rat portion) excitable tissue. Proc ainamide and propafenone shifted the curve upward and to the right and prolonged ERP during AFI, but did not change the duration of the EG. On procainamide, fully excitable tissue was preserved, but on propafen one, in some cases, the fully excitable part of the gap was reduced ma rkedly or even eliminated. In conclusion, both drugs can prolong AFI C L by a direct effect on conduction velocity in fully excitable tissue. In addition, propafenone's effect on refractoriness can contribute si gnificantly in some cases to slowing of AFI.