Conduction over an isthmus of atrial myocardium in vivo: a possible model of Wolff-Parkinson-White syndrome.

Antiarrhythmic drugs appear preferentially to prolong refractoriness of accessory pathways compared with atrial or ventricular muscle in patients with the Wolff-Parkinson-White syndrome. This response may be due to intrinsic properties of accessory pathways or to depressed conduction associated with a narrow strip, or isthmus, of tissue. To test the latter possibility, in 16 anesthetized dogs we surgically isolated a portion of the right atrial myocardium in the form of an ellipse (10 to 25 X 8 to 15 mm). The ellipse was connected to the body of the right atrium by a narrow isthmus (cross-sectional area [CSA] 1 to 13.5 mm2) and was perfused by the sinus node artery or its branch. Diastolic threshold (mean +/- SE 0.16 +/- 0.05 vs 0.13 +/- 0.02 mA) and effective refractory period (ERP; 144 +/- 4 vs 139 +/- 5 msec) were the same proximal and distal to the isthmus. In eight dogs, determination of the ERP of the isthmus was limited by the ERP of the atrial tissue proximal and distal to the isthmus, and the CSA of the isthmus in these dogs was significantly larger than that in the remaining seven dogs in which the ERP of the isthmus could be determined (7.4 +/- 1.4 vs 3.2 +/- 0.6 mm2, p less than .05). The shortest pacing cycle length (PCL) with 1:1 conduction from the proximal to the distal segments did not differ from that in the opposite direction in 16 dogs (154 +/- 9 vs 153 +/- 7 msec). The CSA of the isthmus correlated inversely with the shortest PCL with 1:1 conduction in both directions via the isthmus (r = -.84, p less than .01). Vagal stimulation shortened the shortest PCL with 1:1 conduction from the distal to the proximal segment (153 +/- 14 vs 143 +/- 12 msec, p less than .02), but not in the opposite direction. Procainamide (10 to 20 mg/kg iv, serum concentration 8.6 +/- 0.8 micrograms/ml) prolonged the ERP of the proximal site from 145 +/- 5 to 170 +/- 5 msec (p less than .001), the ERP of the distal site from 143 +/- 6 to 168 +/- 6 msec (p less than .001) in 12 dogs, and the shortest PCL with 1:1 conduction (proximal to distal from 149 +/- 8 to 204 +/- 17 msec, p less than .001; distal to proximal from 149 +/- 7 to 197 +/- 12 msec, p less than .001) in 14 dogs.(ABSTRACT TRUNCATED AT 400 WORDS)