Antiarrhythmic drugs appear preferentially to prolong refractoriness of accessory pathways compared wth 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 < .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 < .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 < .02), but not in the opposite direction. Procainamide (10 to 20 mg/kg iv, serum concentrations 8.6 ± 0.8 μg/ml) prolonged the ERP of the proximal site from 145 ± 5 to 170 ± 5 msec (p <.001), the ERP of the distal site from 143 ± 6 to 168 ± 6 msec (p < .001) in 12 dogs, and the shortest PCL with 1:1 conduction (proximal to distal from 149 ± 8 to 204 ± 17 msec, p <.001; distal to proximal from 149 ± 7 to 197 ± 12 msec, p < .001) in 14 dogs. After procainamide, the increase in the ERP of the isthmus in both directions was greater than the increase in the ERP of the proximal and distal atrial segments in five dogs (37 ± 7 vs 18 ± 6 msec, p < .001). Infusion of isoproterenol (0.1 μg/kg/min) improved conduction over the isthmus, which has been depressed by procainamide in six dogs. We conclude that an isthmus of atrial tissue depresses conduction. The narrower the isthmus, the greater the depression of conduction. Procainamide lengthens the ERP of the isthmus more than it lengthens the ERP of atrial muscle. These findings may help explain the effects of antiarrhythmic drugs on accessory pathways that act as an isthmus in patients with the Wolff-Parkinson-White syndrome.
ASJC Scopus subject areas
- Cardiology and Cardiovascular Medicine
- Physiology (medical)