Pacing threshold changes after transvenous catheter countershock

The serial changes in pacing threshold and R-wave amplitude were examined after insertion of a countershock catheter in 12 patients referred for management of recurrent ventricular tachyarrhythmias. In 6 patients, values before and immediately after catheter countershock were monitored. Pacing threshold increased (from 1.4 ± 0.2 to 2.4 ± 0.5 V, mean ± standard error of the mean, p < 0.05) while the R-wave amplitude decreased (bipolar R wave from 5.9 ± 1.1 to 3.4 ± 0.7 mV, p < 0.01; unipolar R wave recorded from the distal ventricular electrode from 8.9 ± 1.8 to 4.6 ± 1.2 mV, p < 0.01; and proximal ventricular electrode from 7.7 ± 1.5 to 5.0 ± 1.0 mV, p < 0.01). A return to control values occurred within 10 minutes. In all patients, pacing threshold increased by 154 ± 30% (p < 0.001) during the first 7 days that the catheter was in place. It is concluded that catheter countershock causes an acute increase in pacing threshold and decrease in R-wave amplitude. A catheter used for countershock may not be acceptable as a backup pacing catheter.