Conduction occurs through an accessory pathway between atria and ventricles. Baseline ECG typically shows a short PR interval and slurred upstroke of the QRS (“delta” wave). Associated tachyarrhythmias are of two types:

• Narrow QRS “orthodromic” tachycardia (antegrade conduction through AV node). Treat cautiously with IV adenosine or beta blocker, verapamil, or diltiazem (Table 123-2).
• Wide QRS complex “antidromic” tachycardia (antegrade conduction through accessory pathway); wide complex tachycardia may also be associated with AF with a very rapid (>250/min) ventricular rate, which can degenerate into VF. If hemodynamically compromised, immediate cardioversion is indicated; otherwise, treat with IV procainamide or ibutilide (Table 123-2), not digoxin, beta blocker, or verapamil.

Consider catheter ablation of accessory pathway for long-term prevention.