Because of the emergency nature of the arrhythmias associated with WPW syndrome, nurses are often called upon for diagnosis and intervention in critical settings. In such cases the nurse's understanding of mechanisms, ECG recognition, and emergency treatment guarantees the patient the best possible outcome, not only in the critical setting, but in the long term as well. The most common arrhythmias of WPW syndrome are PSVT and atrial fibrillation. In PSVT a differential diagnosis is made on the ECG between (1) CMT using the AV node anterogradely and an accessory pathway retrogradely and (2) AV nodal reentry tachycardia. Helpful clues are location of the P' wave, presence of QRS alternans, the initiating P'R interval, and presence of aberrancy. Atrial fibrillation with an accessory pathway has the morphology of VT but is differentiated because the rhythm is irregular and the rate is more than 200 beats per minute. Emergency treatment consists of blocking the accessory pathway with procainamide. Emergency treatment for both types of PSVT consists of breaking the reentry circuit at the AV node (eg, vagal maneuver, adenosine, or verapamil). Procainamide can also be used to block the retrograde fast pathway in the AV node and to terminate CMT by blocking the accessory pathway. Symptomatic patients with accessory pathways are referred for evaluation and possible radio-frequency ablation.