Understanding the ECG. Part 5: Pre-excitation

• Pre-excitation is caused by an accessory pathway, a strand of myocardium that joins the atria to the ventricles, providing an alternative route of conduction for the electrical impulse. It is often associated with paroxysmal atrioventricular re-entrant tachycardia (AVRT) or atrial fibrillation (AF), in which case it is referred to as Wolff-Parkinson-White syndrome (WPW). It is a congenital condition, often found in otherwise normal hearts. • The ECG features of WPW are a short PR interval, delta wave, wide QRS, and ST and T-wave abnormalities. Changes in ventricular activation also alter the pattern of QRS complexes, and may mimic myocardial infarction. QRS pattern may be helpful in predicting pathway location. • WPW is associated with a 3-4% lifetime risk of sudden cardiac death (SCD). The mechanism for sudden death is rapid conduction of AF to the ventricles, with subsequent degeneration to ventricular fibrillation (VF). Patients that present with pre-excited AF should not be treated with AV nodal blocking agents, as this can increase the heart rate and risk of VF. • Catheter ablation is the first line treatment for symptomatic WPW, and has a 95% success rate with a complication rate of 2-4%. The risk of some complications is universal, the risk of others depends on the location of the accessory pathway. • The treatment of asymptomatic patients is more difficult, and depends on the risk of SCD. Pathway conduction properties and location should be considered, as well as patient age, preference, occupation and leisure pursuits.