ECG Changes Indicating Pre-excitation
Look for a short PR interval (<120 ms), a delta wave (slurred upstroke of the QRS complex), and a widened QRS complex (>120 ms) on the 12-lead ECG. These are the classic electrocardiographic hallmarks of ventricular pre-excitation caused by a manifest accessory pathway 1.
Key ECG Features of Pre-excitation
The pre-excitation pattern reflects early ventricular activation through an accessory pathway that bypasses the normal AV node conduction system 1, 2. The characteristic findings include:
- Short PR interval: Typically <120 ms, reflecting rapid conduction through the accessory pathway rather than the slower AV node
- Delta wave: A slurred, slow initial upstroke of the QRS complex representing the fusion of ventricular activation from both the accessory pathway and the normal His-Purkinje system 1
- Widened QRS complex: Usually >120 ms due to the abnormal ventricular activation sequence 1
The degree of pre-excitation varies depending on the relative contribution of conduction through the normal AV nodal/His-Purkinje system versus the manifest accessory pathway 1.
Important Clinical Caveats
Pre-excitation can be intermittent or subtle, making it easy to miss on a single ECG 1. Even high-risk accessory pathways capable of rapid anterograde conduction may show minimal or inapparent pre-excitation during sinus rhythm 3, 4. In your patient with presyncope and SVT episodes, look for:
- Variation in PR interval >30 ms across different leads on the same ECG, which can help confirm the presence of an accessory pathway even when the delta wave is not obvious 4
- Intermittent pre-excitation: Abrupt loss of the delta wave with normalization of the QRS complex suggests a pathway with a relatively long refractory period (generally lower risk) 5
Critical Warning About Metoprolol
In this patient already on metoprolol therapy, be aware that beta-blockers can be dangerous in pre-excitation syndromes. While metoprolol slows AV nodal conduction 6, it does not affect accessory pathway conduction. If the patient develops atrial fibrillation with pre-excitation (pre-excited AF), beta-blockers like metoprolol will not slow conduction through the accessory pathway and may even be harmful by preferentially blocking the AV node, forcing more conduction down the dangerous accessory pathway 2. This can lead to extremely rapid ventricular rates and potentially ventricular fibrillation.
Additional ECG Clues
During tachycardia episodes, the ECG appearance depends on the mechanism:
- Orthodromic AVRT (most common, 90-95% of cases): Narrow QRS complex with retrograde P waves visible after the QRS, as the accessory pathway conducts only retrogradely during tachycardia 2
- Antidromic AVRT (5% of cases): Wide QRS complex (maximally pre-excited) as the accessory pathway conducts anterogradely during tachycardia 1
The presence of pre-excitation on ECG combined with documented SVT episodes defines Wolff-Parkinson-White (WPW) syndrome 1, which requires risk stratification and consideration for catheter ablation rather than continued medical management with metoprolol alone.