Identifying Pre-Excitation on Baseline ECG
Pre-excitation on a baseline ECG is identified by three characteristic features: a short PR interval, a delta wave (slurring of the initial upstroke of the QRS complex), and a widened QRS complex, reflecting early ventricular activation via an accessory pathway that bypasses the normal AV node conduction. 1
Classic ECG Features of Pre-Excitation
The pre-excitation pattern results from ventricular activation occurring through both the normal AV nodal/His-Purkinje system and an accessory pathway simultaneously, creating a fusion complex with distinctive characteristics: 1
- Short PR interval: The accessory pathway conducts faster than the AV node, resulting in earlier ventricular activation 1
- Delta wave: This is the pathognomonic slurred, slow upstroke at the beginning of the QRS complex, representing initial ventricular depolarization spreading from the accessory pathway insertion point in the ventricular myocardium 1
- Widened QRS complex: The abnormal ventricular activation pattern produces a broader QRS than normal conduction 1
Variable Presentation and Important Caveats
Pre-excitation can be intermittent or subtle, and may not always be easily appreciated on every ECG, even when an accessory pathway capable of anterograde conduction is present. 1
The degree of pre-excitation visible on the ECG depends on the relative contribution of ventricular activation through the normal AV nodal system versus the accessory pathway: 1
- Manifest accessory pathways conduct anterogradely and produce visible pre-excitation on the resting ECG 1
- Concealed accessory pathways conduct only retrogradely and therefore show no pre-excitation pattern during sinus rhythm, appearing completely normal on baseline ECG 1
- Intermittent pre-excitation may appear on some ECGs but not others in the same patient 1, 2
Critical Clinical Implications
The presence of pre-excitation on a resting ECG in any patient with a history of paroxysmal regular palpitations is sufficient for the presumptive diagnosis of atrioventricular reentrant tachycardia (AVRT) and mandates immediate referral to a cardiac electrophysiologist. 1, 3, 4
- All patients with Wolff-Parkinson-White syndrome (pre-excitation combined with arrhythmias) are at potential risk for life-threatening arrhythmias
- Patients with baseline pre-excitation who develop irregular palpitations likely have atrial fibrillation, which can conduct rapidly over the accessory pathway and potentially degenerate to ventricular fibrillation and sudden death
- These patients require electrophysiological evaluation regardless of symptom severity
Common Pitfalls to Avoid
Do not rely on automatic ECG interpretation systems, as they are unreliable and commonly suggest incorrect diagnoses when pre-excitation is present. 1, 4
Additional important considerations: 5, 2
- Pre-excitation has "endless variations" in ECG appearance, and rigid thinking about "classic" WPW patterns can lead to missed diagnoses
- Failure to recognize subtle or atypical pre-excitation patterns may result in misdiagnosis as bundle branch block, myocardial infarction, or ventricular hypertrophy
- Even when pre-excitation is not easily visible, this does not exclude the presence of a potentially dangerous accessory pathway—clinical history of SVT should still prompt referral