What causes a slurring delta wave immediately after the QRS (quantitative racial sensitivity) complex on an electrocardiogram (ECG)?

What Causes a Slurring Delta Wave Right After QRS on ECG

A slurring delta wave immediately after the QRS complex is caused by ventricular pre-excitation through an accessory pathway (Kent bundle) that bypasses the AV node, creating the characteristic ECG pattern of Wolff-Parkinson-White (WPW) syndrome. 1, 2

Pathophysiology of the Delta Wave

The delta wave represents the fusion of two competing electrical wavefronts in the ventricles 1:

• Early ventricular activation occurs through the accessory pathway, which conducts rapidly without the normal AV nodal delay 1, 2
• Simultaneous normal conduction proceeds through the AV node and His-Purkinje system 1
• Collision of these wavefronts creates the characteristic slurred upstroke at the beginning of the QRS complex, known as the delta wave 1, 2

The degree of pre-excitation varies based on the relative contribution from each pathway, resulting in variable delta wave prominence 1.

Classic ECG Findings in WPW Syndrome

The European Society of Cardiology defines the diagnostic triad as: 2

• PR interval < 120 ms (shortened because the accessory pathway bypasses the AV node) 2, 3
• Delta wave (slurred, slow upstroke at the initial portion of the QRS) 1, 2, 3
• QRS duration > 120 ms (widened due to fusion of the two wavefronts) 2, 3, 4

Secondary repolarization changes with ST-T wave abnormalities directed opposite to the delta wave are also typically present 3, 4.

Anatomical Substrate

The accessory pathway is a direct muscular connection between atrium and ventricle across the AV groove 1:

• Most commonly located at the left free wall (67% in one series), followed by right free wall (21%) and septal regions (12%) 5
• Conducts non-decrementally in most cases, allowing rapid transmission without the protective delay of the AV node 1
• Can conduct anterogradely (manifest pathway causing visible delta wave), retrogradely only (concealed pathway), or bidirectionally 1

Important Clinical Distinctions

Delta Wave vs. Terminal QRS Notching

Critical pitfall: Do not confuse the delta wave of WPW with terminal QRS notching seen in early repolarization pattern 1, 6:

• Delta wave occurs at the beginning of the QRS complex as a slurred upstroke 1, 2
• Terminal QRS notch occurs at the end of the QRS complex and represents a different phenomenon related to early repolarization 1, 6

Delta Wave vs. Epsilon Wave

The delta wave must also be distinguished from the epsilon wave of arrhythmogenic right ventricular cardiomyopathy (ARVC) 1:

• Epsilon wave is a low-amplitude, low-frequency deflection occurring after the QRS complex in leads V1-V3 1
• Delta wave is the slurred initial portion of the QRS complex 1, 2

WPW vs. Incomplete RBBB

In right precordial leads, WPW can mimic incomplete right bundle branch block 1:

• True RBBB shows reciprocal S-waves in leads I and V6 with comparable voltage and duration 1
• WPW pattern shows delta waves confined to specific leads without the reciprocal changes of RBBB 1

Associated Conditions

WPW syndrome has increased prevalence in certain structural heart diseases 1:

• Ebstein's anomaly of the tricuspid valve 1
• Hypertrophic cardiomyopathy 1
• Glycogen storage cardiomyopathy (PRKAG2 mutations) 1
• L-transposition of the great arteries 1
• Cardiac tumors 1

Clinical Significance and Risk Stratification

The presence of a delta wave indicates manifest pre-excitation and potential risk for life-threatening arrhythmias 1:

• Paroxysmal supraventricular tachycardia is the most common arrhythmia 1, 3
• Pre-excited atrial fibrillation can degenerate to ventricular fibrillation if the accessory pathway has a short refractory period 1
• Sudden cardiac death risk exists, particularly with rapid conduction during atrial fibrillation 1, 3

Risk Assessment Features 1

• Intermittent pre-excitation or loss of delta wave during exercise suggests a long refractory period and lower risk 1
• Persistent pre-excitation during exercise or with adenosine/verapamil requires electrophysiologic study 1
• Family history of sudden death, syncope, or palpitations increases concern 1

Recommended Evaluation

All patients with delta waves require comprehensive cardiac evaluation 1:

• Echocardiography to exclude associated structural heart disease (Ebstein's anomaly, HCM, glycogen storage disease) 1
• Exercise testing to assess for intermittent pre-excitation and arrhythmia inducibility 1
• 24-hour Holter monitoring to detect paroxysmal arrhythmias 1
• Electrophysiologic study (transesophageal or intracardiac) for definitive risk stratification, including assessment of accessory pathway refractory period and inducibility of tachycardia 1
• Family screening with ECGs in first-degree relatives, particularly siblings 1

Key Clinical Pearls

• Intermittent delta waves are not uncommon in newborns and infants, and may be subtle, detected only in mid-precordial leads 1
• Early repolarization can coexist with WPW syndrome in up to 43% of cases, though the mechanisms may be partially related to the accessory pathway itself 5
• Asymptomatic pre-excitation (isolated delta wave without documented arrhythmias) still warrants evaluation, as symptoms may develop later 1
• Catheter ablation is definitive therapy and should be considered based on electrophysiologic study results and patient symptoms 1