What does a notched S wave on an electrocardiogram (ECG) indicate?

Notched S Wave on ECG

A notched S wave represents a terminal QRS notch—a low-frequency deflection at the end of the QRS complex that is part of the early repolarization pattern and requires differentiation from pathological conditions including arrhythmogenic right ventricular cardiomyopathy (epsilon waves), Brugada syndrome, and early repolarization syndrome associated with sudden cardiac death risk. 1, 2

Definition and ECG Characteristics

• A terminal QRS notch is defined as a low-frequency deflection appearing at the end of the QRS complex, initially described by Osborn as J waves referring to injury 1
• The J point should be measured at the peak of the notch when present, marking where the QRS ends and ST segment begins 1, 2
• Terminal QRS notching is considered part of the QRS complex itself according to the Common Standards of Electrocardiography Working Party 1
• This pattern is commonly seen in early repolarization, which occurs in 50-80% of highly trained athletes and 1-10% of the general population 1

Clinical Significance and Risk Stratification

The presence of terminal QRS notching carries variable risk depending on clinical context and associated features:

• Survivors of idiopathic sudden cardiac arrest demonstrate terminal QRS notching at significantly higher frequency than controls, and this pattern has been associated with increased cardiovascular and all-cause mortality in population studies 1, 2, 3
• When J-point elevation ≥1 mm accompanies the notching in inferior or lateral leads (≥2 contiguous leads), risk may be elevated for ventricular fibrillation, particularly with symptoms like syncope or family history of sudden death 2, 4
• The pattern resembles Brugada syndrome in arrhythmogenicity under certain conditions that predispose to ST-segment elevation 1, 3

Critical Differential Diagnoses

You must distinguish terminal QRS notching from several pathological patterns:

Epsilon Waves (ARVC/D)

• Epsilon waves are low-frequency terminal QRS deflections in anteroseptal precordial leads (V1-V3) seen in arrhythmogenic right ventricular cardiomyopathy 1, 2
• Classical ARVC findings include T-wave inversions in right precordial leads accompanying the epsilon waves 4

Brugada Pattern

• Brugada pattern shows downward coved or saddleback ST-segment elevations in leads V1-V3 associated with sudden cardiac death 1, 2
• Unlike terminal QRS notching, the J-wave in Brugada is confined to right precordial leads without reciprocal S-wave in leads I and V6 1
• Only type I Brugada pattern (spontaneous or drug-induced) is diagnostic; drug challenge with sodium channel blockers may be needed for definitive diagnosis 1

Wolff-Parkinson-White Syndrome

• WPW shows slurred QRS upstroke (delta wave) mimicking terminal notching but represents pre-excitation 4, 5
• The delta wave occurs at the beginning of the QRS, not the terminal portion 5

Evaluation Algorithm

When you encounter a notched S wave, follow this systematic approach:

1. Assess lead distribution and morphology:

   • Determine if notching is in inferior leads (II, III, aVF), lateral leads (I, aVL, V5-V6), or right precordial leads (V1-V3) 1, 2
   • Measure J-point elevation (≥1 mm is significant) 2

2. Evaluate for high-risk features:

   • Syncope, palpitations, or cardiac arrest history 2, 4
   • Family history of sudden cardiac death or early repolarization syndrome 2, 3
   • Symptoms occurring during rest or sleep (higher risk pattern) 3

3. Distinguish from pathological patterns:

   • Check for epsilon waves in V1-V3 with T-wave inversions (suggests ARVC) 1, 4
   • Look for coved ST-elevation in V1-V2 (suggests Brugada) 1
   • Examine for initial QRS slurring (suggests WPW) 4, 5

4. Risk stratify based on clinical context:

   • Asymptomatic with isolated finding: Generally benign, particularly in athletes; odds of malignant arrhythmias are extremely low 1, 3
   • With symptoms or family history: Requires complete cardiac workup including echocardiography, ambulatory monitoring, and possible electrophysiology consultation 2, 3
   • With J-point elevation ≥1 mm in inferior/lateral leads: Consider higher risk, especially if symptomatic 2

Management Recommendations

• Asymptomatic patients: Reassurance is appropriate; avoid generating unnecessary anxiety as the absolute risk remains very low 3
• Symptomatic patients or concerning family history: Obtain echocardiography to exclude structural heart disease, ambulatory ECG monitoring for arrhythmia detection, and cardiology referral 2, 4
• Confirmed early repolarization syndrome (with idiopathic VF): Implantable cardioverter-defibrillator is indicated 3

Common Pitfalls

• Misinterpreting technical artifacts or poor electrode placement as true terminal QRS notching 2
• Failing to compare with prior ECGs—changes over time are more significant than isolated findings 2
• Overlooking the distinction between benign early repolarization in athletes versus potentially pathological patterns—in athletes of African/Caribbean origin, ST-elevation with T-wave inversion in V2-V4 is physiological 1
• Generating excessive anxiety in asymptomatic individuals with isolated findings—the vast majority will never experience adverse events 3
• Missing ARVC by not recognizing epsilon waves—these require specific attention in right precordial leads with associated T-wave inversions 1, 4