What is the clinical significance of a notched ascending S wave in lead V1 on an electrocardiogram (ECG)?

Notched Ascending S Wave in V1: Clinical Significance

A notched ascending S wave in lead V1 most commonly suggests incomplete or concealed right bundle branch block (RBBB), but requires systematic evaluation to exclude arrhythmogenic right ventricular cardiomyopathy (ARVC), which carries significant mortality risk. 1, 2

Primary Diagnostic Considerations

Right Bundle Branch Block Pattern

• A notched S wave in V1 typically represents delayed right ventricular activation or slowed conduction through the right ventricular conduction system. 1
• When this pattern appears in isolation in V1, it may represent "concealed" or incomplete RBBB, particularly if accessory right precordial leads (V3R-V5R) demonstrate a triphasic QRS pattern with a terminal R' wave. 1
• Unlike complete RBBB, incomplete RBBB shows an rSR' pattern in V1 with QRS duration <120 ms and is considered a normal finding in athletes requiring no further evaluation. 3

Arrhythmogenic Right Ventricular Cardiomyopathy

• The critical distinction is whether the notched S wave represents a prolonged S-wave upstroke, which is the most prevalent ECG feature of ARVC (present in 95% of cases). 2
• A prolonged S-wave upstroke in V1-V3 ≥55 ms is highly specific for ARVC and correlates with disease severity and inducibility of ventricular tachycardia. 2
• This finding distinguishes ARVC from benign right ventricular outflow tract (RVOT) tachycardia and should prompt immediate further evaluation. 2

Systematic Evaluation Algorithm

Step 1: Measure QRS Duration and S-Wave Characteristics

• Measure the S-wave upstroke duration in V1-V3: if ≥55 ms, ARVC must be excluded. 2
• Assess total QRS duration in V1-V3: if ≥110 ms without complete RBBB, this supports ARVC diagnosis (present in 64% of ARVC patients). 2
• Look for epsilon waves (small positive deflection between QRS and T wave in V1-V3), present in 33% of ARVC cases. 3, 2

Step 2: Evaluate Associated ECG Features

• Check for T-wave inversions in V1-V3 beyond lead V1 alone, which occur in 85% of ARVC patients without RBBB but are absent in normal individuals. 2
• Assess for PR interval prolongation ≥220 ms, which when combined with R' wave characteristics helps distinguish cardiac sarcoidosis from ARVC. 4
• Examine for prominent S waves in lateral leads (V5-V6), which may suggest biventricular involvement or right ventricular overload. 3, 5

Step 3: Obtain Accessory Right Precordial Leads

• Record leads V3R-V5R to determine if the notched pattern extends rightward with a terminal R' wave, confirming RBBB pattern versus ARVC. 1
• In RBBB, 87% of cases show a triphasic QRS with final R' wave in right accessory leads. 1

Clinical Context and Risk Stratification

High-Risk Features Requiring Immediate Workup

• Any patient with notched S wave in V1 plus T-wave inversions beyond V1, QRS duration ≥110 ms in V1-V3, or S-wave upstroke ≥55 ms requires echocardiography and cardiac MRI to exclude ARVC. 2
• Presence of epsilon waves is pathognomonic for ARVC and mandates comprehensive evaluation including electrophysiology study. 3, 2
• History of syncope, family history of sudden cardiac death, or ventricular arrhythmias elevates concern for life-threatening cardiomyopathy. 2

Lower-Risk Scenarios

• Isolated notched S wave in V1 with normal QRS duration (<110 ms), no T-wave inversions beyond V1, and S-wave upstroke <55 ms likely represents benign incomplete RBBB. 1, 2
• In athletes, incomplete RBBB with rSR' pattern and QRS <120 ms is a normal training-related finding requiring no further evaluation. 3

Critical Pitfalls to Avoid

• Never dismiss a notched S wave in V1 as benign without measuring the S-wave upstroke duration, as this is the most sensitive marker for ARVC. 2
• Do not confuse the terminal R' wave of RBBB with the epsilon wave of ARVC; epsilon waves are distinct low-amplitude signals occurring after the QRS complex. 3
• Avoid attributing T-wave inversions in V1-V3 to RBBB alone, as this combination strongly suggests ARVC in the absence of complete RBBB. 2
• A notched S wave that extends across multiple precordial leads (V1-V3) with prolonged upstroke is pathological until proven otherwise, regardless of patient age or athletic status. 2