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

Notched Ascending S Wave on ECG

A notched ascending S wave most commonly indicates left bundle branch block (LBBB), particularly when seen in the right precordial leads (V1-V4), and serves as a key diagnostic feature distinguishing ventricular conduction abnormalities from normal conduction. 1

Primary Diagnostic Significance

Left Bundle Branch Block

• Notched or plateau-topped R waves in leads I, aVL, V5, and V6, combined with notched S waves in right precordial leads (V1-V4), are pathognomonic for complete LBBB. 1
• The notching pattern reflects abnormal ventricular depolarization sequence, with delayed left ventricular activation causing fragmented QRS morphology 1
• Strict LBBB definitions require monophasic notched R waves in lateral leads alongside the characteristic notched S waves in right precordial leads 1

Ventricular Tachycardia Differentiation

• Notching on the downstroke of the S wave in V1 or V2 is a highly specific criterion (94-100% specificity) for ventricular tachycardia (VT) when evaluating wide complex tachycardias with LBBB morphology. 2
• This finding has 96-100% predictive accuracy for VT, though sensitivity is only 30-64% when used alone 2
• When combined with other criteria (R wave >30ms in V1/V2, any Q in V6, or >60ms from QRS onset to S wave nadir), sensitivity increases to 100% with 89% specificity 2

Secondary Diagnostic Considerations

Myocardial Infarction in LBBB

• Notched S waves in V3 or V4 suggest anterior myocardial infarction in patients with established LBBB. 3
• This finding has high diagnostic specificity but relatively low sensitivity for detecting anterior infarction 3
• Pathologic Q waves or ST shifts in inferior leads similarly indicate inferior infarction with high specificity 3

Concealed Right Bundle Branch Block

• A notched S wave in V1 may occasionally represent atypical or concealed right bundle branch block (RBBB), particularly when accessory right precordial leads (V3R-V5R) show triphasic QRS patterns with terminal R' waves 4
• This pattern suggests slowed conduction or delayed activation of the right ventricular conduction system 4

Clinical Context and Implications

Cardiac Resynchronization Therapy

• The presence of S waves (including notched S waves) in leads V5 and V6 in patients with CLBBB predicts poor response to cardiac resynchronization therapy 5
• Patients with S waves in both V5 and V6 have significantly lower CRT response rates (38.5%) compared to those without (85.2%) 5
• This finding also correlates with higher rates of heart failure rehospitalization and all-cause mortality 5

Ventricular Hypertrophy Assessment

• When LBBB is present with notched S waves, standard voltage criteria for left ventricular hypertrophy become unreliable. 1
• QRS duration >155ms combined with precordial voltage criteria maintains relatively high specificity for LVH in LBBB 1
• The presence of prominent S waves in V5 or V6 alongside LVH criteria may suggest biventricular hypertrophy 1

Critical Diagnostic Algorithm

When encountering a notched ascending S wave:

1. Determine QRS duration and lead location:

   • If QRS ≥120ms with notched S in V1-V4 and notched R in I, aVL, V5-V6 → Diagnose complete LBBB 1
   • If wide complex tachycardia with notched S in V1-V2 → Consider VT (96-100% predictive accuracy) 2

2. Assess for associated findings:

   • Check for Q waves in V6 or prolonged R-S interval in V1-V2 to strengthen VT diagnosis 2
   • Evaluate for pathologic Q waves or ST changes suggesting MI in context of LBBB 3

3. Consider clinical implications:

   • If CLBBB with S waves extending to V5-V6 → Anticipate poor CRT response 5
   • If isolated notched S in V1 with normal QRS → Consider accessory lead evaluation for concealed RBBB 4

Important Caveats

• The distinction between "complete" LBBB and LVH with intraventricular conduction delay can be difficult, as QRS duration in LVH may exceed 120ms without localized bundle lesions 1
• Broader LBBB definitions that don't require strict notching criteria may include cases better classified as LVH with conduction delay 1
• Age, gender, race, and body habitus should be considered when interpreting voltage criteria in the presence of conduction abnormalities 1