RBBB Does Not Typically Mask ST Elevation
No, a slurred S wave in RBBB does not mask ST elevation—in fact, the European Society of Cardiology explicitly states that "RBBB usually will not hamper interpretation of ST-segment elevation" in the context of acute myocardial infarction. 1
Key Guideline Statements
The 2012 ESC Guidelines for STEMI management directly address this question and provide clear guidance:
RBBB preserves ST-segment interpretation: Unlike left bundle branch block (LBBB), which significantly complicates ECG diagnosis of acute MI, RBBB does not interfere with the ability to identify ST elevation 1
Prompt management remains essential: Patients with myocardial infarction and RBBB have a poor prognosis, and prompt management should be considered when persistent ischemic symptoms occur in the presence of RBBB, regardless of whether the RBBB is new or previously known 1
Why RBBB Doesn't Mask ST Elevation
The morphology of RBBB explains why ST segments remain interpretable:
The slurred S wave appears in lateral leads (I, V6), not in the leads where you're primarily looking for ST elevation (precordial leads V1-V4 for anterior MI, or inferior leads II, III, aVF) 1, 2
RBBB characteristics include: QRS duration ≥120 ms, rSR' pattern in V1-V2, and S waves of greater duration than R waves in leads I and V6 1, 2
The terminal forces are directed rightward and anteriorly, creating the characteristic pattern without obscuring the ST-T segments in most leads 1
Clinical Pitfall: Don't Confuse RBBB with LBBB
A critical distinction must be made:
LBBB DOES mask ST elevation: The ESC guidelines note that "in the presence of LBBB, the ECG diagnosis of acute myocardial infarction is difficult" and requires complex algorithms that don't provide diagnostic certainty 1
Concordant ST elevation in LBBB (ST elevation in leads with positive QRS deflections) is one of the best indicators of ongoing MI with an occluded infarct artery 1
RBBB is fundamentally different: The guidelines explicitly contrast RBBB with LBBB, stating that RBBB patients also have poor prognosis but the conduction abnormality doesn't hamper ST-segment interpretation 1
Rare Exception: "Masquerading" RBBB
There is one uncommon scenario where RBBB morphology can be altered:
Masquerading RBBB occurs when high-degree left anterior fascicular block with severe LV enlargement or anterolateral wall fibrosis causes the characteristic slurred S wave in lead I to become smaller or disappear 3
This pattern carries poor prognosis as it always implies severe underlying heart disease, but this is a distinct entity from typical RBBB 3
This is not relevant to routine ST elevation interpretation in typical RBBB patterns 3
Practical Approach
When evaluating an ECG with RBBB for possible STEMI:
Measure ST segments normally: Apply standard STEMI criteria (≥0.1 mV in limb leads, ≥0.2 mV in precordial leads for men >40 years) 1
Don't be falsely reassured by RBBB presence: The conduction abnormality doesn't protect against MI and may actually indicate worse prognosis 1
Consider emergency angiography if persistent ischemic symptoms are present with RBBB, even without diagnostic ST elevation 1
Compare to prior ECGs when available: New RBBB in the setting of chest pain should raise suspicion for acute MI 1