What are the ECG differences between Right Bundle Branch Block (RBBB) and posterior Myocardial Infarction (MI)?

ECG Differentiation Between RBBB and Posterior MI

The key distinction is that RBBB causes ST-T abnormalities in leads V1-V3 as a secondary repolarization effect, while posterior MI produces ST-segment depression in V1-V3 with a prominent R wave and upright terminal T waves (representing a "mirror image" of posterior ST elevation). 1

Primary ECG Features of RBBB

QRS Complex Characteristics:

• Wide QRS complexes (≥0.12 seconds) with characteristic RSR' pattern in V1-V2 1
• ST-T abnormalities in leads V1-V3 are expected and common as part of the bundle branch block pattern itself 1
• These secondary repolarization changes make it inherently difficult to assess for ischemia in the right precordial leads 1

Primary ECG Features of Posterior MI

ST-Segment and R Wave Characteristics:

• Horizontal ST-segment depression ≥0.05 mV in leads V1-V3, which represents the "mirror image" of posterior ST elevation 1
• Prominent R waves in leads V1-V3 (the reciprocal of posterior Q waves) 1
• Upright terminal T waves in the anterior precordial leads—this is a critical distinguishing feature, as the positive terminal T wave represents an ST-elevation equivalent 1
• ST elevation ≥0.05 mV in posterior leads V7-V9 (≥0.1 mV in men >40 years old) confirms the diagnosis 1

Critical Clinical Pitfall: Posterior MI Superimposed on Chronic RBBB

This is the most dangerous scenario clinically:

• A patient with known chronic RBBB can develop acute posterior MI, which manifests as ST-segment depression in V1-V2 that is out of proportion to their baseline RBBB pattern 2
• The key is comparing to a prior ECG—new or worsening ST depression beyond the patient's baseline RBBB-related changes suggests acute ischemia 1, 2
• When new ST elevation or Q waves appear in any leads in an RBBB patient with chest pain, acute MI must be considered immediately 1

Diagnostic Algorithm

Step 1: Assess QRS Duration and Morphology

• QRS ≥0.12 seconds with RSR' in V1 = RBBB present 1
• Normal QRS duration with prominent R waves in V1-V3 = consider posterior MI 1

Step 2: Evaluate ST-Segment Pattern in V1-V3

• ST-T abnormalities expected with RBBB alone (secondary repolarization) 1
• Horizontal ST depression with upright terminal T waves = posterior MI pattern 1

Step 3: Obtain Posterior Leads V7-V9

• Strongly recommended when clinical suspicion for circumflex occlusion exists or when initial ECG shows ST depression in V1-V3 1
• ST elevation ≥0.05 mV in V7-V9 confirms posterior MI 1

Step 4: Compare to Prior ECG

• Essential for patients with known RBBB to identify new changes suggesting acute ischemia 1, 2
• New ST elevation or Q waves in any leads mandate consideration of acute MI 1

Management Implications

RBBB with Acute MI Requires Immediate Reperfusion:

• RBBB patients with acute MI have a 64% increased odds of in-hospital death compared to those without bundle branch block 1, 3
• These patients are systematically undertreated—only 32% receive fibrinolytic therapy versus 65.5% without BBB 1, 3
• Do not dismiss chest pain in RBBB patients because ST-segments are difficult to interpret 3
• RBBB should trigger the same urgency for reperfusion as LBBB when acute MI is suspected 1, 4

Posterior MI Recognition:

• Isolated posterior MI should be treated as a STEMI with immediate reperfusion therapy 1
• The ST depression in V1-V3 with upright terminal T waves is an ST-elevation equivalent 1
• Emergency coronary angiography is indicated when ongoing ischemic symptoms persist despite medical therapy, even without diagnostic ST elevation 1