CONTENTS

• RBBB (right bundle branch block)
  • Diagnosis of RBBB
  • Axis
  • Chambers
  • Morphology in leads with RSR'
  • Morphology in other leads
  • Causes & significance of RBBB
• RBBB patterns with MI
  • RBBB + Anterior MI
• Shark Fin MI patterns
  • Anterior shark fin (RBBB + LAFB + AMI)
  • Inferior shark fin (RBBB + LPFB + IMI)

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RBBB (right bundle branch block)

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diagnostic criteria for RBBB

[1/4] QRS >120 ms (otherwise may be incomplete RBBB: 📖)

• This can be difficult to determine.
• Consider erring on calling RBBB if:
  • The terminal S-wave is blunted.
  • There is unusual QRS notching.
  • There is TWI in the right precordial leads consistent with RBBB.

[2/4] V1 and/or V2 has one of the following

• [a] rsr', rsR' or rSR'. The R' or r' is usually wider than the initial R-wave. 
• [b] Wide R-wave, which is often notched (>50 ms to peak, with normal R peak time in V5-V6)
  • This includes a qR pattern. qR in V1 suggests RBBB plus either anterior Q-wave MI or severe RVH. (19281930)

[3/4] Prolonged terminal S-wave in I and V6, either:

• S duration > R duration, or
• S duration > 40 ms.

[4/4] No clear evidence of RBBB mimics, such as:

• Short PR (consider WPW).
• QRS >~160 ms (4 boxes). This suggests hyperkalemia, Na blocker, or ventricular complexes. (3190044, 🌊)
• Features of hyperkalemia 📖 or Na-channel blocker 📖
  • QRS >160 ms (4 boxes).
  • QT may be extremely long (which may help sort these out from RBBB+LAHB).
  • Peaked T-waves (can occur in either).
  • P-waves is widened with increased PR interval (hyperkalemia).
  • Tall R-wave in aVR (R' >3 or R'>S) is classically associated with sodium-channel poisoning. However, this can occur in RBBB (especially with concurrent LAHB).
  • (Sorting this out is discussed further here: 📖)

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RBBB interpretation algorithm: axis

RBBB doesn't affect the net QRS axis, although it does produce some terminal right-axis deviation. When determining the axis focus on the initial 60-80 ms (ignore the terminal, low-voltage components at the end of the QRS complex). (O'Keefe 2021)

LAD has four major causes 

• [1] Fascicular VT. 📖
• [2] Inferior Q-wave MI.
  • Inferior complexes have a QS or Qr morphology. However, 1/3 of patients with prior IMI can regrow R-waves, so an absence of Q-waves doesn't exclude prior IMI.
  • QRS complex fragmentation suggests prior infarction.
• [3] RBBB + LAHB, is defined by:
  • [i] Frontal axis between -45° to -90°:
    • Positive in Lead I.
    • Negative in aVF.
    • Substantially net negative in Lead II.
  • [ii] qR in aVL (often the most positive deflection in this lead).
  • [iii] rS in the inferior leads.
  • 💡 Clinical significance:
    • [i] RBBB and LAHB commonly occur together because they share perfusion by the LAD.  New-onset RBBB with LAHB may reflect a proximal LAD occlusion.
    • [ii] RBBB + LAHB + prolonged PR (“trifascicular block”) may be an indication for pacemaker insertion if combined with symptoms of syncope.
• [4] LVH:
  • May coexist with LAHB (these aren't mutually exclusive).
  • More on LVH below 👇

RAD has three major causes

• [1] Lateral Q-wave MI.
• [2] RVH and/or PE.
• [3] LPFB:
  • rS in Lead I (wave with considerable negativity).
  • qR in III and aVF.
  • ⚠️ rS in Lead I and qR in Lead III may also be caused by right ventricular enlargement. Thus, as always, LPFB is a diagnosis of exclusion.
  • 💡 Clinical significance: RBBB plus LPFB is a bifascicular block. In the context of symptoms, this may indicate the insertion of a permanent pacemaker.

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RBBB interpretation algorithm: chambers

atria: unaffected

• LAA 📖
• RAA 📖
• Biatrial abnormality 📖

RVH suggested if:

• P-wave criteria for RAA.
• Right axis deviation with terminal S-wave in lead I.
• V1 with either:
  • R' >12-15 (suggests RVH or posterior MI, especially in the absence of globally high voltages due to LVH). (O'Keefe)
  • qR (suggests RVH or prior anterior MI).
• RV strain pattern: TWI and STD extending beyond leads with RSR' (e.g., V3-V4) and deeper than expected for isolated RBBB.

LVH:

• RBBB generally doesn't interfere substantially with the diagnosis of LVH. (O'Keefe 2021) LVH may be suggested if the following are encountered:
• R in aVL >7.5-12 mm (7.5 cutoff has greater sensitivity; 12 has greater specificity). (2523183)
• Romhilt-Estes score minus the QRS duration criterion (≧4 indicates probable LVH; this score retains specificity but has reduced sensitivity in RBBB). (2523183)
  • Voltage criteria (3 points): any of the following
    • R or S in limb leads ≧20 mm.
    • S in V1 or V2 ≧30 mm.
    • R in V5 or V6 ≧30 mm.
    • (⚠️ Voltage decreases with age and obesity.)
  • ST-T shows LV strain pattern: 3 points (1 with digoxin).
  • Left atrial enlargement in V1 (terminal P-wave is >40ms and >1 mm): 3 points.
  • Left axis deviation: 2 points.
  • Delayed intrinsicoid deflection in V5 or V6 (>50 ms): 1 point.

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RBBB interpretation algorithm: leads with rSR' (e.g., V1-V3) 

in leads with rSR' pattern: 

• Expected findings:
  • STD is shallow (<1 mm) unless R' is huge.
  • STD is gently down-sloping.
  • (STD in V1) > ( STD in V2) > (STD in V3).
  • TWI is shallow.
• Deviations raise the possibility of ischemia:
  • Any degree of STE may represent a substantial elevation (relative to baseline).
  • Upright or flat T-waves may suggest ischemia (e.g., posterior OMI).
• But, RBBB combined with pathologies may create confusion:
  • ⚠️ (RBBB + LVH) or (RBBB + BER) may create patterns that are impossible to diagnose on a single ECG. In the presence of unusually high voltages, STE may be less specific for ischemia.
  • ⚠️ (RBBB plus diffuse subendocardial ischemia) may create dramatic ST depression in V1-V3 that mimics a posterior MI.

if there are Q-waves (qR pattern in V1): 

• Are Q-waves pathological? RBBB may cause Q-waves in V1-V2 (especially in the setting of RV overload), but they shouldn't extend past V2 or be wide.
• Interpreting RBBB with pathological Q-waves in V1-V3:
  • ⚠️ It may be impossible to differentiate acute anterior MI versus chronic anterior aneurysm in this context (both may cause STE).
  • Features that suggest a chronic aneurysm:
    • An expected amount of TWI in V1-V3.
    • Lack of other ischemic findings in the remainder of the ECG.
    • Stable ECG findings compared to prior ECGs. 🌊
  • Features that suggest active MI:
    • Upright T-waves in V1-V3.
    • STE & T-wave abnormalities extend beyond leads with deep Q-waves.

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RBBB interpretation algorithm: morphology in other leads

• RBBB shouldn't generally confound the interpretation of the remainder of the ECG.

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causes & clinical significance of RBBB

new-onset RBBB

• Acute PE.
• Anterior MI 📖 involving the proximal LAD, especially with associated LAHB.
• PA catheterization.
• Rate-related aberrancy: the right bundle is longer than the left bundle, making it more likely to suffer from rate-related conduction block.
• Myocarditis.

chronic causes of RBBB

• Longstanding RV strain (e.g., COPD, pulmonary hypertension).
• Conduction system disease (e.g., degenerative disease of the conduction system).
• Hypertensive heart disease.
• Cardiomyopathy.
• Normal adults (~1/500) – unlike LBBB, RBBB can be seen in people without underlying structural heart disease. (O'Keefe 2021)

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anterior MI + RBBB

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differential diagnosis:  

• (1) LAD infarct + new RBBB
  • It is uncommonly seen in anterior MI, but may indicate a severe proximal infarct affecting the septal perforators (which supply the RBBB and LAHB, or less commonly the LPFB).
  • ECG findings = RBBB plus proximal LAD occlusion:
    • Loss of septal Q-waves in lateral leads
    • STE in V1-V3 or V1-V4, aVR, and aVL
    • STD may occur in inferior leads, V5-V6
• (2) LAD infarct + old RBBB
  • ECG findings:  RBBB plus an anterior MI.  Unlike #1, the anterior MI is located in the mid- or distal LAD.
• (3) RBBB plus anterior aneurysm. 📖
  • This combination may very closely mimic RBBB plus anterior infarction.
• (4) Hyperkalemia. 📖

clinical significance of anterior MI plus new RBBB

• This is very bad, since it reflects a large proximal LAD infarction.
• Bifascicular block refers to RBBB plus either LAFB or LPFB (usually RBBB + LAFB).  Bifascicular block with an acute anterior MI carries up to 30% risk of progression to third degree block.
  • Consider the need for transvenous pacing or transcutaneous pacemaker pad placement.

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Shark Fin MI patterns

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• Shark Fin MI patterns refer to ECGs with broad QRST complexes that resemble fins. They are often caused by a combination of ST elevation, hyperacute T-waves, and a bundle branch block.
• If the end of the QRS complex can be identified on one lead, it can be used to time-stamp the end of the QRS and find other leads using vertical lines.

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anterior shark fin: RBBB + LAHB + anterior MI

• A fusion of several components generates shark fins:
  • RBBB and LAHB create broad positive complexes in the right precordial leads.
  • Hyperacute T-waves also generate positive deflections.
• The summation of these forces leads to a shark-fin appearance. This is similar to the more classic “tombstone” appearance, but the presence of a bundle-branch block causes the upslope to be more gradual than that of straight “tombstones.”

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inferior shark fin: RBBB + LPFB + inferior MI 

• “Shark Fin” morphology can be generated by a superposition of the following three components:
  • RBBB plus LPFB generates an upright, wide QRS complex in the inferior leads.
  • STE in the inferior leads.
  • Hyperacute T-waves in the inferior leads.

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questions & discussion

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