CONTENTS
- RBBB (right bundle branch block)
- RBBB patterns with MI
- Shark Fin MI patterns
diagnosis of RBBB
diagnostic criteria for RBBB
- [1] 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] 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] Prolonged terminal S-wave in I and V6, either:
- S duration > R duration, or
- S duration > 40 ms.
- [4] No RBBB-mimic characteristics
- Short PR (consider WPW).
- QRS >~160 ms (4 boxes). This suggests hyperkalemia, Na blocker, or ventricular complexes. (3190044, 🌊)
- Peaked T-waves (consider hyperkalemia). 📖
- Sodium channel blocker: 📖
- 🔑 QRS and QT may be wider than expected for RBBB.
- 🔑 Terminal RAD (prominent S-waves in I and V6).
- 🔑 Tall R-wave in aVR (R' >3 or R'>S).
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 may be caused by:
- [1] Fascicular VT. 📖
- [2] RBBB + LAHB, which may be supported by:
- rS in the inferior leads.
- qR in aVL (often the most positive deflection in this lead).
- (⚠️ RBBB and LAHB commonly occur together because they share perfusion by the LAD. New-onset RBBB with LAHB may reflect a proximal LAD occlusion.)
- [3] 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.
- [4] LVH (more on this 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.
RBBB interpretation algorithm: chambers
atria: unaffected
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.
- Voltage criteria (3 points): any of the following
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.
RBBB interpretation algorithm: morphology in other leads
- RBBB shouldn't generally confound the interpretation of the remainder of the ECG.
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)
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.
- 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.
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.”
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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References
- 18426442 Adesanya CO, Yousuf KA, Co C, et al. Is wider worse? QRS duration predicts cardiac mortality in patients with right bundle branch block. Ann Noninvasive Electrocardiol. 2008;13(2):165-170. doi:10.1111/j.1542-474X.2008.00216.x [PubMed]
