CardiologyTier 1Disease (DEADMAN)

Cardiac arrhythmias - bundle-branch and fascicular block

Description

Conduction delay below the His bundle, in one or more of three fascicles.

The trifascicular system

Right bundle - single, thin, subendocardial -> easily injured, hence RBBB is common and often benign
Left bundle - fans into left anterior fascicle (thin, single blood supply, anterosuperior) and left posterior fascicle (thick, dual blood supply, posteroinferior)
- -> LAFB common, LPFB rare (isolated LPFB implies extensive disease)

Terminology

Term	Meaning
Bifascicular block	RBBB + LAFB (commonest), RBBB + LPFB, or LBBB alone
"Trifascicular block"	Bifascicular block + first-degree AV block. A misnomer - the long PR is usually AV nodal, not the third fascicle. Do not use it to justify pacing
Incomplete	QRS 110-119 ms with the same morphology
Rate-related / aberrancy	Block appearing only above a critical rate
Masquerading BBB	RBBB in V1 with LBBB pattern in the limb leads - severe conduction disease

RBBB ~1-2% of adults, rising with age; often in structurally normal hearts
LBBB ~0.4%, and almost always implies underlying structural heart disease
LAFB ~1-2%; LPFB <0.1%
Bifascicular block ~1-1.5% of adults >60
Progression of bifascicular block to complete AV block: only ~1-2%/yr

RBBB - causes

Normal variant (esp. incomplete RBBB in young)
Right heart strain/volume load - ASD (with LAD = ostium primum; with RAD = secundum), PE (acute RBBB + S1Q3T3), pulmonary hypertension, cor pulmonale
Ischaemic heart disease, cardiomyopathy, myocarditis
Degenerative conduction disease
Brugada syndrome (RBBB-like coved ST elevation V1-V2), ARVC
Iatrogenic - right heart catheterisation (transient; dangerous if pre-existing LBBB -> complete block)

LBBB - causes

*Rarely normal* - assume structural disease
IHD, dilated cardiomyopathy, hypertensive heart disease, aortic valve disease, myocarditis, infiltration, degenerative fibrosis
New LBBB with ischaemic symptoms = treat as a STEMI equivalent pathway (activate the cath lab; note "new LBBB" alone is no longer a STEMI criterion - use Sgarbossa)

LAFB / LPFB

RBBB - criteria

QRS >120 ms
rSR' / RSR' ("M" or rabbit-ear) pattern in V1-V3
Wide slurred S wave in the lateral leads I, aVL, V5-V6
Secondary T wave inversion in V1-V3 (normal - do not call ischaemia)
Mechanism: LV depolarises normally, then the RV is activated late through myocardium

LBBB - criteria

QRS >120 ms
Broad monophasic R wave with absent S in I, aVL, V6
Deep S (QS or rS) in V1
Absent septal q waves in I, V5-V6 (reversed septal activation)
Secondary ST-T changes discordant with the terminal QRS deflection
Left axis common
"Strict" LBBB (for CRT selection): QRS >=140 ms (M) / >=130 ms (F), QS or rS in V1-V2, mid-QRS notching/slurring in >=2 of I, aVL, V1, V2, V5, V6

Fascicular blocks - QRS is NOT wide (<120 ms)

	LAFB	LPFB
Axis	Left (-45 to -90)	Right (+90 to +180)
Leads I, aVL	qR	rS
Leads II, III, aVF	rS	qR
Other	Poor R progression V1-V3	Diagnosis of exclusion - must exclude RVH, lateral MI, PE, vertical heart

Diagnosing MI or ischaemia in LBBB/paced rhythm - Sgarbossa criteria

Criterion	Points
Concordant ST elevation >=1 mm	5
Concordant ST depression >=1 mm in V1-V3	3
Excessively discordant ST elevation >=5 mm	2

>=3 points = specific for MI (spec ~90%, sens poor)
Modified Sgarbossa (Smith): replaces the third criterion with ST/S ratio <= -0.25 - better sensitivity
The same applies to ventricular-paced rhythm

Work-up of new LBBB

A. Asymptomatic block - do not pace

RBBB alone - reassure; no restriction
LBBB alone - investigate for structural disease and treat that; the block itself needs no therapy
Bifascicular block - progression to complete block only 1-2%/yr; no prophylactic pacing
- Including with a long PR ("trifascicular"). Symptoms, not morphology, drive the decision

B. Syncope with bifascicular block - this is the exam scenario

Presumed intermittent complete heart block until excluded
Options:
- EPS with HV interval: HV >70 ms -> permanent pacing (Class I); HV >100 ms or infra-His block on atrial pacing is definite
- Implantable loop recorder if EPS negative
- Empirical pacing is reasonable in recurrent unexplained syncope with bifascicular block
Do not accept "vasovagal" in a patient with bifascicular block and sudden syncope

C. Alternating bundle branch block

D. Post-procedural

New LBBB after TAVI - high risk of late high-grade AV block; monitor, ambulatory ECG, low threshold for pacing
New block after cardiac surgery - observe; many resolve within a week

E. Heart failure implications

LBBB + LVEF <=35% on optimal GDMT >=3 months -> assess for CRT (Class I if QRS >=150 ms)
- This is the one setting where the bundle branch block itself is treated
RBBB does not respond to CRT in the same way - benefit is marginal

F. Practical

LBBB makes exercise ECG uninterpretable -> use stress echo or perfusion imaging (and even then, septal perfusion defects are common false positives; prefer vasodilator stress or stress echo)
LBBB obscures LVH criteria and Q waves

ASD - incomplete RBBB with right axis (secundum) vs left axis (primum)
PE - new RBBB, S1Q3T3, right axis
Brugada syndrome - RBBB-like coved ST elevation V1-V2, no wide S in lateral leads
ARVC - epsilon wave, T inversion V1-V3, RBBB morphology VT
Myotonic dystrophy, LMNA cardiomyopathy, sarcoidosis - progressive conduction disease
Aortic stenosis and TAVI - LBBB
Hyperkalaemia - diffuse QRS widening mimicking BBB
Lev/Lenegre degenerative disease, Chagas (RBBB + LAFB is classic)

RBBB in a structurally normal heart: benign, no excess mortality
- RBBB acquired later in life is associated with modestly increased cardiovascular mortality - reflecting the disease that caused it
LBBB: independently associated with increased cardiovascular mortality, HF and SCD - the marker, not the mechanism
- Can itself cause LBBB-induced cardiomyopathy (dyssynchrony-mediated), which may reverse with CRT/conduction system pacing
Bifascicular block: 1-2%/yr to complete block; most deaths in this group are from the underlying disease or VT, not bradycardia
Monitor: annual review, symptom enquiry (syncope, presyncope), ECG, and echo if new symptoms or a change in QRS

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