Left Bundle Branch Block

Left bundle branch block (LBBB) is delayed or absent conduction through the left bundle, so the left ventricle is activated late from the right, with reversed septal activation.

Criteria for complete LBBB in sinus rhythm (strict criteria) are: QRS duration of 140 ms or more in men and 130 ms or more in women; QS or rS in V1 and V2; and mid-QRS notching or slurring in at least two of V1, V2, V5, V6, I and aVL.

The conventional criteria are a QRS of 120 ms or more, broad notched R waves in I, aVL, V5 and V6, no septal Q waves, and ST-T discordance. Incomplete LBBB has a QRS of 110 to 119 ms. LBBB is found in about 0.1 to 0.8% of the general population, and prevalence rises steeply with age.

Causes are ischaemic heart disease, hypertension, dilated cardiomyopathy, aortic valve disease (including after surgical or transcatheter replacement), degenerative fibrosis of the conduction system, infiltrative disease (sarcoidosis, amyloidosis), myocarditis, hyperkalaemia, septal myectomy or ablation, and idiopathic disease.

LBBB can itself cause dyssynchronous heart failure.

Pathophysiology. Septal activation is reversed and the lateral wall is activated late, giving septal flash, apical rocking, reduced ejection fraction, functional mitral regurgitation and, in some patients, a dilated cardiomyopathy that recovers with resynchronisation.

The late and prolonged ventricular activation changes repolarisation, giving ST-T discordance.

• Often asymptomatic.
• Symptoms of heart failure, ischaemic heart disease or aortic stenosis.
• Syncope from paroxysmal high grade atrioventricular block or ventricular tachycardia.
• Examination: paradoxical (reversed) splitting of the second heart sound, soft first heart sound, and murmurs of the causative valve lesion.

Red flags.

• Syncope or presyncope
• New LBBB with chest pain or ST changes meeting the Sgarbossa criteria
• Reduced ejection fraction, especially 35% or less
• LBBB in a young patient or with PR prolongation or intermittent high grade block
• QRS of 150 ms or more with heart failure
• New LBBB after valve intervention

• 12-lead ECG with strict and conventional criteria. Compare with old tracings to establish whether the LBBB is new.
• Acute myocardial infarction in LBBB (Sgarbossa criteria): concordant ST elevation of 1 mm or more in a lead with a positive QRS (5 points); concordant ST depression of 1 mm or more in V1 to V3 (3 points); excessively discordant ST elevation of 5 mm or more in a lead with a negative QRS (2 points). A total of 3 or more is specific for infarction. In the modified version, ST elevation of 25% or more of the preceding S wave depth replaces the 5 mm rule and raises sensitivity. New LBBB alone, without these findings or typical symptoms, is not a reason for emergency reperfusion.
• Echocardiography for ejection fraction, dyssynchrony, valve disease and thrombus. Cardiac MRI with late gadolinium enhancement when ejection fraction is reduced or in younger patients (septal mid-wall fibrosis, sarcoidosis, scar).
• Ischaemia testing (stress imaging, CT coronary angiography or invasive angiography) when ischaemia is suspected.
• Ambulatory ECG or implantable loop recorder for paroxysmal atrioventricular block.
• Electrophysiology study for unexplained syncope: HV interval of 70 ms or more or induced infra-Hisian block indicates pacing, and inducible sustained monomorphic ventricular tachycardia indicates a defibrillator.
• Laboratory: potassium, thyroid function, natriuretic peptide.

Differential diagnosis.

• Ventricular paced rhythm (pacing spikes)
• Left ventricular hypertrophy with repolarisation abnormality
• Pre-excitation with a right sided accessory connection
• Ventricular tachycardia or accelerated idioventricular rhythm with left bundle morphology
• Hyperkalaemia
• Non-specific intraventricular conduction delay
• Right bundle branch block
• Brugada pattern

• Isolated LBBB with normal ejection fraction and no symptoms: no specific treatment. Echocardiography once to exclude structural disease, and control hypertension and cardiovascular risk. No pacemaker.
• Unexplained syncope: electrophysiology study or implantable loop recorder. Pacemaker for HV interval of 70 ms or more or induced infra-Hisian block. Defibrillator for inducible sustained monomorphic ventricular tachycardia or reduced ejection fraction.
• Heart failure with reduced ejection fraction (ejection fraction 40% or less): full tolerated medical therapy for at least 3 months before device decisions.
  • Sacubitril/valsartan 49/51 mg orally twice daily, doubled at 2 to 4 weeks to the target 97/103 mg twice daily (maximum 194/206 mg daily). Start 24/26 mg twice daily if not on an ACE inhibitor or angiotensin receptor blocker, on low doses, with eGFR under 30 mL/min/1.73 m2, or with moderate liver impairment. Stop any ACE inhibitor 36 hours before. Contraindicated with previous angio oedema, severe liver impairment and pregnancy. Check potassium and creatinine at 1 to 2 weeks after each change.
  • Bisoprolol 1.25 mg orally once daily, increased every 1 to 4 weeks to the target 10 mg once daily, maximum 10 mg daily in heart failure. Contraindicated in cardiogenic shock, second or third degree atrioventricular block without a pacemaker, sick sinus syndrome and severe asthma.
  • Spironolactone 25 mg orally once daily (12.5 mg once daily at eGFR 30 to 49 mL/min/1.73 m2), up to 50 mg daily. Check potassium and creatinine at 1 week and 4 weeks and every 3 months. Withhold at potassium over 5.5 mmol/L and avoid at eGFR under 30 mL/min/1.73 m2.
  • Dapagliflozin 10 mg orally once daily, long term. Start at eGFR 25 mL/min/1.73 m2 or more. Withhold during acute illness with poor oral intake. Contraindicated in type 1 diabetes because of ketoacidosis risk.
• Cardiac resynchronisation therapy: for ejection fraction of 35% or less, NYHA class II to IV despite 3 months of optimal therapy, sinus rhythm and LBBB with QRS of 150 ms or more. It is considered at QRS of 130 to 149 ms. A defibrillator capable device is used when defibrillator criteria are also met (ejection fraction 35% or less after 3 months of optimal therapy and expected survival over 1 year). Conduction system pacing is an alternative when coronary sinus lead placement fails.
• Atrioventricular block needing a pacemaker with ejection fraction of 50% or less and expected ventricular pacing over 40%: resynchronisation or conduction system pacing, to avoid pacing induced cardiomyopathy.
• Acute myocardial infarction: apply the Sgarbossa criteria. Emergency angiography for criteria met, or haemodynamic instability with ongoing ischaemia.
• After valve replacement: monitor new LBBB with ambulatory ECG. A QRS of 150 ms or more, or PR over 240 ms, warrants electrophysiology review. Pacing for high grade atrioventricular block.
• Avoid drugs that slow atrioventricular conduction (verapamil, diltiazem, digoxin) in LBBB with syncope or PR prolongation, unless a pacemaker is present.

Special groups.

• Pregnancy: asymptomatic LBBB with normal ejection fraction needs no change. Stop sacubitril/valsartan, spironolactone and dapagliflozin before conception.
• Older adults: LBBB is often degenerative, and the main task is excluding structural heart disease and syncope.

Monitoring. ECG and clinical review yearly, or at any new symptom. Echocardiography 3 months after new LBBB with reduced ejection fraction and after each heart failure drug titration. Device checks every 3 to 6 months with remote monitoring, aiming for biventricular pacing over 98%. Potassium, creatinine and blood pressure after each titration.

Referral and escalation. Refer to cardiology for new LBBB, and to electrophysiology for syncope, ejection fraction of 35% or less, or atrioventricular block. Urgent for syncope, acute ischaemia, or heart failure with QRS of 150 ms or more.