Bifascicular Block

Bifascicular block is conduction block in two of the three fascicles of the His-Purkinje system. The usual pattern is right bundle branch block with left anterior fascicular block (the commonest), or with left posterior fascicular block (rare, implying more extensive disease).

ECG criteria for left anterior fascicular block are left axis deviation between minus 45 and minus 90 degrees, qR in aVL, rS in II, III and aVF, R-peak time in aVL of 45 ms or more and QRS under 120 ms without the right bundle block.

Left posterior fascicular block shows right axis deviation of +90 to +180 degrees, rS in I and aVL, qR in III and aVF, after excluding right ventricular hypertrophy, lateral infarction and lung disease. It is present in about 1% of adults and rises with age.

Causes are degenerative fibrosis (Lenègre and Lev diseases), coronary artery disease (septal perforator supply from the left anterior descending artery), hypertension, cardiomyopathy, aortic valve disease, infiltrative disease, Chagas disease, cardiac surgery and valve intervention, and hyperkalaemia.

Progression to complete atrioventricular block occurs in about 1 to 4% each year, higher with syncope and PR prolongation. Mortality is driven more by the underlying heart disease than by the block.

Pathophysiology. The left anterior fascicle is thin with a single blood supply from the left anterior descending artery, so it is the most vulnerable. The left posterior fascicle is thicker with dual supply, so its involvement implies extensive disease.

When two fascicles are blocked, the third carries all ventricular activation. Intermittent failure of the third produces complete atrioventricular block with an unreliable wide-QRS escape rhythm at 20 to 40/min.

• Frequently asymptomatic.
• Syncope or presyncope from paroxysmal high grade atrioventricular block, dizziness, falls and fatigue.
• With reduced ejection fraction, ventricular tachycardia is a second cause of syncope.
• Examination: wide splitting of the second heart sound. Bradycardia during complete block.

Red flags.

• Unexplained syncope
• First degree atrioventricular block (PR over 200 ms) with the block, or alternating bundle branch block
• Mobitz type II or intermittent complete block
• Ejection fraction of 35% or less
• New block with anterior myocardial infarction
• Infiltrative disease (sarcoidosis, amyloidosis)
• Hyperkalaemia or drugs slowing atrioventricular conduction

• 12-lead ECG: RBBB plus left anterior or posterior fascicular block pattern, PR interval and QRS duration. Compare with old tracings.
• Ambulatory ECG for 2 to 4 weeks, or an implantable loop recorder (battery up to 3 years) for infrequent syncope.
• Exercise testing: exercise induced atrioventricular block suggests infra-Hisian disease and indicates pacing.
• Electrophysiology study: normal HV interval is 35 to 55 ms. An HV interval of 70 ms or more is prolonged. 100 ms or more is markedly prolonged. Incremental atrial pacing or a drug challenge (procainamide 10 mg/kg intravenously over 10 minutes at not over 100 mg/min) is positive when it prolongs HV to 100 ms or more or causes second or third degree infra-Hisian block. Ventricular stimulation for inducible tachycardia when ejection fraction is reduced.
• Echocardiography for ejection fraction. Cardiac MRI or PET for sarcoidosis or amyloidosis in younger patients or with other clues.
• Laboratory: potassium, thyroid function, troponin.

Differential diagnosis.

• Left bundle branch block
• Trifascicular block
• Left axis deviation from inferior infarction, left ventricular hypertrophy or pre-excitation
• Ventricular pacing
• Right ventricular hypertrophy (for left posterior block)
• Congenital causes of left axis deviation (tricuspid atresia, atrioventricular septal defect)
• Mobitz type I atrioventricular block
• Neurally mediated syncope, orthostatic hypotension and ventricular tachycardia as causes of syncope

• Asymptomatic bifascicular block: no pacemaker and no specific treatment. Annual ECG, treat the underlying disease and counsel to report syncope or presyncope. Drugs that slow atrioventricular conduction (beta blockers, verapamil, diltiazem, digoxin, amiodarone, flecainide, procainamide) are used only when clearly needed and with monitoring.
• Syncope:
  • Alternating bundle branch block: permanent pacemaker.
  • Unexplained syncope without alternating block: electrophysiology study. Pacemaker for HV interval of 70 ms or more, or infra-Hisian block (spontaneous or induced). If negative, implantable loop recorder.
  • Ejection fraction of 35% or less or structural disease: assess for ventricular tachycardia. Defibrillator with pacing capability if inducible or if no other cause is found.
  • Recurrent injurious syncope in older patients with bifascicular block: pacing may be reasonable even when the study is negative.
• Asymptomatic with HV interval of 100 ms or more, or pacing induced infra-Hisian block: pacing is considered.
• Pacing mode: dual chamber pacemaker with algorithms to minimise ventricular pacing when conduction is intact. Biventricular or conduction system pacing when ejection fraction is 50% or less and ventricular pacing over 40% is expected.
• Acute high grade block with compromise:
  • Transcutaneous pacing pads on and then pacing, increasing the output until mechanical capture is confirmed, with analgesia and sedation.
  • Adrenaline 2 to 10 microgram/min intravenously, titrated to response, until pacing is established.
  • Atropine 500 micrograms intravenously every 3 to 5 minutes, maximum 3 mg, is unreliable in infra-nodal block, because it acts on the node and not the His-Purkinje system, and it may speed the sinus rate and worsen block.
  • Transvenous temporary pacing, then permanent pacemaker.
• New block with anterior myocardial infarction: temporary pacing for alternating bundle branch block, Mobitz type II or complete block.
• Cause specific treatment: treat hyperkalaemia, stop offending drugs, revascularise ischaemia, and refer infiltrative disease for specialist therapy.

Special groups.

• Chagas disease: right bundle branch block with left anterior fascicular block is the commonest early ECG sign. Risk of ventricular tachycardia is high, so assess for defibrillator.
• Pregnancy: permanent pacing is feasible when indicated.

Monitoring. ECG every 6 to 12 months. Device checks 2 to 12 weeks after implant, then every 3 to 12 months with remote monitoring. Loop recorder interrogation every 3 to 6 months. Echocardiography yearly if ejection fraction is reduced.

Referral and escalation. Refer to cardiology or electrophysiology for syncope, PR prolongation, alternating block or reduced ejection fraction. Urgent for syncope, Mobitz type II or complete block, and new block with myocardial infarction.