Trifascicular Block

Trifascicular block is evidence of impaired conduction in all three fascicles of the His-Purkinje system.

Usual definitions are: bifascicular block (RBBB with left anterior or posterior fascicular block) with first degree atrioventricular block (PR interval over 200 ms), and bifascicular block with intermittent or permanent third degree atrioventricular block.

Alternating bundle branch block (RBBB alternating with LBBB, or RBBB with alternating left fascicular block) also proves trifascicular disease.

First degree block with bifascicular block does not by itself prove infra-Hisian disease, since the delay can sit in the atrioventricular node, and electrophysiology study is needed in doubtful cases.

Causes are degenerative fibrosis (Lenègre and Lev diseases), coronary disease, hypertension, cardiomyopathy, infiltrative disease, Chagas disease, Lyme carditis, cardiac surgery or valve intervention, hyperkalaemia and drugs.

Pathophysiology. Diffuse disease of the infra-nodal conduction system leaves little reserve. A fixed or intermittent block of the remaining fascicle causes Stokes-Adams attacks. Escape rhythms from the distal conduction tissue are slow and unreliable, so asystole is possible.

• Syncope, presyncope, dizziness, falls, fatigue and heart failure from bradycardia.
• Stokes-Adams attacks: sudden loss of consciousness without warning, with pallor and quick recovery.
• Examination: slow regular pulse in complete block, variable first heart sound, cannon a waves from atrioventricular dissociation, wide splitting of the second sound.
• Some patients are asymptomatic and the block is found on an ECG taken for another reason.

Red flags.

• Syncope or Stokes-Adams attacks
• Alternating bundle branch block
• Mobitz type II or advanced atrioventricular block
• Pauses of 3 seconds or more while awake
• Wide-QRS escape rhythm
• Ejection fraction of 35% or less
• Acute anterior myocardial infarction
• Hyperkalaemia, drugs or Lyme disease as reversible causes

• 12-lead ECG: the blocked fascicles, PR interval (over 200 ms), alternating patterns. Compare with old ECGs.
• Ambulatory ECG and implantable loop recorder to capture paroxysmal block and pauses.
• Exercise test for exercise induced infra-Hisian block.
• Electrophysiology study: HV interval, atrial pacing to unmask infra-Hisian block, and procainamide challenge (10 mg/kg intravenously over 10 minutes at not over 100 mg/min). A positive result is an HV interval of 100 ms or more, or second or third degree infra-Hisian block. HV interval of 70 ms or more is prolonged, and a short HV in a patient with prolonged PR places the delay in the node.
• Echocardiography for ejection fraction. Cardiac MRI or PET for sarcoidosis and amyloidosis. Troponin, potassium, thyroid function. Lyme serology in endemic regions. Chagas serology in patients from endemic regions.

Differential diagnosis.

• Bifascicular block with AV nodal first degree block
• LBBB with first degree atrioventricular block
• Mobitz type I atrioventricular block (narrow QRS)
• Vagally mediated or sleep related atrioventricular block
• Drug induced block (beta blockers, verapamil, diltiazem, digoxin, amiodarone)
• Hyperkalaemia
• Lyme carditis
• Acute myocardial infarction with block
• Sick sinus syndrome
• Neurally mediated syncope, orthostatic hypotension, carotid sinus hypersensitivity, ventricular tachycardia and epilepsy

• Alternating bundle branch block: permanent pacemaker whether or not symptoms are present.
• Intermittent third degree or advanced second degree atrioventricular block, or Mobitz type II with wide QRS: permanent pacemaker whether or not symptoms are present.
• Syncope with bifascicular block and first degree block, other causes excluded: permanent dual chamber pacemaker. Electrophysiology study first when the cause is uncertain, with pacing for HV interval of 70 ms or more or infra-Hisian block. With ejection fraction of 35% or less, assess for ventricular tachycardia and consider a defibrillator.
• Asymptomatic trifascicular block (bifascicular block with first degree block only): no pacemaker. Annual ECG, avoid drugs that slow atrioventricular conduction unless needed, and counsel to report syncope. Electrophysiology study is considered when ejection fraction is reduced or the PR interval is lengthening.
• Acute haemodynamic compromise:
  • Transcutaneous pacing, increasing the output until mechanical capture is confirmed, with analgesia and sedation.
  • Adrenaline 2 to 10 microgram/min intravenously, titrated to response, as a bridge.
  • Atropine 500 micrograms intravenously every 3 to 5 minutes, maximum 3 mg, is unreliable in infra-nodal block.
  • Transvenous temporary pacing, then permanent pacemaker.
• 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.
• Reversible causes: treat hyperkalaemia, stop offending drugs, revascularise ischaemia. Lyme carditis recovers with antibiotics in most cases, and temporary pacing is used until recovery.
• Infiltrative disease: specialist immunosuppression for sarcoidosis, with pacing as indicated.
• Device complications: pocket haematoma, infection, lead dislodgement and pneumothorax. Counsel before implant and avoid anticoagulant interruption bridging where possible.

Special groups.

• Anterior myocardial infarction with new trifascicular block: temporary pacing, followed by permanent pacing if block persists.
• Older adults: pacing treats syncope, but ejection fraction and ventricular tachycardia risk guide device choice.
• Pregnancy: permanent pacing is feasible when indicated.

Monitoring. Wound check at 2 weeks and first device check at 2 to 12 weeks, then every 3 to 12 months with remote monitoring. Check lead thresholds, battery and ventricular pacing percentage. Echocardiography yearly with reduced ejection fraction or high ventricular pacing. ECG every 6 to 12 months if no pacemaker, with loop recorder interrogation every 3 to 6 months if one is implanted.

Referral and escalation. Refer to cardiology or electrophysiology for trifascicular block with symptoms. Urgent for syncope, alternating bundle branch block, Mobitz type II or complete block, and block with acute myocardial infarction.