What is trifasicular block
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"bifascicular block" OR "trifascicular block"
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Trifascicular block is conduction disease affecting all three fascicles of the infranodal (His-Purkinje) system at the same time. The three fascicles are:
- The right bundle branch (RBB)
- The left anterior fascicle (LAF)
- The left posterior fascicle (LPF)
Definition
Bifascicular block means two of the three fascicles are partially or completely blocked. If all three are affected, a trifascicular block exists. - Morgan and Mikhail's Clinical Anesthesiology, 7e
How it appears on ECG
- Delay or partial block in all three fascicles gives a prolonged PR interval (first-degree AV block) or alternating LBBB and RBBB. - Morgan and Mikhail's Clinical Anesthesiology, 7e
- Complete block in all three fascicles gives third-degree (complete) AV block. - Morgan and Mikhail's Clinical Anesthesiology, 7e
- Alternating bundle branch block, or alternating left hemiblock with RBBB, is termed trifascicular block. It should prompt a search for the cause of syncope. - Fuster and Hurst's The Heart, 15th Edition
The classic pattern people mean is RBBB plus left anterior or posterior fascicular block plus a prolonged PR interval. Fuster and Hurst's The Heart, 15th Edition, says this term evolved for two fascicles with delay or block plus PR prolongation. The PR prolongation does not prove the third fascicle is diseased, because the delay can be in the AV node. The textbook also warns that this is not the same as "trifascicular LBBB", which refers to the left bundle having more than two divisions.
Why it matters
- It can progress to complete heart block, causing syncope, Stokes-Adams attacks or sudden death.
- Patients with bifascicular or trifascicular block and symptoms, especially syncope not explained by another cause, should get a permanent pacemaker. - Harrison's Principles of Internal Medicine, 22E
- A pacemaker is also indicated in asymptomatic patients who have intermittent third-degree block. - Harrison's Principles of Internal Medicine, 22E
- In acute settings, new trifascicular block (as with new Mobitz II block) can warrant temporary transvenous pacing. - The Washington Manual of Medical Therapeutics
- In anesthesia, asymptomatic bifascicular block rarely progresses to complete block perioperatively. Trifascicular block with symptoms raises the concern for pacing.
- Tricyclic antidepressants are contraindicated in significant conduction disease such as bifascicular or trifascicular block after recent MI. - Campbell Walsh Wein Urology
My PubMed check of recent meta-analyses and systematic reviews found nothing that updates or contradicts these textbook points.
Bifascicular block + Increased PR Interval/ Mobitz Block is Trifasicular block. Explain
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bifascicular block with first-degree AV block is not trifascicular block PR prolongation AV node HV interval
That statement is a common teaching shortcut, and it is only partly correct. It is a traditional definition, but it doesn't hold up on physiology. Your two examples need to be treated separately.
Where the idea comes from
The older teaching was that RBBB plus a left fascicular block (bifascicular) plus a prolonged PR means the third fascicle is delayed. That would make the block "incomplete trifascicular". Fuster and Hurst's The Heart, 15th Edition, notes the term "evolved when there was a combination of two fascicles showing conduction delay and/or block and additional PR interval prolongation."
Why it is inaccurate
1. A long PR interval does not localize the delay to the third fascicle.
The PR interval covers the atria, AV node, His bundle, bundle branches and Purkinje fibers. Delay at any of these sites prolongs it. In first-degree AV block, the delay is in the AV node in about 60% of cases, in the His-Purkinje system in some, and in both in others. - Fuster and Hurst's The Heart, 15th Edition
The Merck Manual calls this combination "sometimes incorrectly called trifascicular block", because first-degree block is usually AV nodal in origin. It reserves "true" trifascicular block for RBBB with alternating left anterior and posterior hemiblock, or alternating LBBB and RBBB.
2. Prognosis is not different.
Fuster and Hurst say first-degree AV block has an excellent prognosis even with chronic bifascicular block, because progression to third-degree block is low. It needs no specific therapy. LITFL cites similar progression to complete heart block for bifascicular block alone and for bifascicular block with first-degree AV block or Mobitz I (about 1% per year).
What to do with each of your two examples
| Finding with bifascicular block | Likely site | Interpretation |
|---|---|---|
| Prolonged PR (first-degree AV block) | Usually AV node, sometimes His-Purkinje | Not true trifascicular block. Low risk, observe. |
| Mobitz I (Wenckebach) | Usually AV node | Not trifascicular. Low risk. |
| Mobitz II | Infranodal (His-Purkinje) | Real evidence of severe distal disease. High risk of complete heart block, so pace. |
| Alternating RBBB and LBBB | All fascicles intermittently | True trifascicular block. Pace. |
| Complete (third-degree) block with RBBB plus fascicular block pattern | Infranodal | True trifascicular block. Pace. |
So Mobitz II with bifascicular block is the more convincing infranodal evidence, but Mobitz I is not. Harrison's Principles of Internal Medicine, 22E, lists type II second-degree AV block, intermittent third-degree block and alternating bundle branch block as pacemaker indications in asymptomatic bifascicular or trifascicular block.
How to tell if the PR delay is nodal or infranodal
An electrophysiology study measures the His-ventricular (HV) interval. A markedly prolonged HV interval, or block below the His, indicates infranodal disease and supports pacing. - Harrison's Principles of Internal Medicine, 22E. Symptom to Diagnosis: An Evidence Based Guide, 4th Edition, lists unexplained syncope with chronic bifascicular block as an indication, especially if the HV interval is 70 ms or more.
Exam tip
In many exams, "bifascicular block plus first-degree AV block" is still keyed as "trifascicular block". If the question asks for the textbook label, use it. The clinical meaning is different: a long PR is not proof of infranodal disease, so the decision to pace rests on symptoms (syncope), Mobitz II, alternating bundle branch block, complete block, or a prolonged HV interval.
I only searched the web once and did not find a guideline document for the exact wording. The guideline points above come from textbook and secondary sources, so check the current ACC/AHA/HRS bradycardia guideline if you need exact recommendation classes.
Find the latest ACC/AHA/HRS guideline for bundle branch block
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ACC/AHA/HRS guideline bradycardia cardiac conduction system disease 2025 bundle branch block bifascicular block recommendations
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bradycardia cardiac conduction delay guideline
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new ACC/AHA/HRS guideline conduction system disease pacing update 2026 replaces 2018 bradycardia guideline
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ACC/AHA/HRS bradycardia conduction delay guideline update focused update 2025 OR 2026 Circulation JACC Heart Rhythm
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https://www.guidelinecentral.com/guideline/6988
The latest ACC/AHA/HRS guideline I could find is the 2018 one: Kusumoto FM et al., 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. It was published in JACC in November 2018 and has not been replaced. A JACC hub page lists it as current. It supersedes the pacemaker recommendations in the 2008 device therapy guideline and the 2012 focused update. A PubMed search of the last 3 years and two web searches turned up no newer US guideline on this topic. The ACC page shows "last updated July 2026", but it still lists the 2018 document, so I read that as a page update, not a new guideline.
The guideline covers adults only and gives no specific recommendations for pediatric patients. The recommendations below come from Guideline Central's summary and excerpts of the full text, not from the full text itself.
Bundle branch and fascicular block (section 7)
These apply to block with 1:1 AV conduction and a normal PR interval.
Evaluation (section 7.4)
- Class I: a transthoracic echocardiogram for newly detected LBBB, to look for structural heart disease.
- Class I: ambulatory ECG monitoring in symptomatic patients with conduction system disease when AV block is suspected.
- Class IIa: an electrophysiology study (EPS) is reasonable if symptoms suggest intermittent bradycardia (such as lightheadedness or syncope), the ECG shows conduction disease, and AV block has not been demonstrated.
- Class IIa: echocardiography for block other than LBBB if structural heart disease is suspected.
- Class IIa: advanced imaging (cardiac MRI, CT or nuclear studies) in selected LBBB patients when structural disease is suspected and the echocardiogram is unrevealing.
- Class IIb: ambulatory ECG recording in selected asymptomatic patients with bifascicular or trifascicular block, to look for higher-degree AV block.
- Class IIb: stress testing with imaging in selected asymptomatic LBBB patients when ischemic heart disease is suspected.
Pacing (section 7.5)
- Class I: permanent pacing for syncope with bundle branch block and an HV interval of 70 ms or more, or infranodal block at EPS.
- Class I: permanent pacing for alternating bundle branch block.
- Class IIb: cardiac resynchronization therapy (CRT) for LVEF 36 to 50%, LBBB, QRS over 150 ms and NYHA class II or higher heart failure symptoms. This is from the guideline's flow chart, so check the exact wording in the full text.
- Neuromuscular and metabolic disease:
- Kearns-Sayre syndrome with conduction disorders: pacing is Class IIa.
- Anderson-Fabry disease with QRS over 110 ms: pacing is Class IIb.
- Myotonic dystrophy with PR over 240 ms, QRS over 120 ms or fascicular block: pacing is Class IIb.
Related recommendations from other sections
- Acquired Mobitz II, high-grade or third-degree AV block not due to a reversible cause: permanent pacing is Class I regardless of symptoms (section 6).
- Acute MI with a new bundle branch block or isolated fascicular block and no second- or third-degree AV block: permanent pacing should not be performed (Class III: Harm).
- Marked first-degree or Mobitz I block with symptoms clearly due to the AV block: pacing is Class IIa.
Link to your earlier question
The guideline does not use "trifascicular block" as a pacing indication in itself. It bases pacing on syncope with an HV interval of 70 ms or more, alternating bundle branch block, and Mobitz II or higher-grade AV block. It makes no recommendation to pace asymptomatic bifascicular block with first-degree AV block. This matches the point that a long PR interval alone does not prove infranodal disease.
Other documents to know
- The 2023 HRS/APHRS/LAHRS guideline on cardiac physiologic pacing is a separate guideline for avoiding and mitigating heart failure (PMID 37283271). It covers His bundle and left bundle area pacing and CRT.
- A 2026 Canadian CCS/CHRS guideline on selecting and implanting cardiac implantable electronic devices exists (PMID 41539743). I haven't read it.
I didn't read the full text of the guideline, so confirm class and level-of-evidence labels in the ACC guideline page before quoting them.
What are the parts of PR Segment
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PR segment PR interval ECG waveform labeled P wave QRS atrial depolarization AV node delay
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normal ECG waveform P wave PR interval QRS complex ST segment T wave
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The PR segment is the flat line between the end of the P wave and the start of the QRS. The question is often really about the PR interval, so I cover both.
PR interval vs PR segment
- PR interval runs from the start of the P wave to the start of the QRS. It is the P wave plus the PR segment. The normal value is 120 to 200 ms.
- PR segment runs from the end of the P wave to the start of the QRS. It is isoelectric, because the AV node, His bundle and Purkinje fibers produce too little voltage to show on the surface ECG.
What the PR segment contains
By the time the PR segment starts, both atria are fully depolarized. The segment therefore represents the impulse travelling through the specialized conduction tissue:
- AV node: conduction is slow here. This accounts for most of the PR interval, and it gives the atria time to empty into the ventricles. - Goldman-Cecil Medicine, International Edition
- Bundle of His: the impulse then enters this bundle.
- Right and left bundle branches: the left bundle splits into the left anterior and posterior fascicles.
- Purkinje fibers: conduction here is fast, about 2 to 3 m/s compared with 0.3 to 0.4 m/s in myocardium. - Harrison's Principles of Internal Medicine, 22E
Harrison's describes AV nodal conduction time as "the time between the atrial inscription and the subsequent ventricular, or QRS, complex". The PR interval physiologically shortens with exercise and higher adrenergic tone.
Matching intracardiac (EPS) intervals
The PR interval can be split into three parts. These are my own summary, not taken from the library.
| Interval | From | To | Normal | Corresponds to |
|---|---|---|---|---|
| PA | Start of P wave | Low right atrial deflection | About 25 to 45 ms | Intra-atrial conduction (inside the P wave) |
| AH | Low right atrium | His deflection | About 50 to 120 ms | AV node |
| HV | His deflection | Earliest ventricular activation | About 35 to 55 ms | His-Purkinje system |
The PR segment is roughly the AH interval, the His deflection and the HV interval together. This links to your earlier question. A long PR interval with bifascicular block could come from a long AH (AV node, usually benign) or a long HV (infranodal, concerning). An HV of 70 ms or more with syncope is a Class I pacing indication in the 2018 ACC/AHA/HRS guideline.
PR segment changes
- Depression occurs in acute pericarditis, most often in lead II, with PR elevation in aVR. - ROSEN's Emergency Medicine and Fuster and Hurst's The Heart, 15th Edition
- Elevation in aVR is the reciprocal of that finding.
The textbook search returned no figure for the PR segment, so I have not included an image.
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