What Is A Trifascicular Block?

  • Jason Ha Bachelor of Medicine, Bachelor of Surgery - MBBS, University of Bristol

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The heart's conduction system 

The heart consists of an intricate electrical circuit that coordinates the contraction of the various chambers of the heart, which leads to the subsequent flow of blood.1 

The circuit begins at the junction between the superior vena cava and right atrium, where a cluster of specialised cells - referred to as “cardiac pacemaker cells” - form the sinoatrial node (SA node); responsible for initially generating an electrical impulse.2 This acts like a wave of excitation, propagating across both atriums, stimulating the corresponding myocardium - muscular tissue - to contract. Thereafter, the electrical impulse travels down specialised pathways (internodal tracts) to the interatrial septum - tissue that separates the atria - where it’s conducted by the atrioventricular node (AVN). Once gathered, the AVN delays impulse propagation to the ventricles (lower chambers), ensuring complete unloading of blood from the atria. After a brief hold-up (~120ms), the electrical impulse is conducted by the ‘bundle of His’ - colloquially known as the atrioventricular bundle - situated in the interventricular septum, tissue separating the two ventricles. This structure splits into two, the right bundle branch (RBB) and left bundle branch (LBB), with the latter splitting further into the left anterior fascicles (LAF) and left posterior fascicles (LPF). Therefore, once the electrical impulse is conducted by the bundle of His, it disseminates down the RBB, LBB, and further branches, until conducted by specialised ventricular cells known as Purkinje fibers. As a result, contraction of the ventricle chambers is triggered, starting at the heart's apex and progressing upwards to ensure blood is effectively pumped out of the heart to either the lungs for re-oxygenation or the rest of the body. 

Conduction disorders: Heart blocks

Heart blocks, otherwise known as atrioventricular (AV) blocks, occur when there is a delay or disturbance in the conduction of an electrical impulse from the atria to the ventricles.3 Depending on the degree of severity, heart blocks can be split into three different classifications:

First-degree: Conduction of an electrical impulse from the atria (SA node) to the ventricles (AVN) is delayed.3 This is a benign condition, rarely causing the development of any symptoms (asymptomatic) or the need for treatment - only monitoring and preventative strategies to avert progression of degree severity.   

Second-degree (Incomplete blockage): There is intermittent conduction between the atria and ventricles, which can lead to a slowed and irregular heartbeat (arrhythmia).3 They can be further classified into two categories:

Mobitz Type I (Wenckebach): The cells that form the AVN progressively fatigue until they are unable to transmit an electrical impulse to the ventricles.3 However, as most patients are asymptomatic with minimal haemodynamic disturbance, they don’t require treatment. Those who do show symptoms typically respond positively to atropine, a medicine that improves AVN conduction. 

Mobitz Type II: This block occurs below the AVN (E.g., Bundle of His, RBB, LAF, or LPF), where the conduction of an impulse abruptly fails, disrupting ventricle contraction.3 As a result of the unpredictable haemodynamic instability and severe bradycardia, syncope (fainting) and sudden cardiac death can occur without warning. Therefore, patients require a permanent pacemaker. 

Third-degree (Complete blockage): There is complete dissociation between the atria and ventricles as no electrical signals can be transmitted between them.3 Therefore, they conduct independently of one another, resulting in two rhythms occurring simultaneously. As a result, patients with a complete heart block are at an increased risk of developing asystole, ventricular tachycardia, and sudden cardiac death. 

Trifascicular block

The prefix- ‘tri’ indicates three, referring to the presence of a conduction delay/block in all three fascicles below the AV node (RBB, LAF, and LPF) - responsible for propagating impulses to the ventricles - classified clinically as a complete (third-degree) heart block.4 Currently, this conduction disorder is not well understood, with a lack of large population-based studies resulting in no correlation between age, race, or gender and the risk of developing a trifascicular block. However, current knowledge about this disorder is explained hereinafter and should be carefully considered if anything is found to be relatable in regard to your own experience. 

Causes and risk factors

The majority of heart blocks are acquired later in life, although some are present at birth as congenital heart defects. The factors that contribute to the development of an tri fascicular block include the following:2

Structural cardiac diseases are defined as abnormalities or disorders in the heart's physiological structure or function, either present at birth or developed later in life.5 These can range from acute diseases - such as myocardial infarction, endocarditis, or myocarditis - to more chronic, including hypertension, cardiomyopathy, aortic stenosis, and congenital heart diseases.2

Additional medical conditions: Autoimmune (e.g., systemic sclerosis or systemic lupus erythematosus), inflammatory (e.g., Lyme disease or rheumatic fever), and infiltrative (e.g., haemochromatosis, sarcoidosis, or amyloidosis).2

Iatrogenic: Unintentionally induced deleterious effect of a therapeutic or diagnostic regime6. This can be post-cardiac surgery complications or medication-induced, including beta-blockers, calcium channel blockers (non-dihydropyridine), digoxin, adenosine, and antiarrhythmic medications (e.g., quinidine, procainamide, and disopyramide).2

Other: Digoxin toxicity, hyperkalemia (electrolyte balance), and an overactive vagus nerve.2


As expected with a complete blockage of the heart's conduction pathway, patients with a tri fascicular block can expect unpleasurable and potentially life-threatening symptoms, including:3

  • Angina (chest pain)
  • Presyncope (dizziness) or syncope (fainting)
  • Fatigue (extreme tiredness)
  • Heart palpitations (awareness of your heartbeat)
  • Nausea
  • Dyspnoea (shortness of breath)
  • Sudden cardiac arrest 

If you or anyone you know is experiencing any of the aforementioned symptoms, you should seek medical attention immediately. 

Diagnostic tests

Patients clinically presenting with symptoms indicative of a third-degree heart block, in addition to signs of cardiovascular disease, will often require a detailed history and baseline electrocardiogram (ECG) to be conducted2. If a trifascicular block is present, one of the two following ECG patterns will be discovered: third-degree AV block + RBB block + LAF block or third-degree AV block + RBB block + LPF block.7

However, if the initial results are considered non-diagnostic - unable to distinguish between a trifascicular block and other conduction disorders - an electrophysiological study (EPS) will be recommended, enabling a His bundle electrogram recording to be made, ensuring a definitive diagnosis is determined.2

Additionally, blood tests may be performed to check for metabolic or electrolyte imbalances, as this finding could be indicative of hyperkalemia, a condition that has the potential to be a reversible cause of the diagnosed tri fascicular block.8 


The American College of Cardiology (ACC), American Heart Association (AHA), and Heart Rhythm Society (HRA) recommend that patients with a third-degree heart block - not caused by reversible or physiological causes - regardless of symptoms, should receive permanent pacing via a pacemaker or implantable cardioverter defibrillators (ICD).9 The prior functions are to send a regular, low-energy electrical pulse to the heart to ensure a steady heart rate is maintained. The latter monitors the heart for abnormal beating, delivering an electrical shock to the heart if any dangerous rhythm is detected or in the case of an emergency (e.g., sudden arrest). Depending on various factors, your cardiologist and associated healthcare team will decide which device is best for your condition and what you can expect regarding the procedure. 


The long-term prognosis following treatment of a tri fascicular block is not well studied but is likely dependent upon various factors, including age, severity at the time of diagnosis, additional medical conditions (e.g., Diabetes), and any treatment-related complications (e.g., pacemaker lead malposition or dislodgement).10

Preventative strategies

Although this conduction disorder is not fully understood, to prevent the initial development of a heart block or progression from a second-degree to a complete blockage, a heart-healthy lifestyle should be adopted. This includes:

  • A balanced diet, following recommendations by the British Heart Foundation11
  • Limit alcohol consumption (<14 units per week)11
  • Physical activity. The NHS recommends at least 140 minutes of moderate-intensity activity, or 75 minutes of vigorous-intensity activity per week12
  • Weight management (Between 18.5-24.9 BMI for adults)13
  • Smoking cessation


A trifascicular block is a severe and possibly life-threatening conduction disorder that disrupts electrical impulse propagation in the lower chambers of the heart (ventricles). Due to the extent of disruption affecting all three fascicles below the AVN, acute symptoms are often apparent and sometimes fatal. If promptly treated using permanent pacing - in addition to living a healthy, risk-reducing lifestyle - prognosis following surgery is favourable. However, this is largely dependent on various underlying factors, including age, comorbidities, and possible intra- or post-surgical complications. 


  1. Oberman R, Shumway KR, Bhardwaj A. Physiology, Cardiac. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2024 Jan 6]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK526089/.
  2. Subsection: Basic Science Trifascicular Block: Diagnosis and Anaesthetic Considerations. WFSA Resource Library [Internet]. [cited 2024 Jan 6]. Available from: https://resources.wfsahq.org/atotw/subsection-basic-science-trifascicular-block-diagnosis-and-anaesthetic-considerations/.
  3. Kashou AH, Goyal A, Nguyen T, Ahmed I, Chhabra L. Atrioventricular Block. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2024 Jan 6]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK459147/.
  4. Ashley EA, Niebauer J. Arrhythmia. In: Cardiology Explained [Internet]. Remedica; 2004 [cited 2024 Jan 6]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK2219/.
  5. Structural Heart Disease. Yale Medicine [Internet]. [cited 2024 Jan 6]. Available from: https://www.yalemedicine.org/conditions/structural-heart-disease.
  6. Krishnan N, Kasthuri A. Iatrogenic Disorders. Med J Armed Forces India [Internet]. 2005 [cited 2024 Jan 6]; 61(1):2–6. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4923397/.
  7. Buttner R, Larkin J, Larkin RB and J. Life in the Fast Lane • LITFL [Internet]. 2021. Trifascicular Block; [cited 2024 Jan 6]. Available from: https://litfl.com/trifascicular-block-ecg-library/.
  8. RCEMLearning [Internet]. Bifascicular Block; [cited 2024 Jan 6]. Available from: https://www.rcemlearning.co.uk/reference/bifascicular-block/.
  9. Kusumoto FM, Schoenfeld MH, Barrett C, Edgerton JR, Ellenbogen KA, Gold MR, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay: Executive Summary: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines, and the Heart Rhythm Society. J Am Coll Cardiol. 2019; 74(7):932–87.
  10. Knabben V, Chhabra L, Slane M. Third-Degree Atrioventricular Block. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2024 Jan 6]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK545199/.
  11. Healthy eating. British Heart Foundation [Internet]. [cited 2024 Jan 8]. Available from: https://www.bhf.org.uk/informationsupport/support/healthy-living/healthy-eating.
  12. Physical activity guidelines for adults aged 19 to 64. nhs.uk [Internet]. 2022 [cited 2024 Jan 8]. Available from: https://www.nhs.uk/live-well/exercise/exercise-guidelines/physical-activity-guidelines-for-adults-aged-19-to-64/.
  13. Managing your weight. British Heart Foundation [Internet]. [cited 2024 Jan 8]. Available from: https://www.bhf.org.uk/informationsupport/support/healthy-living/managing-your-weight.

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This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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