Conduction system disease
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Nehal Eid, M.D.[2]
Cardiac conduction system disease (CCD), also referred to as cardiac conduction disease or cardiac conduction delay, is a heterogeneous group of disorders characterized by impaired electrical impulse propagation through the cardiac conduction system, including the sinus node, atrioventricular node, His bundle, bundle branches, fascicles, and Purkinje fibers. CCD may manifest as sinus node dysfunction, atrioventricular block, bundle branch block, fascicular block, or combinations thereof. The disease may be congenital or acquired, isolated or associated with structural heart disease, and static or progressive.[1]
Overview
The normal cardiac conduction axis consists of the sinus node, atrial muscle, atrioventricular node, His bundle, bundle branches, fascicles, Purkinje fibers, and ventricular muscle. Conduction system disease results from disruption at any level of this axis. The pathophysiology may be developmental, hereditary/genetic, metabolic, infectious, inflammatory, infiltrative, traumatic, ischemic, malignant, or degenerative. It may be helpful to characterize the process as static or progressive.[1]
Historical Perspective
Progressive cardiac conduction disease was first described by Jean Lenègre in 1964 and Maurice Lev in 1964, who independently characterized idiopathic fibrotic degeneration of the His-Purkinje system. Lenègre disease refers to primary sclerodegeneration of the conduction system in the absence of other cardiac disease, while Lev disease describes extension of calcification from the cardiac skeleton into the conduction system, typically in the elderly.[2]
Classification
Conduction system disease can be classified by anatomic level, etiology, and clinical presentation.
By Anatomic Level
Sinus node dysfunction (SND): Formerly referred to as sick sinus syndrome; includes sinus bradycardia, sinus pauses, sinoatrial exit block, and tachy-brady syndrome
Atrioventricular (AV) block: First-degree, second-degree (Mobitz type I and type II), high-grade, and third-degree (complete) AV block
Intraventricular conduction delay:
Left bundle branch block (LBBB)
Right bundle branch block (RBBB)
Left anterior fascicular block (LAFB)
Left posterior fascicular block (LPFB)
Bifascicular block (RBBB + LAFB or LPFB)
Trifascicular block (bifascicular block with first-degree AV block)
Alternating bundle branch block
Nonspecific intraventricular conduction delay (IVCD)
By Etiology
| Category | Examples |
|---|---|
| Degenerative | Lev disease, Lenègre disease, age-related fibrosis |
| Ischemic | Acute myocardial infarction, chronic ischemic cardiomyopathy, coronary ischemia without infarction |
| Inflammatory/Infiltrative | Myocarditis, cardiac sarcoidosis, amyloidosis, systemic lupus erythematosus, rheumatoid arthritis, systemic sclerosis |
| Infectious | Lyme carditis, Chagas disease, bacterial endocarditis with perivalvular abscess, acute rheumatic fever, toxoplasmosis |
| Congenital/Genetic | Congenital AV block (associated with maternal SLE), congenital heart defects (e.g., L-TGA), SCN5A mutations, LMNA mutations, TRPM4 mutations |
| Iatrogenic | Medications (beta-blockers, verapamil, diltiazem, digoxin, antiarrhythmic drugs), catheter ablation, cardiac surgery, transcatheter aortic valve replacement (TAVR), alcohol septal ablation |
| Metabolic/Endocrine | Hypothyroidism, hyperthyroidism, acid-base disorders, hyperkalemia, pheochromocytoma |
| Neuromuscular | Myotonic dystrophy, Kearns-Sayre syndrome, Emery-Dreifuss muscular dystrophy, Erb dystrophy |
| Vagotonic | Sleep, obstructive sleep apnea, high-level athletic conditioning, neurocardiogenic |
| Other | Lymphoma, cardiac tumors |
Pathophysiology
Degenerative Conduction Disease
Sinus node dysfunction is most often related to age-dependent, progressive, degenerative fibrosis of the sinus nodal tissue and surrounding atrial myocardium. This results in abnormalities of sinus node and atrial impulse formation and propagation. The same milieu of degenerative fibrosis may also involve the specialized atrioventricular conduction system, and evidence suggests that heart block occurs in a proportion of patients who initially required permanent atrial pacing for SND.[1]
Genetic Conduction Disease
Inherited progressive cardiac conduction disease (PCCD) follows an autosomal dominant inheritance pattern in most families. The overall pathogenic variant detection rate in index cases exceeds 50%, with SCN5A and LMNA as core genes, each accounting for approximately 20% of cases, and TRPM4 accounting for 5–10%.[3]
Key genes implicated in CCD include:
| Gene | Protein / Function | Associated Phenotype |
|---|---|---|
| SCN5A | Cardiac sodium channel α subunit (Nav1.5); loss-of-function | Isolated PCCD, Brugada syndrome, long QT syndrome type 3, SND |
| LMNA | Lamin A/C | PCCD with dilated cardiomyopathy, atrial fibrillation, Emery-Dreifuss muscular dystrophy |
| TRPM4 | Transient receptor potential melastatin 4 channel; gain-of-function | Isolated PCCD |
| NKX2-5 | Transcription factor Nkx2.5 | PCCD with atrial septal defect, ventricular septal defect |
| TBX5 | Transcription factor TBX5 | Holt-Oram syndrome (hand-heart syndrome) with PCCD |
| GLA | Galactosidase α | Fabry disease with CCD |
| PRKAG2 | AMP-activated protein kinase γ2-subunit | Cardiac preexcitation (WPW) with CCD |
| Mitochondrial DNA | Multiple mitochondrial genes | Kearns-Sayre syndrome with progressive CCD |
Ischemic Conduction Disease
Acute myocardial infarction may cause conduction disturbances depending on the coronary territory involved. Inferior MI may cause transient AV nodal block due to increased vagal tone or right coronary artery occlusion affecting the AV nodal artery. Anterior MI may cause infranodal block due to extensive septal necrosis involving the His bundle and bundle branches, which carries a worse prognosis.[1]
Post-TAVR Conduction Disease
Conduction disturbances remain the most common complication after transcatheter aortic valve replacement. New LBBB occurs in 19% to 55% of patients, and new high-degree AV block occurs in approximately 10%. Up to half of new bundle branch block and complete heart block may resolve before discharge. Predictors of post-TAVR conduction disturbances include pre-existing RBBB, short membranous interventricular septum, deep valve implantation, and valve type.[1][4]
Epidemiology and Demographics
The prevalence of conduction system disease increases markedly with age. Both SND and AV block are most common in individuals in their 70s and 80s. In one study, 855 men aged 50 years and older were followed for 30 years, with the prevalence of bundle branch block increasing from 1% to 17%. At age 80, the prevalence of RBBB was 11.3% and LBBB was 5.7%.[5]
LBBB is present in approximately 1% of the general population and in 20% to 30% of patients with heart failure and reduced left ventricular ejection fraction.[6] In young, apparently healthy populations, RBBB is approximately 4 times more common than LBBB (0.2% vs. 0.05%).[7]
Ischemic heart disease, heart failure, valvular heart disease, cerebrovascular disease, and atrial fibrillation are common concurrent conditions in patients with conduction system disease requiring treatment.[1]
Risk Factors
Advanced age
Heart failure and cardiomyopathy
Valvular heart disease (especially aortic stenosis)
Prior cardiac surgery or TAVR
Myocarditis and inflammatory conditions
Infiltrative diseases (amyloidosis, sarcoidosis)
Neuromuscular disorders
Medications (beta-blockers, calcium channel blockers, digoxin, antiarrhythmic drugs)
Family history of progressive conduction disease
Natural History, Complications, and Prognosis
The natural history of conduction system disease varies by type and etiology.
LBBB: Cohort studies have demonstrated an association between LBBB and the development of coronary artery disease and heart failure. Progression of LBBB and bifascicular block to atrioventricular block and bradycardia is low, approximately 1% per year. LBBB is associated with higher mortality than other forms of conduction disorders. In a longitudinal cohort of 4,541 adults aged 65 years or older with structurally normal hearts, LBBB at baseline was associated with a higher cumulative risk of incident heart failure compared with those without LBBB (48% vs. 12.2%; HR 4.98; 95% CI 2.18–11.39) over a median follow-up of 14.6 years.[6][1]
RBBB: Isolated RBBB in the absence of structural heart disease has generally not been associated with increased cardiovascular mortality.[7]
Bifascicular block: Associated with a risk of subsequent advanced AV block, syncope, and sudden cardiac death, particularly in the presence of alternating bundle branch block or transient high-degree AV block.[5]
Alternating bundle branch block: Implies unstable conduction disease in both bundles and carries a high risk of developing sudden complete heart block with a slow or absent ventricular escape rate.[1]
Diagnosis
History and Symptoms
Symptoms of conduction system disease depend on the degree and level of conduction impairment and may include:
Syncope or presyncope
Dizziness and lightheadedness
Dyspnea on exertion
Exercise intolerance
Heart failure symptoms (in the setting of ventricular dyssynchrony)
Asymptomatic conduction disease is common and may be detected incidentally on electrocardiogram.
Physical Examination
Physical examination findings may include:
Irregular pulse
Cannon A waves in the jugular venous pulse (in complete AV block)
Variable intensity of the first heart sound
Signs of heart failure (if present)
Electrocardiogram
The electrocardiogram (ECG) is the primary diagnostic tool. Key findings include:
SND: Sinus bradycardia, sinus pauses, sinoatrial exit block
First-degree AV block: PR interval >200 ms
Second-degree AV block:
Mobitz type I (Wenckebach): Progressive PR prolongation with eventual dropped QRS
Mobitz type II: Fixed PR interval with intermittent dropped QRS
Third-degree (complete) AV block: Complete AV dissociation
LBBB: QRS ≥120 ms with broad notched R waves in leads I, aVL, V5–V6 and rS or QS in V1–V3
RBBB: QRS ≥120 ms with rsR' in V1–V2 and wide S wave in I, V5–V6
Fascicular blocks: Left axis deviation (LAFB) or right axis deviation (LPFB)
Alternating bundle branch block: QRS complexes with alternating LBBB and RBBB morphologies
Diagnostic Workup
The 2018 ACC/AHA/HRS Guideline provides the following recommendations for evaluation of conduction disorders:[1][8]
| COR | LOE | Recommendation |
|---|---|---|
| I | B-NR | In patients with newly detected LBBB, a transthoracic echocardiogram to exclude structural heart disease is recommended. |
| I | C-LD | In symptomatic patients with conduction system disease in whom AV block is suspected, ambulatory electrocardiographic monitoring is useful. |
| IIa | B-NR | In selected patients with intraventricular conduction disorders other than LBBB, echocardiography is reasonable if structural heart disease is suspected. |
| IIa | B-NR | In patients with symptoms suggestive of intermittent bradycardia with conduction system disease identified by ECG and no demonstrated AV block, electrophysiology study (EPS) is reasonable. |
| IIa | C-LD | In selected patients with LBBB in whom structural heart disease is suspected and echocardiogram is unrevealing, advanced imaging (e.g., cardiac MRI, CT, or nuclear studies) is reasonable. |
| IIb | C-LD | In selected asymptomatic patients with extensive conduction system disease (bifascicular or trifascicular block), ambulatory electrocardiographic recording may be considered. |
| IIb | C-LD | In selected asymptomatic patients with LBBB in whom ischemic heart disease is suspected, stress testing with imaging may be considered. |
Cardiac MRI detected subclinical cardiomyopathy in one-third of patients with asymptomatic LBBB and a normal echocardiogram in one study, and identified significant abnormalities in 42% of patients with connective tissue disease and new-onset LBBB with normal echocardiograms in another study.[1]
Electrophysiology Study
An EPS may be helpful in selected patients with demonstrated conduction abnormalities in whom other testing has been unrevealing. In patients with fascicular or bundle branch block, a prolonged HV interval at EPS predicts a higher risk for complete heart block. However, EPS has low overall specificity and sensitivity.[1]
Genetic Testing
The 2022 EHRA/HRS/APHRS/LAHRS Expert Consensus Statement recommends genetic testing in patients with progressive cardiac conduction disease, particularly when familial or occurring at a young age, or when associated with cardiomyopathy or neuromuscular disease. SCN5A, LMNA, and TRPM4 are the core genes for testing.[3]
Differential Diagnosis
| Condition | Distinguishing Features |
|---|---|
| Sinus bradycardia (physiologic) | Asymptomatic, occurs in athletes and during sleep; normal response to vagal tone |
| Drug-induced bradycardia/conduction delay | Temporal relationship with beta-blockers, calcium channel blockers, digoxin, antiarrhythmic drugs; reversible with drug discontinuation |
| Hyperkalemia | Peaked T waves, widened QRS, sine wave pattern; correctable with potassium-lowering therapy |
| Hypothyroidism | Sinus bradycardia, low voltage, prolonged QT; elevated TSH |
| Cardiac sarcoidosis | AV block (often in young patients), ventricular arrhythmias; diagnosed by cardiac MRI and/or PET, biopsy |
| Lyme carditis | Fluctuating AV block, often in young patients with tick exposure; serologic testing |
| Chagas disease | RBBB, left anterior fascicular block; endemic exposure history; serologic testing |
| Myocarditis | Acute onset, often with chest pain and troponin elevation; cardiac MRI findings |
| Cardiac amyloidosis | Low voltage on ECG, increased wall thickness on echocardiography; tissue biopsy or nuclear imaging |
| Congenital heart block | Present from birth; associated with maternal anti-Ro/SSA antibodies in neonatal lupus |
Treatment
Management of conduction system disease depends on the type, severity, symptoms, and underlying etiology. The 2018 ACC/AHA/HRS Guideline emphasizes a patient-centered approach with shared decision-making.[1][8]
Initial Management
Identification and Treatment of Reversible Causes
Before considering permanent pacing, reversible causes of conduction disease should be identified and treated:
Discontinuation or dose reduction of offending medications (beta-blockers, calcium channel blockers, digoxin, antiarrhythmic drugs)
Treatment of hyperkalemia, hypothyroidism, or other metabolic derangements
Treatment of Lyme carditis with appropriate antibiotics
Evaluation for myocarditis, cardiac sarcoidosis, or other inflammatory conditions
Screening for obstructive sleep apnea, as treatment may reduce nocturnal bradycardias[8]
Acute Unstable Management
For patients with hemodynamically significant bradycardia due to conduction system disease:
Pharmacologic therapy:
Atropine 0.5 to 1.0 mg IV may be given in increments for second-degree or third-degree AV block believed to be at the AV nodal level associated with symptoms or hemodynamic compromise (Class IIa). Atropine is unlikely to improve AV block at the His bundle or His-Purkinje level and should be used judiciously in patients with wide QRS complexes suggesting significant His-Purkinje disease.[1]
Beta-adrenergic agonists (e.g., isoproterenol, dopamine, dobutamine, epinephrine) may be considered for symptomatic second-degree or third-degree AV block in patients with low likelihood for coronary ischemia (Class IIb).[1]
Intravenous aminophylline may be considered for AV block in the setting of acute inferior myocardial infarction (Class IIb).[1]
Temporary pacing:
Temporary transvenous pacing is reasonable for persistent hemodynamically unstable bradycardia refractory to medical therapy (Class IIa).[1]
Temporary transcutaneous pacing may be considered as a bridge to transvenous or permanent pacing in patients with severe symptoms or hemodynamic compromise (Class IIb). Analgesic and/or anxiolytic agents should be considered in conscious patients.[1]
For patients who require prolonged temporary transvenous pacing, an externalized permanent active fixation lead is reasonable over a standard passive fixation temporary pacing lead (Class IIa).[8]
Temporary pacing should not be performed in patients with minimal and/or infrequent symptoms without hemodynamic compromise (Class III: Harm).[1]
Medical Therapy
There is no pharmacologic therapy that provides long-term treatment for conduction system disease. Medical therapy is limited to acute stabilization as described above and treatment of underlying conditions. Chronic medical management focuses on:
Optimization of underlying cardiac conditions (heart failure, coronary artery disease)
Avoidance of medications that worsen conduction (when clinically feasible)
Treatment of contributing conditions (obstructive sleep apnea, thyroid disease)
Procedural / Surgical Therapy
Permanent Pacemaker Implantation
The 2018 ACC/AHA/HRS Guideline provides the following recommendations for permanent pacing in conduction disorders with 1:1 AV conduction:[1][8]
| COR | LOE | Recommendation |
|---|---|---|
| I | C-LD | In patients with syncope and bundle branch block who are found to have an HV interval ≥70 ms or evidence of infranodal block at EPS, permanent pacing is recommended. |
| I | C-LD | In patients with alternating bundle branch block, permanent pacing is recommended. |
| IIa | C-LD | In patients with Kearns-Sayre syndrome and conduction disorders, permanent pacing is reasonable, with additional defibrillator capability if appropriate and meaningful survival of greater than 1 year is expected. |
| IIb | C-LD | In patients with Anderson-Fabry disease and QRS prolongation >110 ms, permanent pacing may be considered. |
| IIb | C-LD | In patients with heart failure, mildly to moderately reduced LVEF (36%–50%), and LBBB (QRS ≥150 ms), CRT may be considered. |
| III: Harm | B-NR | In asymptomatic patients with isolated conduction disease and 1:1 AV conduction, permanent pacing is not indicated (in the absence of other indications for pacing). |
For AV block, the following key principles apply:[8]
In patients with acquired second-degree Mobitz type II AV block, high-grade AV block, or third-degree AV block not caused by reversible or physiologic causes, permanent pacing is recommended regardless of symptoms (Class I).
For all other types of AV block, in the absence of conditions associated with progressive AV conduction abnormalities, permanent pacing should generally be considered only in the presence of symptoms that correlate with AV block.
In SND, there is no established minimum heart rate or pause duration where permanent pacing is recommended. Establishing temporal correlation between symptoms and bradycardia is important.
Cardiac Resynchronization Therapy (CRT)
Cardiac resynchronization therapy is recommended for patients with symptomatic heart failure despite optimal medical therapy, LVEF ≤35%, sinus rhythm, and LBBB with QRS duration ≥150 ms (Class I). The CARE-HF trial demonstrated that CRT reduced all-cause mortality (20% vs. 30%; HR 0.63; 95% CI 0.51–0.77) compared with optimal medical therapy over a mean follow-up of 29.4 months.[9]
Subgroup analysis from the MADIT-CRT trial found that CRT reduced all-cause mortality or heart failure hospitalization only in patients with QRS duration ≥150 ms (HR 0.48; 95% CI 0.37–0.64) and LBBB morphology (HR 0.47; 95% CI 0.37–0.61). CRT is of no benefit when the QRS complex is narrow.[9]
In patients with AV block who have an indication for permanent pacing with LVEF 36%–50% and are expected to require ventricular pacing >40% of the time, pacing methods that maintain physiological ventricular activation (e.g., CRT or conduction system pacing) are reasonable over right ventricular pacing to prevent heart failure (Class IIa).[8]
Conduction System Pacing (CSP)
Conduction system pacing, encompassing His bundle pacing and left bundle branch area pacing (LBBAP), has emerged as a more physiological alternative to right ventricular pacing and as an alternative to biventricular CRT.[10]
See Also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 Kusumoto FM, Schoenfeld MH, Barrett C, Edgerton JR, Ellenbogen KA, Gold MR, Goldschlager NF, Hamilton RM, Joglar JA, Kim RJ, Lee R, Marine JE, McLeod CJ, Oken KR, Patton KK, Pellegrini CN, Selzman KA, Thompson A, Varosy PD (2019). "2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay". J Am Coll Cardiol. 74 (7): e51–e156. doi:10.1016/j.jacc.2018.10.044. PMID 30412709.
- ↑ 2.0 2.1 Baruteau AE, Probst V, Abriel H (2015). "Inherited progressive cardiac conduction disorders". Curr Opin Cardiol. 30 (1): 33–9. doi:10.1097/HCO.0000000000000134. PMID 25426816.
- ↑ 3.0 3.1 3.2 Wilde A, Semsarian C, Márquez MF, Sepehri Shamloo A, Ackerman MJ, Ashley EA, Sternick EB, Barajas-Martinez H, Behr ER, Bezzina CR, Breckpot J, Charron P, Chockalingam P, Crotti L, Davis MR, Gollob MH, Goodship J, Heathcote K, Ingles J, James CA, Juang JJ, Kääb S, Kaufman ES, Krahn AD, Lal S, Macciardi F, Marchal GA, Marquis-Gravel G, Milting H, Morales A, Nazer AN, Pilichou K, Ratbi I, Saffitz JE, Schulze-Bahr E, Tester DJ, Thiene G, van Tintelen JP, Ware JS, Wierzbowska-Drabik K, Zeppenfeld K (2022). "European Heart Rhythm Association (EHRA)/Heart Rhythm Society (HRS)/Asia Pacific Heart Rhythm Society (APHRS)/Latin American Heart Rhythm Society (LAHRS) Expert Consensus Statement on the State of Genetic Testing for Cardiac Diseases". Heart Rhythm. 19 (7): e1–e60. doi:10.1016/j.hrthm.2022.03.1225. PMID 35890826 Check
|pmid=value (help). Vancouver style error: initials (help) - ↑ Auffret V, Puri R, Urena M, Chamandi C, Rodriguez-Gabella T, Philippon F, Rodés-Cabau J (2017). "Conduction Disturbances After Transcatheter Aortic Valve Replacement: Current Status and Future Perspectives". Circulation. 136 (11): 1049–1069. doi:10.1161/CIRCULATIONAHA.117.028352. PMID 28893961.
- ↑ 5.0 5.1 Curtis AB, Karki R, Hattoum A, Sharma UC (2018). "Arrhythmias in Patients ≥80 Years of Age: Pathophysiology, Management, and Outcomes". J Am Coll Cardiol. 71 (18): 2041–2057. doi:10.1016/j.jacc.2018.03.019. PMID 29724357.
- ↑ 6.0 6.1 Ternes CM, Kim JA, Basu A, Poole JE, Chelu MG (2026). "Cardiac Resynchronization Therapy". JAMA. doi:10.1001/jama.2026.4893.
- ↑ 7.0 7.1 Sumner G, Salehian O, Yi Q, Engert J, Engert JC, Bhatt DL, Bhatt DL, Bhatt DL, Bhatt DL, Bhatt DL (2009). "The Prognostic Significance of Bundle Branch Block in High-Risk Chronic Stable Vascular Disease Patients: A Report from the HOPE Trial". J Cardiovasc Electrophysiol. 20 (7): 781–7. doi:10.1111/j.1540-8167.2009.01440.x. PMID 19490429.
- ↑ 8.0 8.1 8.2 8.3 8.4 8.5 8.6 Kusumoto FM, Schoenfeld MH, Barrett C, Edgerton JR, Ellenbogen KA, Gold MR, Goldschlager NF, Hamilton RM, Joglar JA, Kim RJ, Lee R, Marine JE, McLeod CJ, Oken KR, Patton KK, Pellegrini CN, Selzman KA, Thompson A, Varosy PD (2019). "2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay: Executive Summary". J Am Coll Cardiol. 74 (7): 932–987. doi:10.1016/j.jacc.2018.10.043. PMID 30412710.
- ↑ 9.0 9.1 Murphy SP, Ibrahim NE, Januzzi JL Jr (2020). "Heart Failure With Reduced Ejection Fraction: A Review". JAMA. 324 (5): 488–504. doi:10.1001/jama.2020.10262. PMID 32749493 Check
|pmid=value (help). - ↑ {{cite journal |vauthors=Glikson M, Burri H, Abdin A, et al |title=European Society of Cardiology (ESC) Clinical Consensus Statement on Indications for Conduction System Pacing |journal=Europace |volume=27 |issue=4