What are the types, symptoms, and treatments of heart conduction disorders, including atrioventricular (AV) block, bundle branch block, and sick sinus syndrome, in patients with symptoms such as dizziness, fainting, or palpitations?

Heart Conduction Disorders: Types, Symptoms, and Treatment

Major Categories of Conduction Disorders

Heart conduction disorders encompass three primary categories: sinus node dysfunction (sick sinus syndrome), atrioventricular block, and bundle branch block, each requiring distinct diagnostic and therapeutic approaches based on symptom correlation and ECG findings. 1

Sinus Node Dysfunction (Sick Sinus Syndrome)

Pathophysiology and Presentation:

• Results from age-dependent progressive fibrosis of sinus nodal tissue and surrounding atrial myocardium 1
• Manifests as sinus bradycardia, sinus arrest, sinoatrial block, or "tachy-brady syndrome" (alternating tachycardia and bradycardia) 1, 2
• Symptoms include dizziness, syncope, fatigue, and palpitations when bradycardia correlates temporally with symptoms 1
• Chronotropic incompetence (failure to achieve age-appropriate heart rate with exercise) may occur 1

Critical Diagnostic Principle:

• No established minimum heart rate or pause duration mandates pacing; temporal correlation between symptoms and documented bradycardia is essential 1
• Nocturnal bradycardia alone does not indicate pacing need; screen for sleep apnea first 1

Treatment Algorithm:

• Permanent pacemaker implantation is indicated when clear association exists between significant symptoms and documented bradycardia 3
• Physiological pacing (atrial or dual-chamber) is superior to single-chamber ventricular pacing 3, 2
• Atrial-based rate-responsive pacing (DDDR) is preferred to minimize exertion-related symptoms 3, 2
• Eliminate medications exacerbating bradycardia before attributing symptoms to intrinsic disease 3, 2
• Despite adequate pacing, syncope recurs in approximately 20% of patients due to vasodepressor mechanisms 3, 2

Medication Contraindications:

• Beta-blockers are contraindicated 3, 2
• Non-dihydropyridine calcium channel blockers (verapamil, diltiazem) are contraindicated unless functioning ventricular pacemaker present 3, 2
• Class IC antiarrhythmics can unmask underlying sinus node dysfunction 2

Natural History:

• Progression to AV block occurs at approximately 0.6-0.85% per year 4, 5
• Atrial fibrillation develops more frequently in sick sinus syndrome (35%) compared to AV block patients (17%) 6

Atrioventricular Block

Classification and Management:

Second-Degree Mobitz Type II, High-Grade, or Third-Degree AV Block:

• Permanent pacing is recommended regardless of symptoms when not caused by reversible or physiological causes 1
• These represent infranodal conduction disease with high risk of progression 1

First-Degree and Mobitz Type I (Wenckebach) AV Block:

• Pacing indicated only when symptoms clearly correlate with documented block 1
• First-degree AV block alone without symptoms does not require pacing 1

Bifascicular and Trifascicular Block:

• Permanent pacing indicated for bifascicular block with intermittent complete heart block and symptomatic bradycardia 1
• Bifascicular/trifascicular block with intermittent type II second-degree AV block warrants pacing even without symptoms 1
• Bifascicular block with syncope of unclear etiology may warrant pacing when other causes excluded 1
• Isolated bifascicular block without AV block or symptoms does not require pacing 1

Special Populations - Neuromuscular Disorders:

• Permanent pacemaker may be considered for myotonic muscular dystrophy, Kearns-Sayre syndrome, Erb dystrophy, and peroneal muscular atrophy with any degree of AV block (including first-degree) due to unpredictable progression 1
• Maintain low threshold for investigating symptoms or ECG findings in muscular dystrophies once cardiac involvement occurs 1
• Screening with 12-lead ECG and echocardiogram should be routine regardless of symptoms 1

Post-Myocardial Infarction:

• Transient AV block without intraventricular conduction defects does not require permanent pacing 1
• Persistent high-grade block requires permanent pacing 1

Bundle Branch Block

Left Bundle Branch Block (LBBB):

• LBBB markedly increases likelihood of underlying structural heart disease and left ventricular systolic dysfunction 1
• Echocardiography is the most appropriate initial screening test 1
• LBBB criteria: QRS duration ≥120 ms in adults, broad notched/slurred R wave in leads I, aVL, V5, V6, absent Q waves in I, V5, V6, R peak time >60 ms in V5-V6 1
• Incomplete LBBB: QRS 110-119 ms with left ventricular hypertrophy pattern and R peak time >60 ms 1

Right Bundle Branch Block (RBBB):

• RBBB without axis deviation progresses to pacemaker-requiring AV block or sick sinus syndrome at 3.77-fold and 6.28-fold increased risk respectively 7
• RBBB with axis deviation carries 3.03-fold higher risk for pacemaker implantation compared to RBBB without axis deviation 7
• RBBB with axis deviation patients are younger at diagnosis (59 vs 74 years) but take longer to progress (15 vs 6 years) 7

Nonspecific Intraventricular Conduction Delay:

• QRS duration >110 ms without RBBB or LBBB morphology criteria 1
• Does not require specific intervention unless associated with symptoms or structural disease 1

Monitoring and Follow-Up

Symptomatic Patients Awaiting Pacemaker:

• Continuous ECG monitoring required until definitive pacing established 3
• Patients with complete heart block or long sinus pauses prone to torsades de pointes 3

Asymptomatic Bradycardia:

• In-hospital monitoring not required; untreated sinus node dysfunction does not influence survival 3

Medication Initiation:

• Avoid out-of-hospital initiation of Class IC agents in symptomatic sick sinus syndrome due to risk of worsening sinus node dysfunction and AV block 3
• Dofetilide requires mandatory 3-day inpatient ECG monitoring per FDA requirements 3
• Sotalol requires 48-72 hours ECG monitoring with QT interval measurement; discontinue if QTc exceeds 500 ms 3

Common Pitfalls to Avoid

• Do not pace based solely on heart rate or pause duration without symptom correlation in sinus node dysfunction 1
• Do not attribute symptoms to bradycardia without first eliminating bradycardia-exacerbating medications 3, 2
• Do not overlook sleep apnea as cause of nocturnal bradycardia before considering pacing 1
• Do not use single-chamber ventricular pacing when physiological pacing options available 3, 2
• Do not delay pacemaker implantation in neuromuscular disorders once cardiac involvement documented, even with first-degree AV block 1
• Do not miss screening for structural heart disease when LBBB identified 1