Bradycardia

• Sinus bradycardia is a cardiac rhythm originating from the sinus node with a heart rate of fewer than 60 bpm.
• Diagnosis is confirmed with an ECG showing normal sinus rhythm at a rate lower than 60 bpm.

Presentation:

• Most patients are asymptomatic.
• Symptoms, when present, include fatigue, lightheadedness, dizziness, exercise intolerance, syncope, worsening angina, heart failure, or cognitive slowing.

Etiologies of Sinus Bradycardia

Sinus bradycardia can result from a wide range of intrinsic (inherent) and extrinsic factors. Understanding these causes is essential for targeted management.

Intrinsic (Inherent) Etiologies

1. Cardiac and Structural Causes:
   • Chest trauma
   • Ischemic heart disease
   • Acute myocardial infarction
   • Acute and chronic coronary artery disease
   • Repair of congenital heart disease
   • Sick sinus syndrome – dysfunction of the sinus node often due to fibrotic or degenerative changes in the node or surrounding tissues.
   • Radiation therapy affecting the heart
2. Conduction System Disorders:
   • Heart Block (2nd and 3rd Degree)
   • Bifascicular and Trifascicular Blocks
   • Bundle Branch Blocks:
     • LBBB
     • RBBB
     • Bifascicular and Trifascicular Blocks
   • Tachy-Brady Syndrome (Brady-Tachy Syndrome)
   • Atrial Arrhythmias – Slow Atrial Fibrillation (AF)
   • Atrial Standstill: A rare condition characterized by the absence of atrial electrical activity, leading to bradycardia due to reliance on escape rhythms.
   • Idioventricular Rhythm: A slow, ventricular-origin rhythm that takes over when higher conduction system components fail, usually seen in severe AV block or sinus arrest.
   • Junctional Bradycardia: Slowed rhythm originating from the AV node or junction, often due to impaired sinus node function or as a response to high vagal tone.
3. Infectious and Inflammatory Conditions:
   • Myocarditis
   • Pericarditis
   • Lyme disease
   • Rheumatic fever
4. Systemic and Infiltrative Diseases:
   • Amyloidosis
   • Collagen vascular diseases
5. Neuromuscular Disorders:
   • X-linked muscular dystrophy
   • Neuromuscular disorders affecting the cardiac conduction system
6. Genetic and Inherited Conditions:
   • Familial disorders associated with bradycardia
   • Inherited channelopathies affecting cardiac ion channels

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Extrinsic Etiologies

1. Autonomic and Reflex-Mediated Causes:
   • Vasovagal stimulation (e.g., during endotracheal suctioning)
   • Carotid sinus hypersensitivity
2. Medications:
   • Cardiovascular Drugs:
     • Beta-blockers
     • Calcium channel blockers
     • Digoxin
     • Ivabradine
     • Clonidine
     • Reserpine
     • Adenosine
   • Antiarrhythmic Agents (Classes I to IV):
     • Various antiarrhythmics impacting cardiac conduction
   • Other Medications:
     • Cimetidine
     • Lithium
     • Amitriptyline
3. Substance Use:
   • Narcotics
   • Cannabinoids
4. Metabolic and Endocrine Disorders:
   • Hypothyroidism
   • Hyperkalemia
5. Respiratory and Sleep-Related Factors:
   • Sleep apnea
   • Hypoxia
6. Neurological Causes:
   • Intracranial hypertension
7. Nutritional and Psychological Factors:
   • Anorexia nervosa

Evaluation of Sinus Bradycardia

1. History and Physical Examination:
   • Vital Signs: Check respiratory rate, blood pressure, temperature, and heart rate.
   • ECG
   • Assessment of Hemodynamic Stability:
     • Signs of Instability: High blood pressure, altered mental status, difficulty breathing, or syncope.
     • Hemodynamically Stable Patients: If stable and asymptomatic, especially in young, healthy, or athletic individuals, no immediate intervention may be needed.
   • Medical History: Explore any underlying conditions, including heart disease, history of ischemia, recent weight loss (notably in post-bariatric patients), and medication history.
2. Laboratory Investigations:
   • BGL: Evaluate for hypoglycemia, which can contribute to bradycardia.
   • Electrolytes: Check sodium, potassium, and chloride levels, as electrolyte imbalances can impact heart rhythm.
   • Calcium and Magnesium Levels: Imbalances can affect cardiac conduction.
   • Thyroid Function Tests (TSH, T3, T4): Hypothyroidism is a common cause of bradycardia.
   • Troponin Levels: To assess for myocardial infarction, especially if bradycardia occurs alongside chest pain or other ischemic symptoms.
   • Toxicology Screen: Check for drug intoxications (e.g., beta-blockers, calcium channel blockers, digoxin).
3. Imaging and Additional Diagnostics:
   • Chest X-ray: Useful if an infectious or structural etiology is suspected.
   • Blood Cultures and Viral Panel: Indicated if there is a suspicion of infectious etiology.
   • Urinalysis: To rule out infections that may indirectly affect heart rate.
4. Further Evaluation for Stable Patients:
   • Assess for Underlying Etiologies: If no signs of myocardial infarction and the patient is stable, investigate for infectious causes, thyroid dysfunction, or other reversible factors.
   • Sick Sinus Syndrome Evaluation: If recurrent symptoms or bradycardia persists despite correction of underlying issues, consider evaluation for sick sinus syndrome or long-term cardiac monitoring with an implantable loop recorder.

Treatment and Management

1. Initial Management Based on Stability:
   • Hemodynamically Unstable Patients:
     • Administer IV Atropine 0.5 mg every 3-5 minutes, up to a total of 3 mg.
     • If atropine is ineffective, consider temporary pacing
       • transcutaneous pacemaker
       • chemical pacemaker
         • Isoprenaline 0.02 to 0.06 mg/hour, titrate according to clinical response.
   • Stable Patients with Acute Myocardial Infarction (AMI):
     • Manage per standard AMI protocols, including reperfusion strategies, anticoagulation, and monitoring.
2. Hypothermic Patients:
   • Rewarm to normothermia before making definitive decisions on bradycardia management, as hypothermia itself can slow the heart rate.
3. Medication Review:
   • Identify and Withdraw Culprit Medications: Review current medications for potential contributors to bradycardia (e.g., beta-blockers, digoxin, calcium channel blockers).
   • Permanent Pacemaker Indication: Consider in patients requiring medication that causes bradycardia or those whose bradycardia persists despite medication withdrawal.

Differential Diagnosis of Bradyarrhythmias

1. Sick Sinus Syndrome (SSS):
   • Characterized by a malfunctioning sinus node, often seen in older adults, and can be associated with bradycardia, sinus pauses, or tachy-brady syndrome.
2. Wandering Atrial Pacemaker:
   • A variable atrial rhythm with multiple P-wave morphologies, typically benign and transient.
3. Junctional Escape Rhythm:
   • Originates from the AV node or junctional tissue with absent or retrograde P waves; often a compensatory rhythm when sinus node function is depressed.
4. Ventricular Escape Rhythm:
   • A wide QRS complex rhythm that arises from the ventricles, typically seen in severe sinus node dysfunction or high-grade AV block.

Prognosis

• General Prognosis: Good if identified and managed early, especially in transient or reversible causes.
• Sick Sinus Syndrome: Prognosis is variable; five-year survival rates range from 45-70%, with poorer outcomes in patients with significant comorbidities.

Sick Sinus Syndrome (SSS)

Prevalence: Approximately 0.03% of the population; increases with age.

Definition:
Sick sinus syndrome is an irreversible dysfunction of the sinus node, commonly seen in the elderly. It is characterized by impaired impulse formation due to chronic fibrotic degeneration or calcification of the sinus node and surrounding atrial tissues.

Key Features on ECG:

• Long intervals between consecutive P waves (> 2 seconds).
• Pauses may be a multiple of the basic sinus interval (sino-atrial block) or not (sinus arrest).
• Sinus bradycardia and pauses may allow tachyarrhythmias to emerge, known as Brady-Tachy Syndrome.

Arrhythmias Associated with SSS:

• Sinus bradycardia
• Sinus arrest
• Sinoatrial exit block
• Atrial arrhythmias (e.g., atrial flutter, atrial fibrillation)
• Bradycardia-tachycardia syndrome
• Atrioventricular blocks
• Inadequate heart rate response to stressors (pain, fever, exercise)

Diagnosis:

• A single ECG is generally inadequate; multiple ECGs or electrophysiologic studies are required.

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Atrioventricular (AV) Conduction Block

https://www.mygpnotes.com/cardiology/heart-block/

Definition:
AV conduction block occurs when there is a failure in the conduction of electrical impulses from the atria to the ventricles, typically due to disorders at the AV node, bundle of His, or bundle branches.

Classification of AV Block:

1. First-Degree AV Block: Minimal conduction delay; often asymptomatic.
2. Second-Degree AV Block:
   • Mobitz Type 1 (Wenckebach Phenomenon): Progressive delay with eventual dropped beat.
   • Mobitz Type 2: Intermittent dropped beats without prior lengthening of PR interval.
   • 2:1 Second-Degree Block: Every other impulse fails to conduct.
3. Third-Degree (Complete) AV Block: No conduction between atria and ventricles; requires urgent intervention.
4. High-Degree AV Block: Multiple consecutive non-conducted P waves.
5. Paroxysmal AV Block: Intermittent occurrence.

Prevalence:
Ranges from 0.015% to 0.1%; may co-occur with sinus node dysfunction.

Management:

• Dual-chamber pacing (maintains AV synchrony) is preferred over single-chamber pacing to mimic physiological heart rhythms, especially in sick sinus syndrome without AV block.