bradycardia
BRADYCARDIA

Bradycardia is having a heart rate of <60 beats/minute that may not affect the hemodynamic status of some patients.
A heart rate that is inadequate for the patient's current condition and may not be able to support life is a clinically significant bradycardia.

Bradycardia can be caused by problems in the sinoatrial node, problems in the conduction pathways of the heart, metabolic problems or heart attack/disease that caused damage to the heart.

The different types of bradycardia are sinus bradycardia, sick sinus syndrome, tachycardia-bradycardia syndrome, hypersensitive carotid sinus syndrome, sinus pause/arrest, sinoatrial node exit block and atrioventricular block.

Classification

Types of Bradycardia
Sinus bradycardia

  • Results from decreased sympathetic tone or increased vagal stimulation
  • May also be a physiologic response in young healthy individuals especially in well-trained athletes
  • Pathophysiologic causes includes:
    • Cardiac disease [ischemic/hypertensive heart disease, acute myocardial infarction (MI), cardiomyopathy]
    • Surgery (eyes, congenital heart disease, transplant)
    • Infections/inflammation (meningitis, Lyme disease, viral myocarditis, pericarditis)
    • Trauma, hypothermia, severe hypoxia, collagen vascular disease
    • Medications (antiarrhythmic agents, cardiac glycosides, antihypertensive agents, antipsychotic agents)
    • Electrolyte imbalances (hyperkalemia)
    • Inherited diseases (Friedreich ataxia, x-linked muscular dystrophy, familial disorders)
    • Increased intracranial pressure
    • Intracranial tumors, cervical/mediastinal tumors

Sick sinus syndrome (SSS)

  • A type of arrhythmia involving the sinus node and surrounding tissues
  • Encompasses sinoatrial (SA) node dysfunction that is accompanied by relevant clinical symptoms (lightheadedness, syncope, palpitations)
  • Includes sinus bradycardia, sinus pause/arrest, sinoatrial (SA) node exit block, ectopic atrial bradycardia, atrial fibrillation with slow ventricular response
  • Common causes include degenerative sinus node fibrosis, cardiac diseases (cardiomyopathies, myocarditis, pericarditis, rheumatic heart disease), medications (eg beta blockers, Digoxin, anti-arrhythmics, calcium channel blockers), hereditary diseases

Tachycardia-bradycardia syndrome

  • A variant of sick sinus syndrome (SSS) that features slowing of the heart rate alternating with increased heart rate

Hypersensitive carotid sinus syndrome

  • Also called carotid sinus hypersensitivity, carotid sinus syncope
  • Bradycardia due to vagal stimulation & sympathetic withdrawal brought about by increased sensitivity of the afferent/efferent limbs of the carotid sinus reflex arc
  • May result from sinus pause/arrest or sinoatrial (SA) node block

Sinus pause (Sinus arrest)

  • Characterized by absence of impulse generation by the sinoatrial (SA) node resulting in failure of atrial depolarization
  • May be benign or pathologic
  • Etiologic agents include acute myocardial infarction (MI), excessive vagal tone, degenerative fibrotic changes, medications (ie Quinidine, Procainamide, Digitalis)

Sinoatrial (SA) node exit block

  • Characterized by failure of atrial depolarization caused by the blockage of an impulse out of the sinoatrial (SA) node
  • Subtypes include 2nd degree type 1 (Wenckebach) sinoatrial (SA) exit block, 2nd degree type 2 sinoatrial (SA) exit block, and 3rd degree sinoatrial (SA) exit block
  • Etiologic agents include acute myocarditis, myocardial infarction (MI), excessive vagal stimulation, medications (ie Quinidine, Procainamide, Digitalis)

Atrioventricular (AV) Block

  • May be brought about by drugs, electrolyte imbalance, or structural anomalies resulting from myocardial infarction (MI) or other myocardial diseases
  • Brought about by delayed or interrupted impulse conduction originating from the atria to the ventricles
  • Usual causes include history of myocardial infarction (MI) and myocarditis
  • Risk of asystole is indicated by:
    • Recent asystole
    • Mobitz type II atrioventricular (AV) block
    • Complete (3rd degree) heart block
    • Ventricular standstill of >3 seconds

Classification of Atrioventricular (AV) blocks

1st Degree Atrioventricular (AV) Block

  • Prolonged PR interval of >0.20 seconds
  • Involves the atrioventricular (AV) node or His-Purkinje system

2nd Degree Atrioventricular (AV) Block

  • Mobitz I (Wenckebach)
    • Progressive prolongation of PR interval before and a shortened PR interval following a blocked impulse
    • Block located at the atrioventricular (AV) node
    • Mostly transient
    • Patient may be asymptomatic
  • Mobitz II
    • Fixed PR interval prior to & following a blocked impulse accompanied by a wide QRS complex
    • Block is located at the Bundle of His or bundle branches
    • Patient is often symptomatic
    • May progress to 3rd degree block

3rd Degree Atrioventricular (AV) Block

  • Complete atrioventricular (AV) block
    • Absence of atrial impulse traveling to the ventricles
    • R-R interval longer than P-P interval
  • May occur at the atrioventricular (AV) node, bundle of His, or bundle branches
  • May be congenital or acquired as a result of acute ischemia, medications, or other pathologic diseases

Evaluation

Establish that the cause of the following is bradycardia

Signs and Symptoms of Poor Perfusion and Clinical Instability

  • Hypotension
  • Shock
  • Congestive heart failure (CHF)
  • Altered mental status
  • Chest pain
  • Seizures
  • Syncope

Laboratory Tests

Electrocardiogram (ECG)

  • 12-Lead ECG is the main test used to detect and diagnose cardiac arrhythmias
  • Ambulatory ECG (Holter or external loop recorders) monitoring may be considered when signs and symptoms are suggestive of paroxysmal or intermittent bradyarrhythmias, especially atrioventricular (AV) blocks

Electrophysiological Study (EPS)

  • Used to confirm if syncope is secondary to bradyarrhythmic mechanisms especially in patients w/ bundle branch block, sinus bradycardia, history of myocardial infarction (MI)
  • Invasive electrophysiological study (EPS) may be used to assess conduction of impulse from the atrium to the ventricle while patient is at rest
    • Also used to identify patients at increased risk of sudden cardiac death, as treatment for termination of tachycardia, and to evaluate the efficacy of current intervention

Other Studies

  • Exercise testing may be considered in patients who experience syncope during or after exertion
    • Not to be used as the sole diagnostic test for measurement of heart rate ranges during exercise
  • Upright tilt-table testing may be used to identify patients with syncope secondary to vasodepressor or cardioinhibitory response

Interpretation of ECG

Sinus Bradycardia

  • Occurs when the sinus node discharge rate is <60 beats per minute
  • Features normal P waves which occurs before each QRS complex
  • Negative P wave in lead aVR & upright P wave in leads I, II & aVL indicates sinus node origin
  • PR interval usually >120 milliseconds

Sinus Pause (Sinus Arrest)

  • Absence of P waves on ECG due to failure of impulse generation

Sinoatrial (SA) Node Exit Block

  • Interruptions with similar duration to a normal P-P interval signifies impulse interruption brought about by an exit block

Hypersensitive carotid sinus syndrome

  • Normal ECG findings but with ventricular asystole of >3 seconds duration during carotid sinus stimulation
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