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 Node Dysfunction (SND)

  • Previously referred to as sick sinus syndrome (SSS) 
  • A type of arrhythmia involving the sinus node and surrounding tissues
  • Encompasses various sinus nodal abnormalities including persistent spontaneous sinus bradycardia not due to drugs and inappropriate for the physiologic circumstance, sinus arrest or exit block, combinations of sinoatrial and atrioventricular conduction disturbances, and alteration of paroxysms of rapid regular or irregular atrial tachyarrhythmias and periods of low atrial and ventricular rates 
  • Encompasses sinoatrial node dysfunction that is accompanied by relevant clinical symptoms (lightheadedness, syncope, palpitations)
  • 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 node dysfunction (SND) 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 and 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 (Heart 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 with wide QRS
    • 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 and 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 with 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

Echocardiography

  • Transthoracic echocardiography (TTE) is recommended in patients with newly identified 2nd-degree Mobitz type II atrioventricular (AV) block, high-grade atrioventricular block, or 3rd-degree atrioventricular block with or without coronary artery disease or structural heart disease
    • Some structural heart diseases (eg cardiomyopathy, congenital anomalies, tumors, valvular heart disease) are known to be associated with bradycardia or conduction disturbances

Other Studies

  • Advanced imaging such as cardiac computed tomography (CT), cardiac magnetic resonance imaging (MRI) or nuclear imaging may be done in selected patients with sinus node dysfunction when structural disease is suspected and not detected in other diagnostic modalities
  • 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 and upright P wave in leads I, II and 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 electrocardiogram (ECG) findings but with ventricular asystole of >3 seconds duration during carotid sinus stimulation
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