Written by Dr. Vidya Viswanath, M.B.B.S, M.D. | 
Medically Reviewed by Dr. Nithin Jayan, MBBS, DNB on Mar 11, 2020

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Abnormal rhythms of the heart can adversely affect the pumping ability of the heart. A heart rate of more than 100/min is called tachycardia and a rate of less than 60/min is called bradycardia, or a slow heart rate.

Causes of bradycardia may be physiological as seen during sleep and in athletes. When the bradycardia is pathological it results from a failure of impulse initiation or impulse conduction. This is due to the Sino Atrial (SA) Node Dysfunction or due to AtrioVentricular (AV) block.

The patient may not have any symptoms at all; some present with a wide range of symptoms. Electrocardiographic recordings during an attack are most helpful in diagnosis. The treatment is tailored according to the symptoms by the cardiologist.(1)

Causes of Bradycardia

Bradycardia can be physiological or pathological

Physiological Causes

a) Athletes

An athlete’s heart is considerably large and strong and this allows him to pump a large stroke volume output per beat even during rest. During periods of rest, the large volumes of blood pumped into the arterial tree with each beat initiates feedback circulatory reflexes to cause bradycardia.

Physiological Cause of Bradycardia

b) During Sleep

Pathological Causes

Bradycardia results from a failure of impulse initiation or impulse conduction.

Thus bradycardia can be caused by

  • Reduced Automaticity as in Sinus bradycardia
  • Blocked or automatically slow conduction as in atrioventricular blocks

The most common causes of pathologic bradycardia are

SA NODE DYSFUNCTION: SA (sinoatrial) node is a specialised bundle of neurons that generate electrical impulses in the heart, it is called the pacemaker.

AV CONDUCTION BLOCK: The AV (atrioventricular) node is a small mass of specialized cardiac muscle fibres that receive impulses from the SA node.

Pathological Cause of Bradycardia

SA Node Disease

Causes are Intrinsic and Extrinsic

Extrinsic Causes

Extrinsic is reversible and due to

1. Autonomic causes

  • Carotid sinus hypersensitivity
  • Vasovagal Stimulation
  • Sometimes even a mild external pressure on the neck elicits a strong baroreceptor reflex, causing extreme bradycardia due to vagal-acetyl choline effects on the heart. This happens in carotid sinus syndrome where the pressure receptors (baroreceptors) in the carotid sinus region of the carotid artery walls are very sensitive.

2. Drugs like β blockers, Calcium channel blockers, Digoxin, and others

3. Hypothyroidism

Hypothyroidism: Extrinsic Cause of SA Node Dysfunction

4. Sleep Apnea

5. Endotracheal suctioning

6. Hypothermia

7. Increased intracranial pressure

Intrinsic Causes

1. Myocardial infarction

Heart Attack: Intrinsic Cause of SA Node Dysfunction

2. Sinus node disease ( Sick Sinus Syndrome)

3. Coronary artery disease

4. Cholestatic jaundice

5. Idiopathic fibrotic disease.

6. Congenital heart disease

7. Amyloidosis

8. Chest Trauma

9. Radiation Therapy

10. Post Surgical

Rare Heritable Form of Sinus Node Disease:

Atrioventricular Conduction Disease

This can cause pathologic bradycardia and the causes for Atrioventricular block are-

1. Carotid sinus hypersensitivity

2. Vasovagal

3. Hyperkalemia

4. Hypothyroidism

5. Drugs like β blockers, Calcium channel blockers, Digitalis and others

6. Infections like Endocarditis, Tuberculosis and others

7. Rheumatoid arthritis

8. Amyloidosis

9. Tumors like lymphomas, melanomas and others

10. Coronary artery disease

11. Congenital heart disease and other congenital, heritable conditions(2)

Symptoms of Bradycardia

SA node dysfunction may be completely asymptomatic or the patient may present with syncopal attacks, palpitations or fatigue.

It could also be associated with tachycardia-bradycardia syndrome where symptoms may be due to slow and fast heart rates.

In many cases symptoms can result from concomitant cardiac disease.(3)

Diagnosis of Bradycardia

SA node dysfunction is a clinical or an electrocardiographic diagnosis.

Diagnosis of Bradycardia

Longer term recording Holter or event monitors may permit correlation of the symptoms with cardiac rhythm.

Autonomic nervous system testing is useful in diagnosing carotid sinus hypersensitivity.

Documentation of the sinus pauses during an attack of symptoms provides the best diagnostic information.(4)

Management of Bradycardia

In certain circumstances, sinus bradycardia requires no specific treatment or only temporary rate support.

Pacemaker implantation is the primary therapeutic intervention in patients with symptomatic SA node dysfunction and in patients with atrioventricular dysfunction.

Management of Bradycardia

Chronic pharmacologic therapy for sinus bradyarrhythmias is limited.(5)

References:

  1. Bradycardia: Slow Heart Rate - (https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/bradycardia--slow-heart-rate)
  2. Bradycardia - (https://www.cedars-sinai.edu/Patients/Health-Conditions/Bradycardia.aspx)
  3. Signs and Symptoms of Bradycardia - (https://www.cardiosmart.org/Heart-Conditions/Bradycardia/Understand-Your-Condition/Signs-and-Symptoms)
  4. Diagnosing Bradycardia - (https://www.virginiamason.org/bradycardia)
  5. Treatment of Bradycardia - (https://www.health.harvard.edu/a_to_z/bradycardia-a-to-z)

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