Heart block develops when the normal conduction route that is followed by electrical impulses through the SA node, AV node and bundle branches in the heart becomes interrupted.

The SA node is a small mass of tissue located in the right upper heart chamber (right atrium); it is the area from where electrical impulses are originated for the heart; from there the impulses spread via the His Purkinje and bundle branches to the lower heart chambers and rest of heart).

The AV node is a small mass of tissue located between the upper and lower heart chambers in the wall of the right atrium; the AV node passes impulses received from the atria to the ventricles (lower heart chambers).

Why does this happen and how may it affect the patient’s health?

Blocking of the normal conduction path may occur due to

  • A heart attack
  • Ischaemic (heart muscle does not get enough oxygen) heart disease as a result of atherosclerosis (accumulation of fats/lipids on the walls of blood vessels, causing progressive thickening of these lipid plaques and narrowing or blockage of the blood vessel lumen) providing too little oxygen to the heart muscle
  • Birth defects of the heart
  • High blood pressure
  • Heart failure
  • Degeneration of the conduction system between the upper heart chambers (atria) and lower heart chambers (ventricles); mostly seen in the elderly

The blockage may occur at three places

  • SA node (sinoatrial node)
  • AV node (atrioventricular node), or
  • Bundle branches that conduct the impulse through the ventricles (lower heart chambers)

There are various types of heart block

First degree heart block

  • Delay in conduction from SA to AV node

Second degree heart block

  • Wenckebach/Mobitz I: Progressive lengthening of time between contraction of atria (upper heart chamber) and contraction of ventricles (lower heart chamber), and eventually the atrial contraction falls away before ventricle contraction)
  • Mobitz II

Third degree heart block

  • There is complete disassociation between the atria (upper heart chambers) and the ventricles (lower heart chambers) (the upper and lower heart chambers contract independently of one another and are not synchronized

What symptoms may the patient experience?

  • Very slow heartbeat; it may be so slow that the patient faints or feels like fainting; every now and then the heart completely loses a beat (the heart does not contract)
  • Shortness of breath
  • In many instances the patient may not experience any adverse symptoms

How is the diagnosis made and what special investigations are required?

ECG (electrocardiogram; measuring the electrical output of the heart) demonstrates various types of heart block.

What is the treatment and prognosis?

No treatment is required if the

  • Patient does not experience any adverse symptoms
  • Heart rate is not slower than fifty beats per minute

If the patient experiences adverse symptoms, an electrical pacemaker may be required (definitely necessary in complete heart block).

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