Heart Block

Heart block (HB) is a disorder in the heart's rhythm due to a fault in the natural pacemaker. This is caused by an obstruction in the electrical conduction system of the heart. The condition can sometimes be inherited.

Despite its severe-sounding name, heart block may cause no symptoms at all in some cases, or occasional missed heartbeats in other cases, which can cause light-headedness, syncope (fainting), and palpitations.

In severe cases, it may require the implantation of an artificial pacemaker, depending on the location and severity of the conduction impairment.

Heart block should not be confused with other serious heart conditions such as angina, heart attack, heart failure, cardiogenic shock, arrhythmias, cardiac arrest, or respiratory arrest.

Pathophysiology

The human heart maintains and initiates regular heartbeats using electrical signals. These signals are generated by the sinoatrial node (SA node) and then travel to the atrioventricular node (AV node), which also contains a secondary pacemaker. The conduction path continues to the bundle of His and then via the bundle branches to the apex of the fascicular branches.

Blockages are classified based on their location:

• SA node (Sinoatrial block)
• AV node (AV block or AVB)
• At or below the bundle of His (Intra-Hisian or Infra-Hisian block)

Types of Heart Block

SA Nodal Blocks

SA blocks rarely cause severe symptoms because the AV node's secondary pacemaker can maintain a heart rate of 40 - 60 beats per minute, which is usually sufficient. However, symptoms can still occur, and SA blocks are a common indication for pacemaker implantation in the US.

Types of SA nodal blocks include:

• SA node Wenckebach (Mobitz I)
• SA node Mobitz II
• SA node exit block

AV Nodal Blocks

AV nodal blocks are more serious and are divided into three types:

• First-degree AV block: Delayed conduction without missed beats.
• Second-degree AV block:
  • Type 1 (Mobitz I or Wenckebach): Progressive delay until a beat is missed.
  • Type 2 (Mobitz II): Sudden missed beats without prior delay, often indicating a block in or below the bundle of His.
• Third-degree AV block (Complete heart block): No atrial impulses reach the ventricles, requiring an escape rhythm from a lower pacemaker site.

Conduction system of the heart

Sinus rhythm with acute inferior infarction complicated by Type I AV block manifest in the form of 5:4 Wenckebach periods; R-P/P-R reciprocity.

Sinus rhythm (rate = 100/min) with 3:2 and 2:1 Type II AV block; right bundle branch block

Sinus tachycardia with complete AV block and resulting junctional escape

Infra-Hisian Blocks

Infra-Hisian blocks occur in the distal conduction system and include:

• Type 2 second-degree heart block (Mobitz II): Due to a block within or below the bundle of His.
  • Left anterior fascicular block
  • Left posterior fascicular block
• Right bundle branch block
• Left bundle branch block

Signs and Symptoms

Symptoms of heart block can vary widely depending on the severity and location of the block. They may include:

• Light-headedness
• Syncope (fainting)
• Palpitations
• Possible absence of symptoms in mild cases

Diagnosis

Diagnosis is typically made via electrocardiogram (ECG), which can reveal the specific type and location of the block. The ECG features of different blocks are distinct and can help in pinpointing the exact issue.

Treatment

Treatment depends on the severity and type of heart block. In severe cases, an artificial pacemaker may be required to provide correct electrical impulses to trigger heartbeats. Mild cases might not require any treatment but should be monitored regularly.

Self-assessment MCQs (single best answer)

What is heart block primarily caused by?

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Which of the following is NOT a type of heart block?

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In which type of heart block are no atrial impulses reaching the ventricles?

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Which of the following symptoms is least likely to be associated with mild heart block?

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Which type of heart block might require the implantation of an artificial pacemaker?

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What is the primary diagnostic tool for heart block?

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Which of the following is NOT a type of SA nodal block?

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Which part of the heart's conduction system is responsible for generating the initial electrical signal?

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In which type of AV block is there a progressive delay until a beat is missed?

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Which of the following statements about heart block is TRUE?

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