Heart block, also known as atrioventricular block, is a conduction disorder in which the signals for the regular pumping of the heart are disturbed, leading to various consequences. The transmission of impulses from the atria to the ventricles is disrupted, and arrhythmias occur.

What is the pathophysiology of heart block?

The sinoatrial node or SA node in the heart, located at the right atrium, is called the hearts natural pacemaker. This is attributed to the fact that the SA node can generate the impulses in the heart, i.e., it forms the foundation of the hearts conduction system. The impulses generated here are passed to the atrioventricular node or AV node, which passes through the Bundle of His and Purkinje fibers.

The AV node transmits impulses from the SA node the natural pacemaker to the ventricles. Any disorder in the AV node can lead to a delay in the transmission of impulses. As a result, the hearts regular rhythm is disturbed, generating abnormal impulses and cardiac arrhythmias. This whole condition is termed heart block, as the impulses are unable to transmit correctly.

What are the types of heart block?

Typically, a heart block is divided into three types depending on how the impulses are transmitted. These types are:

First-degree heart block:

In this category, there is only a slight delay in transmitting impulses from the SA node to the AV node, and the symptoms are absent. Such cases do not require any treatment, and the condition is usually temporary.

Second-degree heart block:

Second-degree heart block is a partial blockage in the transmission of impulses. It is further divided into two types, depending on the areas of the heart which are affected.

In Mobitz type 1, there is a disorder in the functioning of the AV node itself that results in the gradual production of slow heartbeats and a skipped heartbeat. It can be due to physiological causes and can occur during rest and sleep. It is also termed the Wencklebach phenomenon.

In Mobitz type 2, the problem usually occurs in the Purkinje fibers or His bundle. In this case, only two situations can occur,i.e., a normal heartbeat or a missed or skipped heartbeat.

Third-degree heart block:

In third-degree heart block, the hearts conduction system fails to perform. As a result, the atria and ventricles beat independently from each other without a synchronization or the normal heart rhythm.

What are the symptoms of a heart block?

As the situation deteriorates and the transmission of impulses is blocked, the symptoms of the heart block begin to manifest. In this case, the most common symptoms are:

• Loss of consciousness
• Dizziness
• Postural hypotension
• Decreased heart rate,i.e., below the average value of 60-100 beats per minute
• Chest pain
• Shortness of breath
• Confusion
• Stokes Adams attacks: These are the characteristic symptoms of a heart block. In these attacks, a sudden loss of consciousness due to decreased cardiac output as a heartbeat or cycle is skipped. As a result, there is a lack of oxygen supply to the brain, resulting in unconsciousness. The situation becomes better when the heart starts beating again. These attacks are very evident as there is a death-like appearance of the patient during this attack. Once the heart starts beating, there is ample blood supply and a flush.

What are the causes of a heart block?

There are several causes of a heart block. Some of these causes are:

• Genetic defects
• Infective endocarditis
• Sarcoidosis
• Trauma
• Myocardial infarction
• Heart surgery
• Ischemia of the hearts tissue
• Idiopathic fibrosis of the heart tissue and nodes
• Use of certain drugs like calcium antagonists and digoxin

What are the possible complications of a heart block?

As the name indicates, heart block can prove to be a hazardous and even fatal condition if not addressed on time. There are various complications associated with a heart block. Some of these complications are:

Cardiac arrest:

Owing to the disruption in the transmission of impulses and blood supply, a cardiac arrest can occur, generally in the cases of third-degree heart block.

Risk of injury:

As the person having a heart block is likely to have a sudden loss of consciousness, there is an increased risk of injury in case the person collapses.

Organ failure:

Due to the failure of the conduction system, the organs may not get their adequate blood supply, leading to the failure of vital organs.

Cerebral ischemia:

Usually, in third-degree heart block, the oxygen and blood supply to the brain is disrupted. This can lead to severe outcomes, even coma, and death, if the heart function is not restored within seconds.

How to diagnose a heart block?

A heart block typically goes unnoticed as the symptoms are not very elaborate. However, there are some specific tests to diagnose a heart block. Some of these tests are:

History and Examination:

A detailed history is crucial to look for any signs and symptoms related to a heart block and to rule out any genetic conditions that can lead to a heart block. The patient should also be asked about the medications used in the past, as certain drugs can also cause a heart block.

During examination, the pulse, irregular heartbeats, heart sounds, and blood pressure must be closely monitored to form a firm diagnosis.

Echocardiography:

An ECG is a standard test for a heart block. The following findings can be seen on an ECG.

In first-degree heart block, PR interval is prolonged, i.e., >0.20 seconds.

In Mobitz type 1, the PR interval is prolonged.

In Mobitz type 2, the PR interval is regular. However, the P wave may be absent or skipped.

In third-degree heart block, narrow or broad QRS complexes are observed.

Electrophysiology test:

This test can be done to accurately determine the area of the heart responsible for a heart block.

What are the treatment options for a heart block?

In mild cases, a temporary artificial pacemaker is inserted in the heart to guide the transmission of impulses. However, in extreme cases, a permanent artificial pacemaker can also be inserted and has a good prognosis generally.

In case of any other underlying condition, it is necessary to treat that before inserting a pacemaker. Lifestyle modifications can also help in some instances.

Final Note:

Heart block is a subtle yet disastrous disease of the heart, which can lead to serious complications and even prove fatal if not treated promptly.