When the heart is beating normally, each beat is triggered by an ‘electrical impulse’, which is generated by a specialised group of cells in the heart muscle, called the Sino-Atrial node (SA node).
This is located near the top of the heart and is responsible for the contraction (squeezing) of the heart’s upper chambers (the atria).
From here, the impulse travels down to another node, the Atrio-Ventricular Node (AV node), which creates a gateway to the specialised electrical conducting system of the ventricles (lower chambers).
The impulse passes rapidly through this system of specialised cells known as the ‘Bundle of His’, which subdivides into two branches (left and right Bundles), causing the lower two chambers (the ventricles), to contract and pump blood out.
If there is an interruption in the impulses between the SA and AV nodes – this is known as Heart Block. If there is an interruption in the impulse transmission between the ‘Bundles’, it is known as Bundle Branch Block’
There are 3 main types of heart block:
First-degree heart block is when there is a very slight disruption in the electrical impulse, usually, there are no symptoms, and no treatment needed.
Second-degree heart block is where there is a pattern of increasing delays in the impulse being passed between the nodes until eventually a beat is ‘dropped’. Second-degree heart block is subdivided into two groups, Mobitz Type I, and Mobitz Type II.
Third-degree (complete) heart block is where there are no impulses passed down between the SA and AV nodes. This means the lower chambers don’t beat normally with the top chambers. There is a backup system in the lower chambers which can take over, but it is at a much slower pace resulting in a weaker contraction of the ventricles. This is the most serious type of heart block.
Those with first-degree heart block are usually symptom-free and require no treatment. People with second or third-degree heart block can present with dizziness and fainting due to the dropped beats or very slow heartbeat.
Dizziness, breathlessness, confusion, extreme tiredness and episodes of collapse are all symptoms of complete heart block. For some, it is a medical emergency requiring immediate treatment, but for others, the block can come and go.
Certain cardiac medications can cause first-degree heart block.
Many people who present with second degree and complete heart block have an underlying heart condition including coronary artery disease (angina; heart attack), cardiomyopathy or a congenital heart condition.
It is also more common in the older person due to wear and tear of the electrical system and can be attributed to some medicines, drug toxicity and electrolyte imbalances.
Unless you are having symptoms, heart block may only be picked up during routine tests for other conditions. Your doctor will take a full history of your symptoms and carry out a physical examination. An ECG will be done to assess your heart rate and rhythm. Other tests include,
Holter Monitor: this is a portable ECG recorder that is worn for 24-48 hours. It gives your doctor a better overview of your heart’s activity.
Event Monitor: this is a similar device to the Holter Monitor, but it can record your heartbeat for a longer period ie, weeks or months.
It can be activated when you feel symptoms that may be related to a slow heart rate.
By doing this, it allows your doctor to evaluate your heart rate and rhythm at the time of your symptoms.
The most common treatment for symptomatic second-degree heart block and complete heart block is the insertion of a pacemaker.