A heart block is when the electrical pulses that control the beating of the heart are disrupted. Usually, only the most serious type of heart block causes symptoms. This type of heart block is known as a complete, or third degree, heart block. Symptoms can include:
- shortness of breath
- palpitations (irregular heartbeat)
- fainting
How the heart beats
When the heart beats, the heart muscle contracts (pulls inwards) in preparation for pumping blood around the body. The contractions are triggered by electrical pulses that are generated by a selection of specialised cells that are known as the sinoatrial node (SA node).
The SA node is often referred to as a natural pacemaker because like artificial pacemakers it generates a series of electrical pulses at regular intervals.
The pulse is then sent to another selection of cells that are known as the atrioventricular node (AV node). The AV node relays the pulse to the two lower chambers of the heart (the ventricles).
A heart block occurs if the transmission of the pulse between the SA node, the AV node and the ventricles is interrupted.
A heart block does not mean that the heart stops beating altogether because other ‘back-up’ systems can take over. However, it can result in an abnormally slow heartbeat (bradycardia), leading to the body’s organs and tissue becoming deprived of oxygen.
Types of heart block
There are three main types of heart block:
- first degree heart block
- second degree heart block
- third degree heart block
These are described in more detail below.
First degree heart block
A first degree heart block is where there is split-second delay in the time that it takes electrical pulses to move through the AV node. First degree heart block does not usually cause any noticeable symptoms and treatment is rarely required.
Second degree heart block
A second degree heart block is where there is a series of increasing delays in the time that it takes the AV node to send the pulse to the ventricle, until eventually a heartbeat is skipped.
There are two sub-types of second degree heart block:
- Mobitz type 1 - this is the least serious type of second degree heart block; it may occasionally cause symptoms of mild dizziness and does not usually require treatment.
- Mobitz type 2 - this is a more serious type of second degree heart block; it may cause symptoms of light headedness, dizziness and fainting in some people and usually requires treatment.
Third degree heart block
A third degree, or complete, heart block is where there is no transmission of electrical pulses between the AV node and the ventricles. As with second degree heart block, there are two sub-types of third degree heart block:
- congenital - where the condition is present at birth
- acquired - where the condition develops as a result of heart damage
A third degree heart block can cause a wide range of symptoms, some of which are life-threatening. This type of heart block is usually regarded as a medical emergency and may require immediate treatment with a pacemaker (an artificial electrical device that is used to regulate heartbeats).
How common are heart blocks?
First degree and Mobitz type 1 heart blocks are uncommon but not rare. It is estimated that 0.5-2% of otherwise healthy adults have these types of heart blocks.
Mobitz type 2 heart block is rare in the general population, but it is more common in people with certain heart conditions. For example, it is estimated that 1 in 30 people with heart failure will develop Mobitz type 2 heart block.
Congenital third degree heart blocks are rare, occurring in 1 in every 20,000 births.
Conversely, acquired third degree heart block is a common complication of heart disease, particularly in older people. It is estimated that 5-10% of people who are over 70 years of age, and have a history of heart disease, will develop a third degree heart block.
Outlook
The outlook for all types of heart block is very good as long as appropriate treatment is given as and when it is required.
The most serious types of heart block respond very well to treatment with a pacemaker, and deaths that are caused by these conditions are very rare.