Heart block is a problem with the way that electrical signals pass from the upper to the lower chambers of the heart. It is a kind of arrhythmia, and usually causes the heart rate to slow down.
There are three main types of heart block: first degree, second degree and third degree, with first degree being the mildest and third degree the most serious. The features of the three types are described in terms of the signal they produce on an electrocardiogram (EKG), so it is worth explaining how the heart functions and how this shows up on an EKG.
The heart has four chambers – the left and right atrium are the upper chambers, while the left and right ventricle are the lower chambers.
When your heart beats, it does so in two main stages known as diastole and systole; the systole stage is further split up into atrial systole and ventricular systole. In the diastole stage, all four chambers of the heart relax and start to fill up with blood. On an electrocardiogram (EKG), this corresponds to the T wave. This is followed by atrial systole, in which an electrical signal produced by the sinoatrial node (SA node) in the right atrium causes the two atria to contract and pump blood into the ventricles. On an EKG this stage – where the electrical signal moves through the atria – corresponds to the P wave.
The electrical signal then travels to the atrioventricular (AV) node between the atria and ventricles, slowing down as it does so; this corresponds to the flat line after the P wave and forms part of the so-called PR interval. The signal then goes down to the ventricles via a path known as the bundle of His. This initiates the last part of the cycle (ventricular systole) where the ventricles contract and pump blood out to the lungs and the rest of the body. The part of an EKG between the signal’s departure from the AV node and the contraction of the ventricles is the QRS complex.
Fast Facts about Heart Block
- Heart block is problem with the way that electrical signals pass from the upper chambers of the heart (the atria) to the lower chambers of the heart (the ventricles)
- There are three main types of heart block: first degree, second degree and third degree
- First degree heart block is mild, rarely causes symptoms and does not usually need treatment. Third degree heart block is the most severe type
- The most important diagnostic test for heart block is the EKG
- A pacemaker is used to treat third degree heart block and the more severe type of second degree heart block
- Symptoms of heart block include breathlessness, fainting, dizzy spells, chest pain and even seizures
- Heart block may be congenital or it may be acquired as a result of other conditions including other types of heart disease, certain infections, sarcoidosis, rheumatic fever, hemochromatosis and some types of muscle wasting disorders
- Heart block may also be caused by toxins or certain types of medication including beta blockers, calcium channel blockers and digoxin
Types of Heart Block
As already mentioned, there are three main types of heart block:
- First degree heart block involves a slowing down of the electrical signals as they travel from the atria to the ventricles. On an EKG, this corresponds to an elongated and more flat line between the P and R waves.
- Second degree heart block involves a greater level of signal slowing, to the extent where some signals don’t reach the ventricles at all. There are two kinds of second degree heart block – Mobitz type I and Mobitz type II. In Mobitz type I heart block (also known as Wenckebach’s block) the PR interval gets longer and longer with each heartbeat, until it gets so long that the next P wave occurs before the QRS complex. In other words, the heart “misses a beat”. In the more serious Mobitz type II heart block, electrical signals to the ventricles are blocked rather than slowed down, but in an irregular way. In EKG terms, the PR interval is of normal length but the QRS complex is sometimes absent.
- Third degree heart block (also known as complete heart block or complete AV block) is where electrical signals do not reach the ventricles at all. There is a back-up system in the ventricles that kicks in when this happens, but it is less efficient and not so regular. In an EKG, the P waves happen more often and are not in synch with the QRS complex. Third degree heart block is very serious (sometimes fatal).
Common Causes of Heart Block
Heart block may be congenital or acquired. Congenital heart block may be inherited (genetic) or it may develop while a baby is still in its mother’s womb, perhaps as a result of the mother having an autoimmune disorder.
Acquired heart block can be caused by a number of things, including damage occurring as a result of other heart diseases like heart attack (myocardial infarction), coronary heart disease, myocarditis, cardiomyopathy, heart infection (endocarditis) or heart failure. Other diseases, including rheumatic fever, sarcoidosis, hemochromatosis, Lyme disease, Chagas disease and certain muscle wasting disorders, can also result in heart block.
Heart block may also be caused by surgery, toxins and drugs. Ironically, some medications used to treat other forms of arrhythmia, like beta blockers, digitalis and calcium channel blockers, may be responsible for heart block.
If the vagus nerves are too active, they can slow down the heart rate to the extent where it causes first degree heart block. There are two vagus nerves in the body, each of which runs from the brain stem to the abdomen.
Risk Factors for Heart Block
Pregnant women who get an autoimmune disorder like lupus are at risk of passing congenital heart block to their unborn child.
The risk of acquired heart block tends to increase as people grow older, especially if they have another heart disorder. Age-related scarring of the electrical system of the heart is the most common reason for heart block.
Other diseases that increase your risk of heart block include sarcoidosis, rheumatic fever, hemochromatosis, Lyme disease, Chagas disease and some types of muscle wasting disorders.
You may be at risk of heart block if you take certain drugs such as beta blockers, calcium channel blockers and digoxin.
Young people and athletes are at increased risk of heart block caused by hyperactive vagus nerves.
Symptoms of Heart Block
First degree heart block is usually symptom-free. Second and third degree heart block may result in breathlessness, fainting, dizzy spells, chest pain and even seizures.
Diagnosing Heart Block
First of all your doctor will want to know about your symptoms, as well as medications you’re on and any possible family history of heart disease. He or she may also take your pulse, listen to your heart with a stethoscope and look at your legs/feet for signs of swelling.
To confirm a diagnosis of heart block, other tests must be carried out. The most important and commonly used of these is the EKG.
The standard EKG is a one-off test that will only detect heart block if it occurs during the test. To get a better idea of how your heart functions over a longer period of time, you may be asked to wear or carry a monitor (known as a Holter monitor or event monitor) as you go about your normal routine. The Holter monitor records your heartbeat continuously, while the event monitor is used when you experience symptoms.
You may also be given a test known as an electrophysiology study, which involves inserting a wire into a vein and threading it up to your heart, where it can be used to record electrical signals.
There are more tests which may be used to check whether another form of arrhythmia is causing your symptoms.
Treatment Options for Heart Block
First degree heart block does not normally need treating. Third degree heart block and the more severe type of second degree heart block (Mobitz type II) is usually treated by insertion of a pacemaker, which regulates the electrical signaling in your heart.
If you’re on drugs that are causing heart block as a side effect, then your doctor will recommend reducing the dose or stopping the drug altogether.
Coping with Heart Block
If you have first degree heart block, you probably won’t need treatment.
If you have a pacemaker fitted to treat your heart block, you should take precautions to avoid strong magnetic and electrical fields, such as those produced by MRI (magnetic resonance imaging), industrial welding machinery, generators and airport security metal detectors – tell airport screeners that you are wearing a pacemaker.
Some everyday devices, like cell phones and iPods, should also be used with caution if you have a pacemaker.
You should carry a card in your wallet that states the kind of pacemaker you have. You might find it worthwhile to wear an ID bracelet or necklace too.
Avoid dangerous activities such as full-on contact sports, and have your pacemaker checked as often as your doctor recommends.
Where to Find More Information: Heart and Stroke Foundation of Canada