Bradycardia (slow heart beat)
Overview - When to see a cardiologist - Symptoms - Causes - Classification - Treatment
Overview
Every heart beat is caused by an electrical impulse travelling through the heart. As the impulse spreads, it causes the heart muscle to twitch and contract generating a heart “beat”. A normal heart will beat between roughly 60 and 100 times a minute at rest. If the heart rate drops below 60, this is termed “bradycardia” (Greek for “slow heart”).
Many things can cause the heart beat to be slower than normal. Some of these things are very safe - such as being physically fit - but others can be quite risky. In extreme cases bradycardia can cause collapse and even death due to insufficient blood being pumped around the body. Serious problems related to bradycardia get more common with age and are more likely to affect people with other cardiac diseases such as heart attacks, high blood pressure and diabetes.
When to See a Cardiologist
Thanks to modern wearables such as smart watches and smartphones containing heart monitors, we are all more aware of our heart rate than in the past. Many people worry about the numbers reported by their wearables and seek advice on whether what they are experiencing is normal.
A persistently slow heart beat may be the first sign of heart disease but in fact most cases of slow heart beat will be related to levels of physical fitness. A “normal” heart rate is defined as 60-100bpm but this normal range is not true for everyone. It is very common for young fit people to have resting heart rates far lower than this. However, teling what is a normal low heart rate versus an abnormal one requires more information than is available from most smart watch data.
A cardiologist can help distinguish a normal low heart beat (due to physical fitness) from a dangerous low beat due to heart disease. This is usually done using detailed electrical tests on the heart to clarify whether a low heart beat is normal or abnormal. These tests may include resting, ambulatory and exercise ECGs to examine how the heart rhythm performs under different conditions.
Symptoms
Severe or dangerously low heart rates related to heart disease usually - but not always - cause symptoms. Typical symptoms of severe bradycardia include breathlessness, dizziness (particularly on exertion) and collapse (also called “syncope”).
Many people however develop quite serious conditions of bradycardia with little or no symptoms, and only become aware of the problem during a routine medical check, or a visit to hospital for another reason. Unfortunately lack of symptoms is not a good guide to how serious the problem is.
Causes
Bradycardia has many causes, the most common being the natural slow heart beat due to physical fitness. However, as age increases, a “healthy” low heart beat becomes rarer, so that most people over 60 with a low resting heart beat will turn out to have a medical cause for bradycardia. Some of these are benign and require little or no treatment. Others can be serious if left untreated.
Common medical causes for bradycardia include:
Thyroid disease - underactive thyroid is a leading cause of slow heart beat. Most cases of hypothyroid bradycardia can be successfully treated by taking thyroid hormone replacement (thyroxine).
Hypertension - high blood pressure is associated with faster than normal wear and tear to the heart’s electrical system, parts of which may eventually fail causing bradycardia.
Coronary artery disease - this condition causes narrowing and blockages in the arteries on the surface of the heart. If one of the arteries supplying the heart’s electrical system is blocked, for example during a heart attack, part of the heart’s electrical system may fail abruptly causing bradycardia or heart block.
Post-athletic bradycardia - high levels of aerobic physical fitness in middle age is unfortunately associated with higher levels of bradycardia in later life. Endurance sports such as long distance running and cycling are particularly potent causes of slow heart beat after retirement from athletics.
Classification
Doctors describe slow heart beats according to the part of the heart’s electrical system that is misbehaving to produce the slow heart rate:
Sinus bradycardia - this is caused by disease or slowing of the sinus node, the main electrical junction box of the heart which starts off the heart beat. In extreme cases the sinus node may fail or stop entirely (“sinus arrest”) in which case another part of the heart’s electrical system will take over as the main pacemaker of the heart. In general, because there is so much redundancy in the heart electrics, sinus bradycardia is not usually very dangerous. It may, however, cause significant symptoms and require treatment.
Atrioventricular block - AV block is caused by disease or slowing of the atrioventricular node. This is the electrical junction between to the top chambers (“atria”) and lower chambers (“ventricles”) of the heart. Most cases of AV block require treatment, due to the risk that they will progress to “complete heart block”.
Complete heart block - CHB is the most extreme case of AV block, in which there is no longer any electrical connection between the atria and ventricles of the heart. In this condition, the heart rate usually drops quite severely and may become unreliable. For this reason patients with complete heart block are usually treated as an emergency with hospital admission and urgent correction of the issue.
Treatments
Treatment of bradycardia involves first addressing any reversible factors that may be worsening the condition. For example, if atrioventricular block has been caused by a blocked artery or heart attack, then re-opening the artery may entirely reverse the problem. Likewise, if sinus bradycardia has been caused by a high potassium level from kidney failure, then normalising the potassium level may be enough to improve the situation.
However, most cases of bradycardia will not be so easily reversed. The treatment of choice for most patients with bradycardia will be insertion of a permanent pacemaker device to take over the job of the part of the heart electrics that has failed.
Pacemaker implantation is a very common and relatively minor medical procedure, albeit with some potentially serious complications. Around 60,000 pacemaker devices are implanted in the UK every year. You can read more about pacemaker implantation elsewhere on our website.