The term bradycardia refers to an abnormally slow heart rate, of under 60 beats per minute (bpm).
Normal heart rate
The heart has a complex system of nerves that conduct the electrical impulses fired by the sino-atrial node (SA node) and spread them throughout the heart, causing it to pump. A normal heartbeat begins with an impulse fired from the SA node reaching the atria which contract and push blood into the ventricles. The impulse then reaches the atrioventricular node (AV node), which regulates the flow of the impulse as it passes between the atria and ventricles. The impulse then flows down into the ventricles causing them to contract and pump blood around the body. This cycle is repeated and continues throughout a person’s lifetime. The SA node therefore sets the rate and rhythm of the heart and is also called the pacemaker of the heart.
A normal adult heart beats 60 to 100 times per minute, in a regular pattern. This is called the sinus rhythm.
What is bradycardia?
Sometimes, the pathway that conducts the impulse through the heart is blocked or damaged and the heart’s rhythm is disrupted. When this causes the heart to beat too slowly, a person is said to have bradycardia. Bradycardia can impair the heart’s ability to pump blood around the body.
Bradycardia occurs if impulses are sent from the SA node at a decreased rate or if their conduction is delayed. Usually, a decline in heart rate to below 50 or 60 bpm is termed bradycardia.
Types of bradycardia
In many cases, bradycardia is not a dangerous condition and does not require treatment. For example, sinus bradycardia is a type of bradycardia that occurs in response to being deeply relaxed or extremely fit. Sinus bradycardia is commonly observed in athletes. On the other hand, bradycardia may be caused by ageing of the heart’s conduction system, heart disease or medications used to treat high blood pressure or arrhythmia.
Another cause of bradycardia is sick sinus syndrome, which occurs when the SA node fails to generate the impulse needed for a heartbeat. Sick sinus syndrome can cause bradycardia, tachycardia (a fast heart rate) or a mixture of the two, referred to as brady–tachy syndrome. Patients with sick sinus syndrome may feel dizzy and tired and be prone to fainting.
Heart block is another cause of bradycardia that occurs when impulses flowing through the AV node to the ventricles are blocked. Also called AV block, this condition is divided into categories depending on its severity. These include:
- First-degree heart block – The impulses are slowed as they pass through the AV node but they still reach the ventricles. Treatment is not usually required.
- Second-degree heart block – Some of the electrical impulses do not reach the ventricles and a pacemaker may be required to control the heart’s rhythm.
- Third-degree heart block refers to a complete failure of signals from the SA node to reach the ventricles. This is usually the result of underlying disease and a pacemaker may be implanted to keep the heart pumping regularly.
Diagnosis and treatment
Bradycardia is often symptomless, but in cases of third-degree heart block, symptoms may be severe and the condition is diagnosed using an electrocardiogram (ECG).
The main approaches to treating bradycardia involve the discontinuation of any drugs that slow the heart rate, treating underlying disease and fitting the patient with a pacemaker.
A pacemakers is a small device that is implanted beneath the collar bone and connected to a wire inside the heart. The device detects when the heart rate is slow, at which point it fires electrical signals to stimulate the organ and ensure it pumps at a normal rate.
Reviewed by Sally Robertson, BSc