What Is Sinus Bradycardia

Introduction 

Sinus bradycardia is a heart rhythm that is slower than the normal range but with a regular rhythm. The word ‘Bradycardia’ comes from the Greek word ‘bradys’ meaning slow and ‘kardia’ meaning heart.¹

There can be many causes of sinus bradycardia, not all of which have any negative consequences, however, it is important to understand how it may be caused and the effect it can have. In order to understand what causes sinus bradycardia, it is helpful to know how the electrical circuitry of the heart works.

The heart's electrical system 

The heart is able to control the tempo at which it beats all by itself. It uses a clever system of nodes and tracts to send electrical signals in a specific order so that the heart pumps blood correctly and with an appropriate frequency. The part of the heart that controls all of this is called the Sino-Atrial Node (SAN)

The Sino-Atrial Node is known as the pacemaker of the heart. It is responsible for initiating the electrical signals which stimulate the heart to contract in ‘sinus rhythm’. In a normal person, this occurs approximately 60-100 times a minute. This means that in an individual with sinus bradycardia, the SAN would only initiate electrical signals less than 60 times a minute.² 

Understanding sinus bradycardia

Sinus bradycardia can be completely normal in certain situations. For example, while you are sleeping, your heart rate slows down naturally as you are not moving. Your body requires less oxygen and energy, therefore the heart does not need to pump as often.³

Athletes may also have sinus bradycardia due to having a strong heart. The heart is, of course, a muscle, so the more it works the stronger it gets. Athletes train hard so the muscles of the heart are often larger and stronger. This means that when the heart contracts to pump blood it can generate much greater force, pumping a larger volume of blood to all areas of the body.

A stronger heart, therefore, improves the efficiency at which blood is pumped, and it will not need to pump as often to meet the demands of the body. However, this type of bradycardia should only occur at rest and the individual will not experience any symptoms. If bradycardia occurs while moving and being active then it may be a result of different health conditions.⁴

Sinus bradycardia can occur in various conditions:⁵

• Myocardial infarction (reduced ability of the sino-atrial node caused by a heart attack)
• Sinus node dysfunction (defect of the sino-atrial node where it fails to produce regular electrical impulses) 
• Drug side effects (beta blockers)
• Hypertension (high blood pressure)
• Hypothyroidism (low thyroid hormones)
• Hypothermia (low body temperature)
• Hyperkalaemia (high potassium levels in the blood)

In most people with sinus bradycardia they do not feel any serious symptoms, especially if they are very active. Other people may experience:⁵

• Dizziness
• Shortness of breath
• Fatigue 
• Difficulty concentrating
• Chest pain

Diagnosis

You can try and find your heart rate yourself at home. If you can feel your pulse (It is easiest to feel with two fingers on the wrist with the palms facing upward) then count the number of pulses with a timer set for 15 seconds. Multiply this number by 4 to get the number of pulses in one minute.

To diagnose sinus bradycardia a doctor may need to do a physical examination where they listen to your heart with a stethoscope. In a hospital setting the main method of diagnosis would be an electrocardiogram (ECG). This measures the electrical activity of the heart through electrodes that are placed on the chest with sticky pads.⁷

The shape of the ECG would be normal, representing the ‘sinus’ rhythm, however, the frequency of electrical pulses would be slower than normal, representing the ‘bradycardia’. If the bradycardia comes in episodes then it may be difficult to catch it on an ECG. Instead, the doctor may provide a ‘Holter monitor’.⁷

This is a device that will measure the electrical activity of the heart over 24 hours continuously allowing short episodes to be recorded for a diagnosis. Another alternative is an ‘event recorder’ which is similar to the Holter monitor but it allows the individual to start a recording when they begin to feel symptoms. Some newer models are also able to automatically record only when it detect a rhythm defect.⁷

Treatment and management 

In most cases some simple lifestyle changes can help to manage sinus bradycardia. Stopping smoking can help to reduce the risk of heart diseases that may further impact heart rhythm. Reducing alcohol intake will help to manage blood pressure, and regular exercise (30 minutes of brisk walking or 1 hour of slow walking every day) can help to raise heart rate and maintain a healthy weight.⁸

If the sinus bradycardia is a more serious problem then medications can be given to treat it. The most common medication used is a combination of Atropine and Adrenaline. These drugs will help to raise the heart rate and will alleviate many symptoms associated with sinus bradycardia such as fatigue and dizziness. Atropine should not be used if the individual has had a heart transplant or any spinal cord injuries. If these drugs have little or no effect after a few weeks of use then having a pacemaker may need to be considered.⁹ 

This is a much more drastic intervention that is only used in very rare, serious cases of bradycardia. There are two methods that are often used. The first is called ‘Transcutaneous Pacing’. This involves placing adhesive electrodes on the chest that supply regular electrical stimulation. This method is usually used as a temporary solution either while waiting for a permanent pacemaker or for short bradycardic ‘episodes’. This is because this method of pacing can cause painful muscle stimulation in other areas of the body such as the arms and legs, therefore it is often done under sedation.⁹ 

The second method is called ‘Transvenous Pacing’ and is used for individuals who require continuous pacing treatment. This is a more invasive treatment as it involves placing the pacemaker leads directly on the heart. The surgeon does this by passing the lead through one of the central veins leading to the heart. Due to the nature of the process, it is not done on individuals who have a prosthetic heart valve.⁹

It is important to consult a medical professional to determine whether having a pacemaker implant is the best course of action. In most cases, it can be resolved with medication, but further episodes may return in the future. It is important to follow the advice of your healthcare provider after a diagnosis of sinus bradycardia. This includes strictly following medication instructions and advice on how to maintain and take care of a pacemaker if it is implanted. 

Summary

Sinus bradycardia is a slower-than-normal heart rhythm. It often poses no problems in everyday life and in certain cases, such as athletes, it is a sign of strong cardiovascular health.¹⁰ However, in some cases, it can cause symptoms such as dizziness, fainting and lack of concentration. This is usually caused by other diseases or may occur from childhood. If it is not resolved with lifestyle changes and daily medication then a pacemaker may need to be implanted to control the heart rate.⁶ 

If you are experiencing any symptoms mentioned earlier (dizziness, weakness, fainting) then it is important that you visit your local healthcare professional for further advice. While it may be a temporary event it is possible that the problem may return, especially if it is caused by other diseases or if it has been an issue from birth. Seeking medical advice if there are any concerns is important to detect any problems at an early stage. There is a positive outlook for most individuals with sinus bradycardia if they present to a doctor early.

Sameer Gonuguntla

MBBS, Imperial College London, UK

I am a medical student at Imperial College London with a keen interest in medical writing. I am interested in a wide range of fields in the world of health from medical technology to advances in surgical care. I have experience in academic writing and I wish to bring the complex world of research into a more digestible form for the public to have a better understanding of their health.

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