Atrial Fibrillation

How does the heart work?

The heart is made up of four main chambers: the right and left atria and the larger right and left ventricles. The atria pump blood into the ventricles, while the ventricles pump blood throughout the body. The right side of the heart pumps blood to the lungs so that they have access to oxygen, and the left side pumps blood to the body and brain.

When the heart is functioning normally, the atria pump blood to the ventricles before the ventricles pump blood to the body, so the atria and ventricles work in sequence. To make sure that the chambers are all in sequence and to control heart rhythm and the speed at which the heart beats, it has an electrical timing system.

Small cells in the sinus node at the top of the heart send an electrical impulse out 60 to 80 times per minute, which means that the normal rate at which the heart beats is between 60 to 80 times a minute, with an increase during exercise. The impulse moves across the atria from right to left, causing the cells of the atria to contract. An insulated line separates the atria and ventricles from each other. When the electrical wave reaches the line, it ends, and the cells wait for the next electrical impulse from the sinus node.

The atria are connected to the ventricles by the atrioventricular node and bundle branches, which act as electrical paths through which the electrical waves can spread, causing the ventricles to contract.  When an electrical wave is sent, it moves from the top to the bottom of the heart before it dies out until the next impulse is sent.

When the heart’s electrical timing system is not working correctly, the normal heart rhythm is upset. This abnormal heartbeat is often referred to as heart arrhythmia. These irregular electrical signals are known as atrial fibrillation (AF).

 

What is atrial fibrillation?

Atrial fibrillation (AF) occurs when there is an irregular heartbeat because the heart's electrical timing system is not working properly, and the normal rhythm of the heart is disturbed, causing a cardiac arrhythmia.

What problems does it cause?

There are two major problems associated with AF. These are:

• The sinus node loses control of the heart
  Instead of a single electrical wave coming from the sinus node, a number of rogue cells generate electrical impulses, creating a series of continuous and chaotic electrical waves around the atria. These cells are generally situated in the left atrium, near the pulmonary veins. The pulmonary veins have triggers that cause abnormal electrical waves.

As a result of the abnormal and chaotic waves, the atria do not contract as they should. Instead, the atria quivers, which means that they don't pump blood properly. This can cause blood to stagnate and clot, increasing the risk of stroke. Certain medications can decrease the risk of stroke in patients with AF – speak to your doctor for more information.

• The efficiency of the heart’s pumping is reduced
  When the atria do not contract as they should, the heart is not able to pump at its optimal rate. The atria are connected to the ventricles by an electrical connection in the atrioventricular node, which will react to the electrical impulses and try to keep up with the chaotic signals. As it tries to keep up, it will contract haphazardly and with varying speeds, causing the heart to contract irregularly.

Atrial fibrillation is a chronic condition, and patients with the ailment often find that they cannot do as much physical activity as those with normal heart rhythm.

What are the risks?

AF increases the risk of the following:

• Stroke
  Stroke, which is the most common cardiac cause of stroke, refers to the damage of the brain (either temporary or permanent) caused by a loss of blood supply. Symptoms of a stroke include weakness on one side of the body and impaired speech and sight. Although patients with AF are at a greater risk of stroke, other factors such as age, diabetes, heart failure, heart disease, and previous stroke all play a role. Speak to your doctor about appropriate medication to reduce the risk of stroke.
• Heart failure
  Heart failure is caused by the weakening of the heart muscle and results in fatigue, swelling and breathlessness. AF can cause heart failure by making the heartbeat at a faster rate for too long. A catheter heart ablation procedure can help to restore a healthy heart rhythm.
• Death
  Patients with AF are at greater risk of death than those with a healthy, normal heart rhythm. There is currently no known reason for this, but it appears that restoring the heart’s normal rhythm may decrease the risk.

What causes it?

AF often occurs as a result of a pre-existing cardiovascular condition, including:

• High blood pressure
• Heart failure
• Coronary heart disease
• Valvular heart disease

Other factors that can cause AF include:

• Excessive intake of alcohol
• Overactive thyroid gland disease

In some cases, there is no known cause. This is called “lone AF”.  Although there is no single known cause of AF, there are three important factors to note. These are:

AF begins with a trigger. The sinus node acts as a pacemaker in the heart, but there is always the chance that cells can activate unexpectedly.

Although it is common for everyone to experience the occasional irregular heartbeat (caused by the heart cells activating at the wrong time), those with AF experience these ectopic beats frequently. In a normal heart, an ectopic beat feels like a skipped beat or heart flutter, while in AF patients, the heartbeat feels irregular for much longer periods. In AF patients, rogue atrial cells, which produce rapid and frequent ectopic beats, are the trigger for AF. These cells gather between the left atrium and the pulmonary veins. Although it is not yet known why the AF triggers are usually located in the pulmonary veins, it may be that the abnormal cells are inside the veins. Patients with high blood pressure are particularly susceptible to ectopic beats because high pressure in the heart distorts the atrial tissue. In those with normal atria, on the other hand, AF can be triggered by ectopic beats, but the AF will stop because the atria are unable to fibrillate for an extended amount of time. This sporadic atrial fibrillation is known as paroxysmal AF.

The substrate refers to an abnormality in the electrical properties of the atrial tissue of the heart. When the chaotic electrical waves move around the atria, they need space to move around. If there is not enough space, the waves will crash into each other and stop moving. If the atrium is large enough, the waves will move around slowly, giving the tissue time to recover. The waves are then more likely to move towards this tissue. Many kinds of heart disease cause this kind of substrate. Enlarged atria can be picked up on tests like echocardiograms. When the heart is severely abnormal, AF can be sustained by the substrate without any triggers. This kind of atrial fibrillation is called persistent AF.

It is possible for patients to experience paroxysmal AF, which then becomes persistent AF. This is because AF has the ability to alter the electrical properties of the atria by causing the atria to dilate. This, in turn, creates the substrate for further, more persistent AF. Because of this, it is important to treat AF early.

What are the symptoms?

Although some patients never experience any symptoms, some signs of the condition include the following:

• Heart palpitations
• Shortness of breath
• Dizziness
• Chest pains
• Lethargy
• Weakness on one side of the body
• Speech or visual impairment
• Irregular heartbeat
• Irregular and fast pulse

Can I be tested for AF?

To determine if you are suffering from AF, your cardiologist will record the electrical activity of your heart with an electrocardiogram (ECG). An ECG can be conducted on a treadmill during exercise, with a Holter monitor or event recorder (which you will carry around as you go about your daily routine), or in a fixed chair.

Once it is established that you have AF, your physician or cardiologist will also conduct several tests to rule out any other potential underlying causes of your symptoms. These tests include:

• Blood pressure test
• Blood test to measure thyroid function
• Echocardiogram to check for structural heart disease

How can it be treated?

There are a number of treatment options for AF. These include:

• Medication to regulate heart rhythm and speed
• Anticoagulants to prevent blood clots
• Catheter ablation
  If medication does not have the desired effect, your doctor may recommend catheter ablation. This is a minimally invasive treatment, which works by destroying the cells that are causing an irregular heartbeat. There are two catheter ablation options available: Cryoballoon ablation and radiofrequency ablation.

Read more information about this procedure and what to expect after catheter ablation.