Atrial Fibrillation

“Atrial Fibrillation is one of the most common forms of abnormal heart rhythm (arrhythmia) and a major cause of stroke.”

British Heart Foundation

“In the UK, more than one million people have Atrial Fibrillation.”

British Heart Foundation

“People with AF are five times more likely to have a stroke.”

British Heart Foundation

“7 in every 100 people over 65 have AF.”

British Heart Foundation

Your heart is a muscular pump, and every heartbeat is immediately preceded by a small electrical impulse which travels through the heart via a system of nerves called the Cardiac Conduction System.

Normal heart rhythm (Sinus rhythm)

Each electrical impulse starts high up in the Left Atrium and spreads across the two top heart chambers (the Atria) causing them to contract simultaneously. This impulse reaches a mid-point between the top and bottom chambers where there is a short delay as the impulse can only travel along one nerve fibre from the atria to the ventricles. Once through, the impulse travels lightning fast along nerve fibres within the muscular walls of the ventricles signalling them to contract.

After a short recharge time, another electrical impulse starts following the same pathway.

A normal heart rate at rest (i.e. after sitting or lying down for 10-15mins) is typically between 60 – 100 beats per minute. If slower than this you have Bradycardia, something commonly seen in very active people. However even with top athletes it is abnormal to have a resting heart rate less than 40 beats per minute.

A heart rate above 100 beats per minute is described as Tachycardia.

Blood flow through a Cardiac Cycle

So how do these electrical impulses relate to the pumping function of the heart and blood flow?

Essentially the atria contract while the ventricles relax (ventricular diastole) allowing the smooth flow of blood from the atria into the ventricles. The short delay at the mid-point of the conduction system ensures the ventricles have time to fill. As the ventricles contract (ventricular systole), the valves between the atria and ventricles close (to prevent the backflow of blood) and blood is pumped out of the heart to the lungs and the body. At the same time the atria relax and fill with blood returning from the body and lungs. After a short recharge time, another electrical impulse triggers the atria to contract starting the next cardiac cycle. In normal sinus rhythm, each impulse makes the atria and ventricles contract regularly.

Irregular heart rhythm (Arrhythmia)

The heart works most efficiently when it is working in a normal Sinus rhythm. Sometimes however it can miss a beat or have a beat occur sooner than expected – people often describe these as Palpitations (if they feel them).

Atrial Fibrillation

Atrial Fibrillation (AF or AFib) is a very different arrhythmia. This occurs when an increased number of electrical impulses fire off from different areas in the top chambers of the heart (the Atria) in a random and disorganised way. These irregular impulses cause both the right and left atria to quiver instead of contracting uniformly as in normal sinus rhythm.

The heart rate is often higher with Atrial Fibrillation as the increased number of electrical impulses compete to travel down to the ventricles. It is not unusual to see resting heart rates of 100-175 beats per minute in people with uncontrolled Atrial Fibrillation in contrast to a normal heart rate of 60-100 beats per minute.

Unfortunately, many people are completely unaware they are experiencing this arrhythmia until it is detected at a routine health check or they experience a sudden unexpected adverse event such as a Stroke or Heart attack.

Causes

The cause of Atrial Fibrillation is often unknown, however its more likely to occur as we get older and more common in people with other heart conditions such as hypertension (high blood pressure), coronary artery disease or previous heart surgery.

Killer consequences

If these blood clots dislodge and travel into the ventricles, they can be pumped out of the heart leading to killer consequences;

1) Stroke – The most common and serious complication of Atrial Fibrillation is a stroke. This is where a blood clot cuts off the blood supply to a part of the brain, damaging brain cells due to a lack of oxygen. A stroke can lead to brain injury, disability or possibly death.

2) Heart Attack – If a blood clot travels down one of the blood vessels supplying the heart muscle (coronary arteries), this can cause a heart attack by blocking the blood supply to an area of heart muscle, starving the muscle of oxygen and causing it to slowly die.

Other consequences of Atrial Fibrillation

Atrial Enlargement:

Due to pooling of blood in the left and right atria, the first early consequence is both top heart chambers increase in size as they are stretched due to an increasing volume of blood.

The size of the atria is something routinely measured with cardiac ultrasound (Echocardiogram) to help assess how long a person has been in Atrial Fibrillation, especially if they have not experienced any symptoms.

Leaking heart valves:

A consequence of the atria increasing in size is the valves controlling the flow of blood between the atria and ventricles are often stretched and fail to fully close. This leads to these valves leaking blood back into the atria during ventricular contraction (Systole) which increases the volume of blood in the atria leading to further atrial enlargement.

Leaking heart valves are graded mild, moderate or severe. The more they leak, the more blood is being misdirected back into the atria and the less efficiently the heart works.

Left Ventricular dysfunction:

If the heart rate is not carefully controlled and continues to beat faster than normal, over time the muscular walls of the ventricles will become tired and weaken. Gradually the Left Ventricle will not contract normally, and the volume of blood pumped from the heart each heartbeat will be reduced. This condition is referred to as Left Ventricular Dysfunction and ultimately leads to Heart Failure.

Heart failure has a dramatic effect on a person’s quality of life; a weakened heart will reduce your ability to play sports, exercise or at its more advanced stages limit your ability to simply walk.

Symptoms

Directly related to the irregular timing of the heart beats and the faster heart rate (more than 100 beats per minute).

Related to the often-faster heart rate (more than 100 beats per minute) with Atrial Fibrillation. In people with Atherosclerosis (a build up of fatty plaque inside of the coronary arteries supplying their heart muscle) an increase in heart rate / work rate of the heart can lead to chest pain or discomfort as the blood supply downstream from the narrowing is less than the heart muscle needs.

Since the resting heart rate is generally higher with Atrial Fibrillation it will invariably go even faster when you go for a walk or climb a set of stairs. Even these low intensity activities will likely increase your heart rate further (to levels more commonly seen when running on a treadmill), in which case we would expect to be breathless.

Due to a combination of;

1) Heart beating faster than normal and never getting a chance to rest.

2) Losing the ability of the atria to contract in Atrial Fibrillation results in a 20-30% loss of heart function as only the ventricles are pumping.

3) Leaking heart valves reduce the smooth flow of blood through the heart resulting in further pumping inefficiency.

4) Left Ventricular Dysfunction will significantly impact the hearts ability to meet the bodies need for oxygen-rich blood.

Related to the faster heart rate often seen with Atrial Fibrillation and an insufficient supply of oxygen-rich blood being delivered to the brain.

When should I go see my doctor?

If you have any concerns about your heart rate, heart rhythm or general health, please consult with your local family doctor.

Active Heart Health aims to alert you to potential silent killers which would need to be investigated further by your family doctor or cardiologist.