An article by Dr Edward Leatham, Consultant Cardiologist, author of The Naked Heart  a series of simple blog and video explanations covering all of the main common medical conditions that can affect us, as we age.

Atrial fibrillation (‘AFib’  or just ‘AF’ for short) is a common and intricate cardiac condition that affects the rhythm and efficiency of the heart. To fully grasp the impact of AFib, it’s essential to understand the normal functioning of the heart, particularly focusing on the crucial moments before a heartbeat occurs.

The Heart’s Architecture

Mammalian hearts, including humans, feature two atria – the left and the right. These chambers serve as reception areas, collecting blood before it’s dispatched to the ventricles. This system bears a resemblance to a turbocharger in an engine, where the atria boost the heart’s efficiency, preparing it for a powerful contraction.

The atrium not only primes the heart but also ensures that the timing of its contraction is in perfect harmony with the ventricular contraction. This coordination relies on an intricate electrical pathway, synchronising the atrial contraction with the mitral and tricuspid valves opening and the subsequent blood transfer to the left and right ventricles.

The Prelude to a Heartbeat

The heart, an engineering marvel, orchestrates a meticulous sequence of events before each beat. Central to this process is the mitral valve’s role, located between the left atrium and the left ventricle. When the mitral valve opens, it allows blood to flow from the left atrium into the left ventricle, the heart’s main pumping chamber.

The journey of blood begins in the lungs, where it receives oxygen and then travels through the pulmonary veins into the left atrium. This step is pivotal for the heart to fulfill its mission: supplying oxygen-rich blood to nourish the body.

When Synchrony Falters: Atrial Fibrillation

AFib is a disorder where the atria lose their organized contraction, descending into a chaotic rhythm that fails to prime the ventricles effectively. The synchrony between atrial and ventricular contractions falters, leading to a less efficient pumping, which can also cause lung pressures to rise and give rise to breathlessness..

In AFib, the atria quiver at an astounding rate of around 600 cycles per minute. This quivering is erratic, undermining the heart’s efficiency as the priming function of the atria is compromised. While the ventricles still receive blood due to their inherent suction power, this mechanism is not as effective as the normal, coordinated contraction. The pulse rate of patients in Afib varies according to a normal distribution due to the variation in effectiveness of the atrioventricular node (AV Node),  which means while some people in Afib have a normal pulse rate at rest, others will have a heart rate that is very fast – upto 160 beats per minute.

How is Atrial Fibrillation diagnosed?

AFib may not always cause any symptoms, however it can be suspected by detecting or hearing an uneven pulse, usually associated with a faster heart rate. It can be confirmed using the electrocardiogram (ECG). The figures below show the normal ECG configuration, with the atrial contraction normally creating an electrical signal just moments (100-200 msec before the main heart beat), detectable on the ECG as the ‘P’ wave.  In Atrial Fibrillation the ‘P wave’ is lost and the interval between each heart beat becomes shorter (ie faster pulse rate) and uneven, hence the term irregularly irregular.

The Consequences of Atrial Fibrillation

The transition to atrial fibrillation has significant repercussions.  The lack of coordinated atrial contraction causes stasis of arterial blood within the left atrial appendage (an outpouching of the left atrium), which can lead to small clots forming that can then detach and be carried in the arterial circulation (embolism) causing stroke, heart attack, ischaemic bowel and limb ischaemia.  The lack of effective atrial contraction also means that the ventricles are less adequately primed, which can lead to a cascade of physiological consequences. For instance, the left atrial pressure  can increase (causing lung flooding), particularly in older or stiffer hearts, a phenomenon that is pronounced in individuals with AF, the heart rate can increase due to the higher atrial rates of contraction, leading to a sensation of palpitation. The heart pump, like an engine, is most efficient at a range of pulse rates – normally ranging 45-90 beats per minute at rest, whereas in atrial fibrillation the pulse becomes uneven and erratic, with pulse rates that can vary widely from normal to very high (up to 180 beats per minute). It’s easy to understand that higher heart rates like this when they occur in older patient, like a classic car, are less well tolerated than when they occur in those under 40 years of age.

Conclusion

In conclusion, the heart’s operation is a testament to the intricacies of human biology. Atrial fibrillation, while common, is a serious condition that disrupts the heart’s rhythm and efficiency. By delving into the mechanics of the heart, we gain a deeper appreciation for this vital organ and the critical need to maintain its health in the face of conditions like AF.

Key messages

1. Atrial Fibrillation is a very common heart condition affecting over a million people in the UK alone.
2. Not everyone affected by atrial fibrillation is aware they have it.
3. For every patient in Atrial fibrillation all the time, there are probably 100s of people with transient episodes – called paroxysmal atrial fibrillation.
4. The main hazard from Afib is the association with left atrial appendage thrombi that can detach and be carried in the arterial circulation to cause embolic complications including stroke.
5. Atrial fibrillation is the cause of approximately 50% of all large and disabling strokes.

Other related articles

1. Atrial thrombus in AFib presenting as a heart attack or stroke
2. Afib: what investigations
3. Afib causes left atrial paralysis 
4. Advice for patients at risk of Afib 

The Naked Heart is an educational project owned and operated by Dr Edward Leatham. It comprises a series of blog articles, videos and reels distributed on Tiktok, Youtube and Instagram  aimed to help educate both patients and healthcare professionals about cardiology related issues.

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