Polygenic score
Influential genes: LINC01681,KCNN3,ZFHX3,SH3PXD2A,NEURL1
KCNN3 gene encodes a potassium channel involved in the regulation of cardiac electrical activity.
ZFHX3 regulates gene expression in cardiac development and function. Changes in this gene have been associated with atrial fibrillation.
Mutations in the SH3PXD2A gene have been linked to the development of atrial fibrillation.
Atrial fibrillation (AF) is the most common cardiac arrhythmia in adults. Simply put, a cardiac arrhythmia is a deviation from the normal heart rate and/or rhythm that is not physiologically justified. (1)
To understand atrial fibrillation, it is necessary to take a look at how the heart works. The heart is to some extent an autonomous organ. Instead of the central nervous system (the brain), the impulses for the heart rhythm originate directly in the myocardium (heart muscle) in modified cardiomyocytes (individual muscle cells), which together form the heart's electrical conduction system. This characteristic feature is called the heart’s automaticity.
Fig 1: The conduction system of the heart | The University of Nottingham (5)
The normal, physiological heart rate is called the sinus rhythm and it is generated and maintained by the sinoatrial (SA) node, a bundle of pacemaker cells in the upper wall of the right atrium (upper chamber of the heart). The pacemaker cells of the SA node generate an action potential (an electrical signal), which then propagates to the surrounding cells to induce synchronized atrial contraction and, after a small time delay, also the contraction of ventricles (lower chambers of the heart). Therefore, if everything works properly the number of impulses generated in the SA node per minute corresponds to the heart rate.
The signal generated by the SA node travels very fast through the right atrium - first to the left atrium so that the contractions of the left and right atria are in sync. After that the signal continues to the atrioventricular (AV) node, which is found at the bottom of the right atrium. The AV node conducts the excitation very slowly, which causes a delay in atrioventricular transmission - it is a desired pause, because it is necessary to complete the contraction of the atria first, and only then to start the contraction of the ventricles. From the AV node the impulses continue down the conduction pathway via the bundle of His into the ventricles. The bundle of His divides into right and left pathways called the bundle branches to stimulate the right and left ventricle. In the ventricles the signal is transferred by the Purkinje fibers which is the final step before transmitting the electric charge to the heart muscle.
Coming back to the matter at hand, the pathology of atrial fibrillation is caused by electrical signals, which do not originate in the SA node but fire from multiple locations in the atria (typically near the exit of pulmonary veins, that transfer oxygenated blood from the lungs to the left atrium), causing them to beat chaotically. Because of the ineffective, uncoordinated contraction of the atria, blood can stagnate and form clots (thrombi) in the heart’s auricle, a small natural outbulging of the atria. From there the thrombus can be expelled into the body's circulation and block another blood vessel, which can affect multiple organs, cause organ damage and even death.
The atrial rate is usually 300-600/min, while the ventricular rate is 80-180/min. That is because the atrioventricular node blocks the transfer of most atrial contractions to the ventricles. This protects the ventricles from exhaustion. Since the AV node doesn't prevent all of these chaotic signals from progressing onto the ventricles, the heart tends to beat faster and in an irregular rhythm. (2)
Fig 2: Atrial fibrillation | Mayo Clinic
Traditionally, atrial fibrillation is divided into five patterns based on presentation, duration, and spontaneous termination of AF episodes:
It is clear from this classification that episodes of atrial fibrillation may come in waves or they may be omnipresent. Although atrial fibrillation itself is usually not life-threatening, it is a serious medical condition that requires proper treatment to prevent stroke and/or other associated complications. (2)
Prevalence of AF increases significantly with age, especially in people over 50 years of age. Between the ages of 60 and 70 it occurs in up to 10% of the population. (1, 3) A gradual rise of prevalence is expected in the upcoming years as the average life expectancy of the general population increases and the search for undiagnosed AF becomes more intense. The lifetime risk depends on age, genetic and (sub)clinical risk factors. Studies suggest that an early intervention and control of modifiable risk factors could reduce the incidence. (4)
Atrial fibrillation is a complex disease whose pathogenesis involves combined environmental and genetic factors. (6) In recent decades, family aggregation of individuals with lone atrial fibrillation has been observed in many studies. In a study of 1137 identical twins, the heritability (fraction of the trait explained by genes) of AF was estimated to be as high as 62%, suggesting that genetic factors play a substantial role in the risk of AF (7). A more recent study also concluded that genetic variation contributes significantly to AF risk. (8) The principle of the relationship between the risk of atrial fibrillation and genomic variability is similar to several other cardiovascular diseases, such as hypertension and myocardial infarction. Established loci (position on a chromosome where a particular gene is located) explain only a small part of the disease risk, suggesting that further genetic discovery (focused on common genome variations) is needed to understand the causal genetic basis of AF (8).
In many people, atrial fibrillation may be asymptomatic (silent) for a long time. However, in others it can cause a broad spectrum of symptoms (1, 3, 4):
Unfortunately, it also happens quite often that the initial manifestation is a serious condition caused by the arrhythmia. These include (4):
Atrial fibrillation is often discovered randomly during a physical exam, when a doctor is listening to the heart with a stethoscope. To confirm the diagnosis and exclude other possible causes of the rapid heart rate, other tests are needed. These tests may include (2):
The aim of the treatment of atrial fibrillation is (2):
From what we know about the disease, not all cases of atrial fibrillation can be prevented. However, by taking steps to avoid underlying clinical conditions which are known to increase the overall risks (such as hypertension, COPD and type 2 diabetes), you can substantially reduce the risk of developing AF due to these causes. These steps include (10):
Most people with AF are able to live normal lives. However, it is important to note that patients with atrial fibrillation have increased morbidity and mortality compared to patients with sinus rhythm. The primary cause of increased morbidity in atrial fibrillation are thromboembolic events. The risk of stroke is increased approximately fivefold, depending on the other risk factors for thromboembolism. Atrial fibrillation is also associated with cognitive decline, including vascular dementia. It can impair quality of life and reduce tolerance to physical exertion. AF itself can also worsen or induce heart failure. (11)
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