Atrial fibrillation (afib) happens when the upper chambers of your heart fire rapid, chaotic electrical signals instead of beating in a steady rhythm. There isn’t one single cause. Afib results from a combination of structural changes in the heart, electrical misfires, and underlying health conditions that create the right environment for those misfires to take hold.
How Afib Starts: Rogue Electrical Signals
Your heart’s rhythm normally begins in a small cluster of cells called the sinus node, which sends an orderly electrical pulse through the upper chambers (atria) and down to the lower chambers. In afib, stray electrical impulses originate from other locations, most commonly from the pulmonary veins, the four blood vessels that carry oxygenated blood from the lungs back into the left atrium.
The tissue where the pulmonary veins connect to the atrium has unusual electrical properties. In people with afib, this tissue recovers from each electrical impulse much faster than normal atrial tissue (about 185 milliseconds versus 253 milliseconds). That faster recovery means the cells are ready to fire again almost immediately, creating a rapid, self-sustaining loop of electrical activity. At the same time, electrical signals traveling through these veins slow down and fragment, which sets up conditions for signals to circle back on themselves rather than moving forward in an orderly wave. This combination of quick recovery and slow, disorganized conduction is what allows a single stray impulse near the pulmonary veins to snowball into full-blown afib.
High Blood Pressure and Heart Disease
Hypertension is the single largest contributor to afib at the population level, accounting for roughly 22% of all cases. When blood pressure stays elevated, the left ventricle has to pump harder, its walls thicken, and it becomes stiffer. That stiffness backs up pressure into the left atrium, stretching it over time. The stretched atrium responds by growing extra fibrous (scar-like) tissue between muscle fibers and enlarging individual heart muscle cells.
This remodeling has direct electrical consequences. Fibrous tissue disrupts the normal connections between muscle bundles, forcing electrical signals to take longer, more erratic paths. Some signals hit dead ends and bounce back, creating the reentry circuits that sustain afib. A hormonal system that regulates blood pressure and fluid balance (the renin-angiotensin system) accelerates this process by promoting fibroblast growth and altering the way calcium and potassium flow through heart cells.
Other forms of heart disease cause similar remodeling. Heart failure, coronary artery disease, and valve disorders all increase atrial pressure and stretch, and together they account for about 5% of the population’s afib burden on top of hypertension’s contribution. Essentially, anything that makes the atrium work harder or grow larger sets the stage for chaotic electrical activity.
Obesity and Epicardial Fat
Obesity increases afib risk by about 28% for every five-unit rise in BMI, and population-level studies attribute 13% to 17% of all afib cases to excess weight. Part of that risk is mechanical: carrying extra weight raises blood pressure and stresses the heart. But a more direct mechanism involves the fat that sits on the surface of the heart itself, called epicardial adipose tissue.
In people with obesity, this fat layer becomes inflamed. It releases signaling molecules that attract immune cells (macrophages) into the neighboring atrial tissue. Those immune cells, along with activated fibroblasts, then deposit collagen and other fibrous proteins directly into the atrial wall. Researchers have shown that the degree of scarring in the left atrium correlates with the concentration of inflammatory proteins in the surrounding fat. In lab studies, exposing rat atrial tissue to the substances secreted by epicardial fat causes fibrosis, and blocking those substances reverses it. The result is the same kind of patchy scarring that disrupts electrical conduction and promotes reentry circuits.
Sleep Apnea
Obstructive sleep apnea raises the risk of afib by about 88% compared to people without the condition, and roughly 10.5% of people diagnosed with sleep apnea also have afib. The risk appears to increase with severity.
During an apnea episode, the airway collapses, oxygen levels drop, and the body mounts a stress response that includes surges in adrenaline and swings in pressure inside the chest. Over time, these repeated episodes promote inflammation, elevate blood pressure, and cause the same kind of atrial stretching and scarring seen in hypertension. If you have afib and haven’t been evaluated for sleep apnea, it’s worth looking into, because treating the apnea can reduce afib recurrence.
Thyroid Disease
An overactive thyroid (hyperthyroidism) roughly doubles the risk of developing afib. Thyroid hormones directly affect heart muscle cells by increasing their tendency to fire spontaneously, speeding up the rate at which cells recover between beats, and shortening the electrical “action potential,” the brief burst of voltage that drives each contraction. These changes create faster, more excitable atrial tissue that is primed to sustain chaotic rhythms.
Interestingly, an underactive thyroid also increases afib risk, though through different mechanisms involving altered electrical connections between heart cells. Afib that appears alongside thyroid dysfunction often improves significantly once thyroid hormone levels are brought back to normal.
Alcohol
Alcohol has both acute and chronic effects on the heart’s rhythm. The acute effect is sometimes called “holiday heart syndrome,” where a bout of heavy drinking triggers an afib episode in someone who may not have had one before. Alcohol directly affects atrial muscle cells and simultaneously activates both branches of the autonomic nervous system. The parasympathetic (calming) branch shortens the atrial recovery period, while the sympathetic (fight-or-flight) branch floods cells with calcium, making them more likely to fire spontaneously. That dual activation creates the electrical instability needed to launch afib.
Chronic drinking compounds the problem. The risk of afib rises in a dose-dependent way: one drink per day increases risk by about 8%, two drinks by 17%, and five drinks per day by 47%. Even a single alcoholic drink doubles the odds of an afib episode within the next four hours in people who already have the condition.
Age, Genetics, and Other Risk Factors
Age is the strongest non-modifiable risk factor. Every five years of aging increases afib risk by 43% to 66%. This reflects decades of cumulative wear on the atria: gradual fibrosis, loss of muscle cells, and stiffening of the heart’s structure. Afib is uncommon under age 50 but affects roughly 10% of people over 80.
Genetics also play a role. Having a first-degree relative with afib increases your own risk, and researchers have identified numerous genetic variants that influence the electrical properties of heart cells or the tendency of the atria to develop fibrosis. You can’t change your genes or your age, but knowing you carry these risks can motivate attention to the factors you can control.
Other established risk factors include diabetes (which raises risk by about 27%), smoking (21% to 43% increased risk), a sedentary lifestyle (which nearly 2.5 times the risk compared to active people), and even height, with risk climbing about 28% for every 10 centimeters of additional stature, likely because taller people have larger atria.
Surgery as a Trigger
Afib develops in roughly 26% of patients after cardiac surgery, with rates ranging from about 7% to 53% depending on the procedure and how closely the heart rhythm is monitored afterward. Heart operations cause direct inflammation and trauma to atrial tissue, and the post-surgical stress response can tip a vulnerable heart into afib. Most post-surgical afib resolves within weeks, though it can increase the risk of stroke during recovery and may signal a higher likelihood of developing afib later in life.
How Multiple Causes Work Together
In most people, afib isn’t caused by a single factor working alone. A common pattern looks something like this: years of high blood pressure gradually scar and enlarge the left atrium, excess weight adds inflammatory signals from epicardial fat, untreated sleep apnea delivers nightly surges of stress hormones, and then one evening a few drinks provide the acute trigger that tips the heart into its first episode. The pulmonary veins, with their inherently unstable electrical properties, are where the spark ignites, but the scarred, stretched, inflamed atrium is the fuel that keeps it burning.
This layered picture explains why managing afib almost always involves addressing multiple risk factors at once. Weight loss, blood pressure control, treating sleep apnea, moderating alcohol, staying physically active, and managing blood sugar each chip away at a different part of the problem. Clinical guidelines now emphasize this comprehensive lifestyle approach as the foundation of afib management, not just an add-on to medications or procedures.