Atrial fibrillation happens when the heart’s upper chambers (the atria) receive chaotic, disorganized electrical signals instead of the steady, rhythmic ones that keep a normal heartbeat. This electrical chaos causes the atria to quiver rather than contract in a coordinated way, which disrupts blood flow and raises stroke risk by about fivefold. Over 52 million people worldwide live with the condition, and that number has more than doubled since 1990.
The reasons behind this electrical malfunction range from structural damage in the heart to hormonal imbalances, lifestyle habits, and conditions like high blood pressure or sleep apnea. Often, multiple causes overlap in the same person.
How the Heart’s Electrical System Goes Wrong
A healthy heartbeat starts with a single electrical impulse from a small cluster of cells called the sinus node, located in the right atrium. That impulse spreads in an orderly wave across both atria, triggering a synchronized contraction that pushes blood into the lower chambers. In atrial fibrillation, this orderly process breaks down in two main ways.
The first is abnormal firing. Cells outside the sinus node start generating their own electrical impulses, essentially competing with the heart’s natural pacemaker. These rogue signals most commonly originate in or near the pulmonary veins, the four blood vessels that carry oxygenated blood from the lungs back to the left atrium. Muscle fibers extend a short distance into these veins, and their unusual cellular arrangement makes them electrically unstable. They can fire rapidly and erratically, bombarding the atria with extra impulses that override normal rhythm.
The second mechanism is called reentry. Instead of traveling in one direction and dying out, electrical signals get caught in loops, circling back through the atrial tissue over and over. These self-sustaining circuits can maintain fibrillation even after the original trigger has stopped. In many people, both mechanisms operate at the same time: a burst of abnormal firing from the pulmonary veins kicks things off, and reentry circuits keep the arrhythmia going.
This distinction matters for treatment. Pulmonary vein isolation, the most common ablation procedure, works by electrically disconnecting those rogue-firing veins from the rest of the heart. It’s most effective for people with newer, intermittent episodes. In long-standing afib, reentry circuits have become so established in the atrial tissue that eliminating triggers alone isn’t enough.
Structural Changes That Fuel Afib
The heart doesn’t just malfunction electrically. Over time, physical changes to the atrial tissue create an environment where afib thrives. The most important of these is fibrosis, the buildup of scar-like tissue between heart muscle cells. Fibrosis disrupts the smooth, cable-like connections between cells that allow electrical signals to travel in an orderly fashion. Signals have to weave around patches of scar tissue, slowing down in some areas, speeding up in others, and creating the kind of uneven conduction that sustains reentry loops.
Heart failure is a powerful driver of this process. The strain it places on the atria triggers the release of signaling molecules that stimulate fibroblasts (the cells responsible for producing connective tissue) to lay down excess collagen. Even after heart failure is treated and the heart’s pumping function recovers, the fibrosis remains, and the atria stay vulnerable to fibrillation. This is one reason afib tends to become progressively harder to treat the longer it persists.
Atrial enlargement also plays a role. A bigger atrium can physically accommodate more simultaneous reentry circuits, and it allows larger circuits that wouldn’t fit in a normal-sized atrium. The atria can stretch from chronic high blood pressure, valve disease, heart failure, or from afib itself. This creates a vicious cycle: afib causes the atria to enlarge and develop fibrosis, which in turn makes afib easier to sustain and harder to stop.
Conditions That Set the Stage
Most people who develop afib have at least one underlying condition driving the process. High blood pressure is the most common. Chronically elevated pressure forces the left atrium to work harder to push blood into the left ventricle, gradually stretching and scarring the atrial walls. It also activates hormonal pathways that directly promote fibrosis in atrial tissue.
Sleep apnea is another major contributor. Roughly half of people with afib also have sleep apnea. The repeated drops in oxygen during the night cause surges in adrenaline-like hormones, swings in pressure inside the chest, and inflammation, all of which stress the atria. Treating sleep apnea with a CPAP machine has been shown to improve afib outcomes, making it one of the more actionable risk factors.
Thyroid disorders affect the heart’s electrical properties directly. An overactive thyroid speeds up repolarization (the electrical “reset” between heartbeats) in atrial cells, shortening the window during which the tissue is protected from extra impulses. This makes it far easier for reentry circuits to form. Interestingly, an underactive thyroid also increases afib susceptibility, though through different electrical changes. Both conditions alter the behavior of ion channels, the tiny gates in cell membranes that control the flow of charged particles responsible for each heartbeat.
Obesity, diabetes, and chronic kidney disease all promote inflammation and fibrosis in the atria. Valve disease, particularly mitral valve problems, places direct mechanical stress on the left atrium. And prior heart surgery or heart attacks can leave behind scar tissue that serves as a substrate for reentry.
Lifestyle Triggers
Some afib episodes have identifiable triggers. Alcohol is the most well-documented. The so-called “holiday heart syndrome” describes afib episodes that occur after binge drinking, even in people with otherwise healthy hearts. Alcohol causes heart muscle cells to swell, has direct toxic effects on atrial tissue (partly through its breakdown product, acetaldehyde), and disrupts the autonomic nervous system that regulates heart rate and rhythm. These effects are dose-dependent: heavier drinking carries higher risk, but even moderate consumption can trigger episodes in susceptible people.
Caffeine was long suspected as a trigger, but more recent evidence suggests moderate coffee intake doesn’t increase afib risk for most people. Intense endurance exercise is a less intuitive risk factor. Marathon runners and competitive cyclists have higher rates of afib than the general population, likely because years of high-volume training enlarge the atria and promote fibrosis through repeated inflammatory stress. Moderate exercise, by contrast, is protective.
Poor sleep, acute emotional stress, dehydration, and illness (especially infections with fever) can all provoke episodes. For some people, identifying and managing personal triggers is one of the most effective ways to reduce how often afib occurs.
Why Afib Often Goes Unnoticed
Not everyone with afib feels it happening. Studies of patients seen by cardiologists have found that 43 to 48 percent of people with confirmed afib report no symptoms at all. They don’t feel palpitations, shortness of breath, or fatigue. This is particularly concerning because even “silent” afib carries the same stroke risk as the symptomatic kind. Blood still pools in the quivering atria, and clots can still form.
Paroxysmal afib, the type that comes and goes, is especially easy to miss. An episode might last minutes to hours and resolve on its own, potentially occurring while you’re asleep. Many people are only diagnosed after a stroke or during a routine medical visit when an irregular pulse is detected. Smartwatches and portable ECG monitors have begun catching cases earlier, though they can also produce false alarms.
Why Afib Tends to Get Worse Over Time
Afib is often described as a progressive condition, and the biology explains why. Each episode causes a small amount of electrical and structural remodeling in the atria. Cells adapt to the rapid, chaotic signals by changing the number and behavior of their ion channels, which paradoxically makes future episodes easier to trigger and harder to stop. Meanwhile, the atria continue to stretch and accumulate fibrosis.
The clinical pattern typically follows a predictable arc. It often starts as paroxysmal afib, with episodes that come and go and resolve on their own. Over months to years, episodes become longer and more frequent, eventually becoming persistent (lasting more than a week) and then long-standing persistent. At each stage, the structural changes become more entrenched, and rhythm-control strategies become less effective. This is why cardiologists often emphasize early treatment and aggressive management of underlying conditions like blood pressure, weight, and sleep apnea, not just to control the arrhythmia, but to slow the remodeling that drives its progression.