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Spencer Moseley is not the type of person you’d expect to have a heart problem. A dedicated runner with 50 marathons under his belt, Moseley kept meticulous track of his pace while logging

45 to 60 miles a week. Usually, he kept a steady pace of 7½ minutes per mile with a heart rate of 140 beats per minute. But last year, he noticed that his heart rate was increasing. He

thought he might be sick, so he scaled back. Or maybe it was just the stress of recent international travel, a transfer of his military-related job and a move. Still, the problem continued

to worsen. “I was winded on easy runs,” said Moseley, who turns 50 in June. “What was I doing walking on a 4-mile run? I was posting heart rates of 220 to 230 heartbeats per minute.” Moseley

was due for an annual physical, and when he mentioned his symptoms, a nurse listened to his heart and gave him an EKG, an electrocardiogram. Then Moseley’s doctor told him he had a

condition called atrial fibrillation — known as A-fib — and that he needed tests at a hospital right away. Moseley’s condition, a quivering or irregular beat in the upper chambers of the

heart, affects about 33.5 million people, including an estimated 2.7 million to 6.1 million in the United States. Researchers expect the number to double by 2050. “A lot of people, when you

say atrial fibrillation, they say, ‘I think I’ve heard of that,’ or ‘My grandfather had that.’ But people don’t understand how common it is today,” said cardiologist Jonathan Piccini,

director of the Center for Atrial Fibrillation at Duke University Medical Center in North Carolina. “If you are older than 40 years old, there’s a 1 in 4 chance that in your lifetime you are

going to get it. It’s very common.” But as common as A-fib may be, it is not something to take lightly: Untreated, A-fib doubles the risk of heart-related death. It increases a person’s

risk of having a stroke fivefold. [embedded content] WHAT CAUSES THE QUIVER? Basically, each heartbeat is controlled by the heart’s electrical system. An impulse begins at the top of the

heart and travels like a wave to the lower chambers, signaling the tissue to contract. In a healthy adult, a node in the top of the heart fires off between 60 and 100 beats per minute. The

electrical wave moves through the atria to another node that acts as a kind of gatekeeper for electrical pulses going from the top to the bottom chambers. But in A-fib, that electrical

system malfunctions. The top chambers, the atria, don’t produce an effective regular contraction, beating either too fast or unevenly. When that happens, the heart fails to squeeze all the

blood from one chamber to the next. That can leave blood to pool in the atria, which can increase the risk of clotting. If a blood clot forms, it can break off and lodge in an artery leading

to the brain, blocking off blood supply, causing a stroke. With weak or irregular contractions, a heart that’s in A-fib can allow blood to back up in the pulmonary veins, causing fluid

buildup in the lungs, which causes fatigue and shortness of breath. And when oxygen-rich blood doesn’t get delivered to the body and brain, physical and mental fatigue can set in. Fluid also

can build up in the feet, ankles and legs. THE MANY FACETS OF A-FIB People who experience A-fib describe it in a wide range of ways — a painful thumping in the chest, a racing heart that

leaves them gasping for air, a fluttering in the chest. Some feel dizzy or nauseated. But many people aren’t even aware of their symptoms.