When does that heartbeat that skips after too many cups of coffee or lurches erratically at night lead to a fatal arrhythmia?
"That's a popular question," said Dr. David Haines, director of the Heart Rhythm Center at Beaumont Hospitals in Royal Oak, Mich. "Almost everybody has a palpitation or skipped heartbeat and the overwhelming majority are benign."
But more than 250,000 people die each year in the United States from sudden cardiac deaths, and most of those are thought to be from ventricular fibrillation.
Just this week, the 44-year-old daughter of famed New York City newspaper columnist Jimmy Breslin died unexpectedly from what doctors think may have been a sudden death arrhythmia.
Kelly Breslin died four days after collapsing at a Park Avenue bistro where she had enjoyed a steak and fries at a friend's birthday celebration. After eating, she put her hand on the leg of the person seated next to her and collapsed.
Her father, who is 79, is one of the most celebrated journalists in New York City. He was contacted by Son of Sam killer David Berkowitz in 1977 and won a Pulitzer Prize in 1986 for columns that championed ordinary citizens.
According to the American Heart Association (AHA), more than 4 million Americans have arrhythmias and an estimated 638,000 hospital admissions are due to the condition. But most, say doctors, are not deadly.
"Most happen when people lie down at night, when all distractions and noise and stimulation goes away," said Haines. "It's when you are in a dark room with your heart beating, you feel it the most."
Moore, 81, was fitted with a pacemaker after blacking out while on stage.
Experts say that, although arrhythmias are common, not all are fatal.
The most common type are premature, abnormal heartbeats that begin in one of the heart's two lower pumping chambers. These extra beats disrupt the regular heart rhythm, which normally starts in the upper right chamber.
"It's just early beats coming from an irritable focus in the heart that fires off and beats on its own accord," said Haines. "Usually they just happen."
"Everybody's heart flutters at some time," he said. "That is the rule, not the exception."
When a person should seek further evaluation is when the fluttering patterns are rapid and sustained, lasting 30 seconds or a minute or 10 minutes.
"If it is sustained racing, or continuous, meaning it's sporadic but happening all the time and at night, of it if was a symptom preceded by a faint or near faint, especially during exertion, that would merit evaluation," said Haines.
The most common form of arrhythmia is atrial fibrillation, which currently affects more than 2.2 million Americans, according to the AMA. About 70 percent are between 65 and 85 years old. That is non-threatening and treatable.
Most fatal arrhythmias occur in association with other heart problems, valve problems, blockages and coronary heart disease, according to Dr. Tristram Bahnson, assistant professor of medicine at Duke University Medical Center.