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.
"Young people often get them in conjunction with congenital heart problems, ranging from hypertrophic cardiomyopathy (thickened heart) to coronary arteries coming off the aorta in an unusual way," he told ABCNews.com.
Though an autopsy has not yet been completed on Kelly Breslin, EKGs done at the hospital indicated a heart irregularity, according to The New York Times, perhaps suggesting a more dangerous form of arrhythmia caused by an underlying genetic defect.
Breslin, who worked in public relations, had no history of heart problems, but her mother, Rosemary Breslin, died in 1981 at age 50. Her sister, also named Rosemary, died of a blood disease in 2004 at 47.
Experts say finding a direct relative with heart problems is often the key to diagnosis.
"The issue with the more uncommon types of rhythm abnormalities is that they strike people with otherwise normal hearts," said Alice Lara, executive director of the Sudden Arrhythmia Death Syndromes Foundation.
"There is a low prevalence in the general population, but they are potentially life threatening if they go undiagnosed," she told ABCNews.com.
The term "arrhythmia" refers to any change from the normal sequence of electrical impulses. The electrical impulses may happen too fast, too slowly, or erratically -- causing the heart to beat too fast, too slowly, or erratically.
When the heart doesn't beat properly, it can't pump blood effectively, affecting the functioning of the lungs, brain and all other organs, which may shut down or be damaged.
In bradycardia, the heart beats too slowly; in tachycardia, too fast. Fibrillation occurs when the heart "quivers," and premature contraction is an early beat, according to the AHA.
"Almost all arrhythmias are hard to detect and you have to catch it right when it happens," said Lara.
Doctors use a monitor to diagnose dangerous arrhythmias, recording every beat in 24 hours. If they detect problems, they can treat the patient with beta blockers.
"Tons of people have it. If it's something that would kill you, you are more likely to have a genetic condition," Lara added.
When arrhythmias strike those under the age of 40, underlying genetic problems are usually to blame. Sometimes fainting is a clue, especially during exercise.
Though rare, long-term arrhythmias can cause a blood clot to form and lead to stroke. About 10 percent of all SIDS babies are born with a heart deformity. Women are more susceptible to these sudden death arrhythmias than men.
"It also happens older in life and women are more susceptible," Lara said. "Sometimes a lot of things come together -- you take cold medications and they can prolong the QT interval."
The QT interval is the "blip" on the EKG when the heart is at rest and is beginning to beat again. "It it's too long, the heart isn't beating right and it spins out of control."
Pacemakers are used when the heart beats too slowly with long QT symptoms, a condition that often kills a person with an arrhythmia in their sleep and affects about 1 in 2,500 Americans.
Other conditions, set off by a loud noise or startling, require a defibrillator that jolts the heart back into rhythm.
An automatic external defibrillator, if administered within 3 to 5 minutes can restore a normal heartbeat to stricken patients, said Lara.