"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.