This "oxygen saturation" drops when air doesn't flow into the lungs. "If the lowest oxygen saturation was 78 percent, or less, their risk of [sudden cardiac death] increased by 80 percent," Gami said. In a healthy person, 95 percent to 100 percent is normal.
Having 20 events an hour would be termed moderate sleep apnea, Gami said.
Gami found a link, not a cause-and-effect relationship, between sleep apnea and sudden cardiac death. He can't explain the connection with certainty, but said there are several possible explanations. For example, sleep apnea is related to the type of heart rhythm problem that causes sudden cardiac death, he said.
The study findings should be taken seriously by those who have sleep apnea or suspect they do, said Dr. Neil Sanghvi, an electrophysiologist at Lenox Hill Hospital, in New York City, who reviewed the findings.
People with sleep apnea are often but not always obese, and many have other heart risk factors such as heart failure or heart disease. Having these other risk factors already puts a person at risk of sudden cardiac death, Sanghvi said. "The sleep apnea may be the tipping point. Each of these factors adds a level of risk. When you add sleep apnea, you could have a worse outcome."
Anyone who suspects they have sleep apnea should ask their doctor about a sleep test, Sanghvi said. Daytime sleepiness and fatigue are frequent symptoms. Another tipoff is a bed partner who complains of snoring.
The study didn't address whether those who used sleep apnea treatments -- such as the CPAP machine (continuous positive airway pressure) prescribed during sleep to help breathing -- would reduce risk. "It would be fair to say we suspect it would," Gami said.
The U.S. National Institutes of Health funded the study. Gami has served as a consultant for Medtronic, Boston Scientific and St. Jude Medical, which make defibrillators and other heart devices. Other study authors have also worked for heart device manufacturers and CPAP makers.