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Overall, bystander CPR was performed in only 11 heart stoppages involving sex, which may explain why only six out of the 34 people survived.
Sex is rarely a trigger for a sudden cardiac arrest but when it happens, only a third of people who collapse during sex are likely to receive CPR from their partners, a new study finds.

Researchers who examined the records of more than 4,500 cases of sudden cardiac arrests found a total of 34 linked to sexual activity, 94 per cent of them involving men.

However, their partners were alarmingly reluctant to attempt CPR. "This likely explains the relatively low survival rates despite mostly shockable initial cardiac arrest rhythms," the researchers report in a study published Sunday in the Journal of the American College of Cardiology.

The finding surprised and puzzled the researchers.

"There is plenty of evidence that performing CPR by bystanders until the ambulance arrives translates to significantly better survival for cardiac arrest," said first author Aapo Aro, of Cedars-Sinai Heart Institute in Los Angeles. "By definition, a cardiac arrest occurring during sexual activity is witnessed by a partner, and if CPR would have been initiated by the partner this would have been likely to save the lives of some of these patients."

Sex isn't without risk, Aro and his co-authors write. In one German study, 0.2 per cent of autopsied natural deaths were linked to sex. "It is also recognized that sexual activity may trigger nonfatal acute cardiac events such as myocardial infarction (heart attack)," the team writes.

Sudden cardiac arrest - a sudden collapse and loss of pulse - isn't the same as a heart attack. A heart attack occurs when blood supply to the heart is slowed or stopped because of a blockage.

With cardiac arrest, the heart abruptly stops beating. It's a "mostly lethal condition," resulting in more than 300,000 deaths annually in the U.S., the authors write, as well as 35,000 to 45,000 in Canada.

Cardiologist Dr. Sumeet Chugh, the study's senior author and director of the heart rhythm centre at the Cedars-Sinai Heart Institute, said patients often ask about the risks of having sex. "There was no data out there to tell them whether their risk is low, medium or high," he said.

So the researchers analyzed data from the Oregon Sudden Unexpected Death Study, ongoing since 2002 and the largest community-based study of its kind involving a population of about one million people in Portland. The study has collected detailed information, including past medical history, blood, genetic and other markers of risk from people who "literally drop dead in their homes or anywhere outside hospital," Chugh said.

In total, the researchers identified 4,557 sudden cardiac arrests in Portland from 2002 to 2015; 34 occurred during or in the minutes after sex. On average, these people were more likely to be male, middle-aged and have a history of heart disease.

Overall, one in 100 cases of cardiac arrest in men was associated with sex, compared with one in 1,000 in women.

The absolute risk appears to be "extremely low," the authors write, even among people with known heart disease. "The first thing is, it's reassuring," Chugh said.

However, men normally account for 70 per cent of all sudden cardiac arrests, and women 30 per cent. In the sex-related arrests, men seem to be hugely over-represented, Chugh said, and it's not entirely clear why. The researchers noted that men might have been using medications, stimulants or alcohol before sex. They may also have been sicker with their heart disease and, therefore, more vulnerable.

Overall, bystander CPR was performed in only 11 heart stoppages involving sex, which may explain why only six out of the 34 people survived.

Another study released in August found men are four times more likely to die from a cardiac arrest if it occurs during sex. French researchers found that these men waited twice as long for resuscitation, and spent an average of 8.4 minutes without any assistance, compared to men who had a cardiac arrest while playing sports, walking or gardening, The Independent reported.

"Here's a situation where you have a captive audience - I mean, it's tough not to have a partner for sexual activity," Chugh said.

Partners may have panicked or worried they didn't know how to perform CPR, he said. (New guidelines recommend that untrained bystanders who witness an adult collapse scrap "rescue breaths" and focus on deep and rapid chest compressions instead - because the evidence doesn't support mouth-to-mouth making a difference.)

"Maybe the setting was not great and the shock was too much," Chugh added. "And, you know, who knows who the partner was - if it's your spouse, or it isn't. We really didn't study those things."

Regardless, "clearly we have to increase the awareness of doing CPR, no matter what the circumstances are," he said.

An abnormality in the heart's electrical system usually causes a cardiac arrest. The chance of survival decreases about 10 per cent for every minute that passes without CPR and defibrillation.

For bystanders, "just try to get out of your shock and get to it," Chugh said. "Here is someone who is a loved one - all you have to do is put your hands on the chest and put your muscles to work.

"It increases the chance of them making it out of this whole thing alive."

Chugh's study was presented Sunday at the American Heart Association's annual scientific meeting in Anaheim, Calif.