HAMBURG (Reuters) - Extramarital sex can increase your risk of having a heart attack, a British cardiologist has told a congress on sexual health in the German city of Hamburg.

Graham Jackson, a heart specialist at St. Thomas's Hospital in London, found couples in long-term relationships were far less likely to have heart attacks while having sex than those having affairs or one-night stands.

"We found that 75% of the cases of sudden death during sexual activity involved people who were taking part in extramarital sexual intercourse," Jackson told the European congress late Monday.

He added the danger of having heart failure during extramarital sex was exacerbated even further when there was a significant difference in age between the partners.

Jackson said that even though heart rates accelerate considerably during all sexual activity, his research found "couples in long-term relationships in general hardly faced any risk of heart failure."

He said measurements on blood pressure and heart rates found sex is generally comparable to a brisk 20-minute walk, with an orgasm putting a strain on the heart similar to an ensuing walk up a flight of stairs.

"All in all, only 1% of all heart attacks are triggered by sexual activity," Jackson said.