DUISBURG, Germany (AP) -- Eight-time Olympic champion Birgit Fischer says cardiac arrhythmia is threatening her attempt to come back from retirement and compete at the London Games at the age of 50.

Fischer was forced to give up plans to compete at the German canoe and kayak Olympic trials this weekend.

Germany's most decorated Olympian told a news conference on Friday that routine checks this week revealed the condition, a problem with the heartbeat's speed or rhythm.

"I had the necessary medical checks on Monday and Tuesday and routine examinations revealed things that I was suspecting but wasn't sure of," Fischer said.

Team doctor Wolfgang Dillmann said Fischer's electrocardiogram produced results "that shouldn't come up in performance sports."

The German canoe federation said on Thursday that Fischer was banned by her personal doctor from competing this week but did not elaborate.

Dillmann said more tests were necessary to determine how serious Fischer's problem was.

She could theoretically still qualify for the London Games at the German nationals at the end of April but her chances appear slim.

"First, Fischer has to be able to compete and then produce outstanding results," team coach Reiner Kiessler said. "And then she would still need a wild card."

Fischer said she would not give up.

"Those who know me know that I will cling to any straw that becomes available," she said. "I feel fitter than I did before my last comeback in 2003."

Fischer has been training at a lake in East Germany, where she lives and runs a canoeing school. She herself has reported inconsistent training times and has been well off the pace needed to make the Olympic team.

She is arguably the best kayaker in Olympic history.

Fischer, who also has 27 world championship golds, won her first Olympic title in Moscow in 1980, competing for East Germany.

She did not compete four years later in Los Angeles because of a boycott by Communist nations but returned in 1988 in Seoul to win another gold. Fischer won at least one gold medal at each subsequent Olympics, competing for a reunited Germany, until her last games in Athens in 2004.

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