Chinese swimmer Zhang Lin's disappointing result at the national championships is probably caused by an asthma attack, said his coach Chen Yinghong on Wednesday.

The 25-year-old Zhang, winner of the men's 800m freestyle at 2009 Rome worlds, only managed a fourth-finish at the men's 400m freestyle Tuesday and his result, three minutes and 49.14 seconds, didn't reach the Olympic A standard 3:48.92, which may make him miss the 400m free at the 2012 London Games.

Zhang Lin talks to media after finishing fourth in the men's 400m freestyle at the national swimming championships in Shaoxing, Zhejiang province, April 4, 2012. [Photo/CFP]

Zhang trailed newly-crowned world champion Sun Yang in the first 200m, but was overtaken by Hao Yun and Li Yunqi in the last 100m and seemed totally out of strength. Both the coach and Zhang himself were astonished about the failure as Zhang always swam well in the training. "It's weird," said Chen.

"According to his physical conditions, I think the poor result is probably caused by his asthma," said Chen. "He has been dealing with it in the past year. The asthma attacks had happened two or three times during his training and affected the results."

"I had expected that he could swim within 3:42 or 3:43 in the final, but the result was surprisingly bad," Chen said. "He told me that he felt stuffy and a little out of breath before the heats earlier Tuesday and didn't feel good before the final either, but I didn't think about the asthma at that time and just gave him some encouragement."

"Zhang could swim within 3:50 even with a shoulder injury when trained in America, and he is in good condition recently. Even if he had not taken part in competition for a long time, the result should not be so bad," Chen said. "We thought about the reason the whole night, and the only explanation is his asthma."

Zhang grabbed a silver medal in the men's 400m free at the Beijing Olympic Games, marking China's first-ever Olympic medal in men's swimming. He won a historic gold in 800m freestyle in 2009 Rome worlds to become the first male Chinese swimmer to break a world record in freestyle in 49 years, and also the first male Asian swimmer to seize gold in a long-distance freestyle event at the Olympic Games or the world championships in 73 years.

Zhang also had an asthma attack in the 1,500m free final at the Rome worlds, which made him fail to win a medal. And since then he began to deal with the illness.

Some top swimmers in the world also suffers from asthma, like Australia's swimming legend Grant Hackott and world champion Lisbeth Lenton.

Chen said Zhang was upset after the 400m final, but he will do his best in the 200m free on Friday.

"We had a long talk today," said the coach. "Now Zhang will focus on the following competitions and try his best to qualify for the London Games. His training is good, and if he can perform his normal level, he's still hopeful in the 200m free."