KUALA LUMPUR, Sept 26 (Reuters) - Former Malaysian prime minister Mahathir Mohamad was stable after emerging from a critical three-day period following surgery for a chest infection, an aide said on Wednesday.

Mahathir, 82, who ruled Malaysia for 22 years until 2003, went back into the operating theatre at Kuala Lumpur's National Heart Institute around midnight last Saturday to have infected tissue cut out of his chest following a recent heart bypass.

"He continues to be stable," the aide told Reuters in an e-mail message.

The 72 hours immediately after the operation were the crucial period in Mahathir's post-operative care, the hospital had said earlier.

Mahathir went through a light physiotherapy routine of deep breathing and muscle toning exercises on Tuesday, his eldest daughter Marina said on her Internet blog.

"Dad's general condition is better and he's alert and cooperative," she added.

Despite his retirement, Mahathir remains an influential political figure, even though he has fallen out with his successor, Abdullah Ahmad Badawi, over policy changes made since his retirement.

Mahathir underwent a quadruple bypass on Sept. 4, his second heart operation since 1989. It followed two heart attacks over the past 10 months.

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