KUALA LUMPUR, Sept 24 (Reuters) - Former Malaysian prime minister Mahathir Mohamad was sedated but in stable condition on Monday after weekend surgery to counter a chest infection that followed a recent heart bypass, his family and an aide said.

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 on Saturday to have infected tissue cut out of his chest.

"We are optimistic but we also don't want to celebrate too early," his eldest daughter, Marina Mahathir, wrote on her Internet blog soon after meeting her father early on Monday.

"He's still very much sedated because he needs to rest," she added. "But he does wake up from time to time and he can hear us and respond to what we say. I told him I was there and he did a typical Dad thing: he winked! I wanted to both laugh and cry."

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, his second heart operation, on Sept. 4 and was reported to be recovering well before he suddenly went back into intensive care in recent days.

"He is stable," an aide said on Monday, but declined to give any further details, adding that doctors planned to issue a bulletin on Mahathir's condition on Tuesday.

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

Mahathir's Sept. 4 operation was his second since 1989, and followed two heart attacks over the past 10 months.

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