The National Heart Institute (IJN) said former premier Dr Mahathir Mohamad is showing improvement and that his condition was stable throughout the night.

"Doctors treating Mahathir are happy with his positive response to the treatment. Today he is scheduled to undergo routine investigation and chest physiotherapy," said the hospital in a statement today.

Mahathir, 81, had spent the past 24 hours in IJN. He was transferred there from Langkawi Hospital, where he was admitted on Sunday after complaining of breathing difficulties.

IJN said Mahathir is expected to remain in the Cardiac Care Unit for monitoring and treatment, while doctors have advised him against receiving any visitors except for close family members.

He was able to have breakfast with his wife Dr Siti Hasmah today, while his son Mirzan visited him briefly. The latter declined to comment when approached.

There is no indication when Mahathir may be discharged.

Good sign

His daughter Marina told a press conference at the hospital that her father's breathing difficulties was unrelated to his heart condition.

"There's some congestion in the lungs, but the fluid was easily drained with medication," she said.

She added that Mahathir was not suffering from lung infection or pneumonia.

"He's getting better. He's breathing on his own. He just asked for some food. He was a little bit hungry. It's a good sign. His appetite's coming back," she said.

She added that it was "not clear" when Mahathir would be discharged.

Eating well

Meanwhile an AFP report quoted Mahathir's son Mokhzani as saying that the former premier was recovering "very well" and was in stable condition.

"My father is recovering very well. He is eating well," Mokhzani told AFP.

Mokhzani said he was confident his father would continue to make good progress.

"He is a workaholic. He was born in the year of the bull. He is a fighter," he said.

He has had a history of heart ailments, most notably in 1989 when he suffered a heart attack and subsequently had a triple bypass surgery.

Last November, he suffered a mild heart attack which forced him out of a much anticipated appearance at the Umno general assembly.