Mahathir Mohamad, Malaysia's former prime minister, has been admitted to hospital after complaining of breathing difficulties while resting in his hotel room.
He was warded in the intensive care unit of Langkawi hospital in Malaysia's northern Kedah state.
Mahathir's son, Mukhriz, said late on Monday that his father may be flown to the National Heart Institute in the capital, Kuala Lumpur, "if his condition improves".
He added that Mahathir's condition could be due to exhaustion.
"He asked for some water to drink. Doctors have given him some oxygen and sedatives to make him rest. This has stabilised his condition," he said. "But I am not sure when he could leave [the hospital]."
Doctors have advised Mahathir to rest and only relatives are allowed to visit him, added Mukhriz.
Mahathir's wife, children and other relatives were at his bedside.
The 81-year-old former prime minister and his wife arrived in Langkawi for a two-day day private visit and were scheduled to return to Kuala Lumpur on Tuesday.
In November, Mahathir suffered a mild stroke caused by a clot that restricted blood flow in one of his arteries, but subsequently recovered.
He also underwent heart bypass surgery after a heart attack in 1989.
Mahathir retired in 2003 after 22 years in office, handing over the reins to Abdullah Ahmad Badawi, whom he has since accused of nepotism and corruption.