Shah, 67, was admitted to the Norvic Hospital late last night.
One of Shah's arteries on the heart's left side was blocked due to the cardiac arrest. Doctors said this was a "mild heart-attack" and he is recovering.
Shah, who had undergone an angioplasty surgery, can walk on his own and he is not having any chest pain, according to The Himalayan Times.
"He went to restroom on his own this morning," senior surgeon Bharat Raut told the daily.
More From This Section
Doctors have kept Shah under observation and if his health remains stable until tomorrow he might be discharged, the report said.
Shah was crowned as king of Nepal in 2001 after his elder brother Birendra Bir Bikram Shah Dev and his family were killed in a massacre at the Royal Palace.
An investigation blamed Birendra's son Dipendra, who was among the dead, for the massacre.
Shah stepped down in 2008, after a popular revolt forced him to abdicate, and the centuries-old monarchy was abolished and the country was turned into a republic by the Constituent Assembly.