COPENHAGEN, Denmark (AP) — Norway’s 83-year-old King Harald V was admitted to the main hospital in Oslo Thursday to undergo an operation to replace a heart valve, the palace said.

“This intervention is necessary to improve the king’s breathing. This type of operation is performed several hundred times a year at Rikshospitalet,” the monarch’s doctor, Bjoern Bendz, said in a palace statement.

The king was hospitalized last month with breathing difficulties. Doctors ruled out COVID-19.

The royal household in Oslo said the monarch is on sick leave until further notice and that his son and heir to the throne, Crown Prince Haakon, has stepped in and taken over his father’s duties.

The palace said the operation will not be open heart surgery. The king will be awake, and the operation will be performed via the groin with local anesthesia. It is scheduled for Friday at the hospital’s Cardiovascular and Lung Clinic.

In 2005, the king’s aortic valve was replaced by an artificial heart valve. Such valves have a lifespan of between 10 and 15 years, the royal household said, adding it was “not uncommon” for such interventions to have to be repeated.

Harald ascended the throne upon the death of his father, King Olav, on Jan. 17, 1991.

The country’s first native-born king since the 14th century, he married a commoner as a prince and won hearts in his egalitarian country by leading the mourning in 2011 for the victims of mass killer Anders Behring Breivik.