Vatican says Pope Francis' prognosis 'remains guarded'

However, the statement also said that Francis' prognosis "remains guarded," which means he is not out of danger.

Francis, the leader of the world's nearly 1.4 billion Roman Catholics, has been in Rome's Gemelli Hospital for more than two weeks.

He was admitted on February 14 with a case of bronchitis that worsened into double pneumonia.

On Tuesday morning, the 88-year-old pontiff "transitioned to high-flow oxygen therapy and underwent respiratory physiotherapy," according to a Vatican statement.

On Tuesday night, the pope was set to resume noninvasive mechanical ventilation throughout the night.

While Francis' heart, kidney and blood measurements are stable, "his health situation remains complex," the Vatican said.

On Monday, the pope underwent two bronchoscopies to remove "a significant accumulation of endobronchial mucus."

The Vatican said Francis remained "alert, oriented and cooperative at all times" during the procedures.

However, Dr. John Coleman, a pulmonary critical care doctor at Chicago's Northwestern Medicine, told The Associated Press, said Francis seems to be "taking little steps forward and then steps back."

"The fact that they had to go in there and remove [the mucus] manually is concerning, because it means that [the pope] is not clearing the secretions on his own," said Coleman, who is not part of the pope's medical team.

This hospital stay is Francis' longest during his time as pope. He is prone to lung infections, having had part of a lung removed when he was a young man.

Francis' hospital stay is not the record amount of time a pope has been hospitalized. In 1981, Pope John Paul II spent 55 days in Gemelli for a minor operation that resulted in a serious infection that extended the pontiff's hospital stay.