ROME — Pope Francis, hospitalized with pneumonia in both lungs, slept all night and was resting on Tuesday after suffering two bouts of acute respiratory failure the day before, the Vatican said.

The 88-year-old was admitted to Rome's Gemelli hospital on Feb. 14 with bronchitis, which developed into pneumonia in both lungs, sparking alarm.

On Monday, the pope "experienced two episodes of acute respiratory failure, caused by a significant accumulation of endobronchial mucus and consequent bronchospasm," the Vatican had said in its evening update.

Acute respiratory failure, which can be life-threatening, occurs when the lungs cannot pass enough oxygen into the blood or when carbon dioxide builds up in the body.

The Holy See added that Francis' prognosis remained "reserved," an indication that doctors cannot predict the likely outcome of his condition. On Tuesday morning, it issued a typically brief update saying: "The pope slept all night long and continues to rest."

It was the third crisis the Argentine pontiff has suffered since his admission to the Gemelli, where he is being treated in a special papal suite on the 10th floor.

On Feb. 22, he suffered a "prolonged asthmatic respiratory crisis," followed on Feb. 28 by "an isolated crisis of bronchospasm" — a tightening of the muscles that line the airways in the lungs.

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Medical experts warned Francis' continued hospitalization — the longest of his 12-year papacy — and the repeated crises were alarming.

"At 88 years old, being in the hospital for two weeks and having repeated episodes of respiratory discomfort is a very bad sign," Bruno Crestani, head of the pulmonology department at Bichat hospital in Paris, said. AFP

Herve Pegliasco, head of pulmonology at the European Hospital in Marseille, added that with double pneumonia, "there is the issue of exhaustion, because he is forced to make much more effort to breathe."

The Vatican said on Monday the pope was alert and cooperative during his crisis, which required two separate bronchoscopies, where doctors look into the air passages using a small camera at the base of a flexible tube.

The head of the world's almost 1.4 billion Catholics has been working during his time in the Gemelli, talking on the telephone and receiving some officials, according to Vatican sources.

But Francis has not been seen in public for almost three weeks and the last photos taken of him were from his private audiences on the morning of his admission to hospital.