Pope Francis has suffered two episodes of "acute respiratory insufficiency", the Vatican says.
The latest detailed medical update said it has been caused by "significant accumulation of endobronchial mucus" on Monday.
To help with breathing, the pope had to use "non-invasive, mechanical ventilation".
The pope also suffered a bronchospasm, which is similar to an asthma attack, the statement said.
It required doctors to perform two bronchoscopies, where they evaluate his air passages.
The Vatican said the pope's prognosis remained "guarded", which means he is not out of danger.
It is another setback for the 88-year-old pontiff as he continues to battle double pneumonia.
The latest news of more breathing difficulties for the pontiff came after relatively upbeat statements during the weekend.
Francis has been in Rome's Gemelli Hospital since February 14, and has not been seen in public since.
He was admitted with a severe respiratory infection that triggered other complications.
Francis has experienced several bouts of ill health over the past two years and is prone to lung infections because he had pleurisy as a young adult and had part of one lung removed.
Reuters
ABC
