At their institution, the investigators routinely assess the individual response of mechanically ventilated patients with acute respiratory failure to higher positive end-expiratory pressure (PEEP) by examining changes in lung aeration through lung computed tomography (CT). They typically obtain two lung CT scans during an end-expiratory hold, one at a PEEP of 10 cmH2O and another at 15 cmH2O. Recruitment and hyperinflation are measured by assessing the decrease in the volume of non-aerated lung regions (with a density greater than -100 HU) and the increase in the volume of hyperinflated lung regions (with a density of less than -900 HU) between the two CT scans. If recruitment exceeds hyperinflation, the response to higher PEEP is considered "positive", and medical doctors are encouraged to treat that patient with a higher PEEP. Conversely, if hyperinflation exceeds recruitment, the response is deemed "negative", and a higher PEEP is discouraged. This assessment is further complemented by a "PEEP test," during which gas exchange (specifically arterial oxygen and carbon dioxide tension) and respiratory system mechanics (including compliance) are evaluated while ventilating with PEEP levels of 10 and 15 cmH2O. In this retrospective analysis, the investigators will focus on patients with severe pneumonia who underwent the two lung CTs as part of their routine clinical practice. They will retrieve data on primary demographic characteristics, significant comorbidities, causes of pneumonia, severity of acute illness, and treatments delivered. Additionally, they will review the results of the quantitative analysis of the lung CT scans taken at 10 and 15 cmH2O, and the PEEP tests. The primary aim of this study is to determine the proportion of patients with net hyperinflation in response to higher PEEP.
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Lung response to a higher PEEP
Timeframe: The effects of a higher PEEP will be assessed by comparing the CT scans taken at ICU admission, with a PEEP of 10 or 15 cmH2O, within the same subject. Usually, the interval between the two CT scans is a few minutes.