The lastest updated surviving sepsis guidelines for COVID-19 patient recommends a high-peep strategy in the intubated, mechanically ventilated patient. As most of these patients present with moderate to severe ARDS, PEEP is used to keep lung areas open and therefor to improve oxygenation. This seems to be especially true in the classical case of ARDS, where the lung become 'wet' and 'heavy' which results in widespread atelectasis of the dependent parts of the lungs, often further complicated by pleural effusions.
Classical CT appearance in the acute phase of ARDS is an opacification with an antero-posterior density gradient. Dense consolidation in the most dependent regions merges into a background of widespread ground-glass attenuation and the normal or hyperexpanded lung in the non-dependent areas (Howling SJ et al. Clin Radiol 1998;53(2):105-109). The theory behind these changes is that the increased weight of overlying lung causes compression-atelectasis posteriorly. The fact that prone positioning these patients quickly redistributes these gradients supports this theory (Desai SR et al. Anaesthesiology 1991;74(1):15-23).
Chest CT's in patients with COVID-19 often show ground-glass opacification with or without consolidations. These are changes often seen in viral pneumonia. Several case series suggest, that CT abnormalities seem to be mostly bilateral and tend to have a peripheral distribution, often involving the lower lobes. In contrast to the classical ARDS pleural thickening, pleural effusion and lymphadenopathy seem to be a less common finding (Shi H et al. Lancet Infect Dis 2020).
The leading problem in COVID-19 patients with ARDS is hypoxemia, while hypercapnia does not seem to be a significant problem. Sometimes profound hypoxemia does not seem to correlate with patient symptoms at all. In regards to the images above, atelectasis might not be the predominant reason for V/Q mismatches in these patients.
Observations of mechanically ventilated patients in our unit and other hospitals in Switzerland have shown, that higher PEEP levels (15cmH2O and higher) often result in significantly reduced compliance values complicating ventilation and favouring the development of pulmonary over-inflation. This observation might support the theory that patients with COVID do not represent the traditional manner of ARDS with distinctive atelectasis. Another observation that supports this theory is that COVID-19 patients often do not respond as clearly to Prone Positioning as classical ARDS patients do.
More probably, V/Q mismatch seems so happen on a more microscopical level in COVID-Patients. Lung compliance is often normal on these patients and, therefore, applying high PEEP-levels does NOT add any benefit at all.
Maybe the principle of less is more also applies to COVID-19 patients we treat (Gattinoni L et al. Intensive Care Medicine; 46, pages780–782(2020))