Advanced Respiratory Monitoring in COVID-19 Patients: Use Less PEEP!

Comment: I read the article by Lisanne Roesthuis et al. with great interest as finding an optimal mode of ventilation for patients with severe Covid-19 pneumonia remains a challenge. The combined findings of high compliance and a severe reduction in gas exchange for both oxygen and CO2 in these patients are not easily explained and make it hard to maintain an adequate, yet lung protective ventilation. Clinical observations combined with analyses such as those performed by Lisanne Roesthuis et al. are needed to improve our understanding of this specific disease. While reducing PEEP based on their observations is tempting, we often see reduced P/F ratio’s when PEEP levels are lowered in Covid-19 patients with moderate to severe ARDS. In my view this challenges the concept of high PEEP causing mainly alveolar hyperinflation without relevant effects on recruitment. An explanation for some of the discrepancies observed in Covid-19 patients could be that high compliance results largely from changes in the airways while compliance remains low at the level of respiratory bronchioli and alveoli. A potential contribution of airway distension to pulmonary compliance in ARDS has previously been demonstrated (Schepens et al.). With instable lungs at the alveolar level due to ongoing inflammation, derecruitment (Knudsen et al) or intermittent airway closure (Chen et al) could cause P/F ratio’s to decline with PEEP reduction. Increased airway compliance may also contribute to biotrauma caused by intermittent airway closure when PEEP is insufficient to maintain open airways during expiration. In this scenario pulmonary conditions may gradually deteriorate as long as inflammation continues, despite ventilation modes intended to be lung protective. A gradual decline in the amount of lung tissue contributing to gas exchange may be hard to detect with hypoxic vasoconstriction masking the effects in arterial blood gas analyses, while circumstances allow few adequate radiological examinations to be made. A direct effect of Covid-19 on bronchial smooth muscle tone has not been reported, but effects may also occur through central nervous system involvement (Li et al, Wu et al.). Further indirect evidence for increased airway compliance stems from CT studies indicating dilation at the bronchiolar level (Ye et al). Dynamic airway closure with retention of air in the alveoli may explain the absence of recruitable lung tissue in CT studies, despite positive effects of PEEP on P/F ratio’s (Chen et al). In all it seems that more studies are needed to improve our understanding of Covid-19 pneumonias. It would be interesting to see if functional respiratory imaging can help to differentiate between effects at the level of airways and alveoli in such studies. With current knowledge, it remains to be seen if reducing PEEP for all Covid-19 patients will translate into better outcomes.

Fred van Steveninck, MD, PhD.
Intensive Care.
Deventer Hospital, the Netherlands.

References:

Schepens et al. Functional respiratory imaging of the airways in the acute respiratory distress syndrome. Anaesthesia Critical Care & Pain Medicine. In press. https://doi.org/10.1016/j.accpm.2019.10.017

Knudsen et al. The micromechanics of lung alveoli: structure and function
of surfactant and tissue components. Histochemistry and Cell Biology. (2018) 150:661–676.

Chen et al. Airway Closure in Acute Respiratory Distress Syndrome: An Underestimated and Misinterpreted Phenomenon. American Journal of Respiratory and Critical Care Medicine.
Volume 197 Number 1 | January 1 2018

Li et al. The neuroinvasive potential of SARS-CoV2 may be at least partially responsible for the respiratory failure of COVID-19 patients. https://doi: 10.1002/jmv.25728

Wu et al. Nervous system involvement after infection with COVID-19 and other corona viruses.
Brain, Behavior, and Immunity. https://doi.org/10.1016/j.bbi.2020.03.031

Ye et al. Chest CT manifestations of new coronavirus disease 2019 (COVID-19): a pictorial review.
European Radiology. https://doi.org/10.1007/s00330-020-06801-0