Over the past few months of managing patients with COVID-19, one thing has become abundantly clear, we are continually learning how to treat this infection and its complications. While the majority of people who contract COVID-19 have mild symptoms and recover, others need further intervention, including hospitalization. Patients with severe symptoms of COVID-19 often go on to develop acute hypoxemic respiratory failure and pneumonia and 17 to 29% of these patients develop adult respiratory distress syndrome (ARDS) (Auwaerter, 2020). Critical care clinicians have managed ARDS for years; however, COVID-19 ARDS does not act like the typical ARDS we’ve known, and we need to learn a new way to manage it.

“Typical” ARDS

ARDS was originally defined in 1994 by the American-European Consensus Conference, however, experts started to doubt the reliability and validity of the definition. In 2011, the European Society of Intensive Care Medicine, the American Thoracic Society and the Society of Critical Care Medicine developed the Berlin definition of ARDS which is based on degree of hypoxemia and four other variables, including chest radiographic severity, respiratory system compliance, positive end-expiratory pressure (PEEP) and corrected expired volume per minute (ARDS Task Force, 2012).

Let’s look at lung compliance

Lung compliance is very important when we are comparing traditional ARDS to COVID-19 ARDS. Lungs expand with inspiration and recoil with expiration. The ability of the lung to expand and recoil is compliance. Compliance can be divided into two types:

• Static compliance
  • Compliance of the lungs when the lungs and the muscles of the lungs are at rest; pressure is the only variable
  • Think “lungs not moving”

• Dynamic compliance
  • Compliance of the lungs during breathing
  • Think “lungs moving”

In patients with traditional ARDS, lung compliance is decreased. Interventions are based on preventing barotrauma and optimizing oxygenation, ventilation and perfusion with interventions such as low tidal volumes (6 mL/kg of predicted body weight), keeping the plateau pressure less than 30 mm H₂O, avoiding oscillatory ventilation, prone positioning, and using higher levels of PEEP in patients with moderate or severe ARDS.

What’s different in patients with COVID-19? Lung compliance is high

Patients with COVID 19 often progress to acute hypoxemic respiratory failure and go on to develop ARDS, however, their ARDS is different. In COVID-19 patients, lung compliance is high; this is not what you see in a traditional ARDS patient (Gattinoni et. al., 2020). So, although the patients with COVID-19 meet the Berlin criteria for ARDS based on their degree of hypoxemia, they do not meet the compliance component.

In a recent anecdotal study conducted in Italy with 16 intubated and mechanically ventilated COVID-19 patients, the authors noted these patients appear to lose their ability to regulate lung perfusion and hypoxic vasoconstriction (Gattinoni et al., 2020). In the ventilated COVID-19 patients, oxygenation increased with high PEEP and prone positioning but, this was not due to alveolar recruitment which is normally seen in traditional ARDS. In these patients, oxygenation increased with high PEEP and prone positioning due to the redistribution of perfusion in response to pressure and gravitational forces. Higher PEEP was sometimes associated with hemodynamic instability. In addition, improvements due to proning were modest and took more time to appreciate. Their recommendation was to use gentler ventilation and the lowest possible PEEP (Gattinoni et al., 2020).

The findings of Gattinoni et al., are somewhat contradictory to other reports which suggest that COVID-19 ARDS with relatively high lung compliance benefit from 10 to 15 cm H₂O of PEEP as compared to 5 cm H₂O (Anesi, 2020).

Patients with COVID-19 may not tolerate traditional modes of ventilation such as volume-limited, low tidal volume ventilation. This is evident by their inability to achieve a plateau pressure that is less than or equal to 30 com H₂O or they experience ventilator dyssynchrony. Using pressure-limited modes or volume targeted pressure-controlled ventilation may be required (Siegel & Hyzy, 2020). Clinicians around the world have recognized that weaning these patients from ventilatory support takes a long time. The weaning process is done slowly with small incremental changes in FiO2 and pressure support (PS) since decreasing ventilation support appears to hasten decompensation even in a patient that appears quite comfortable  on PS settings. These patients end up being on the ventilator for 2 weeks or more and may require tracheostomy placement.
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COVID-19 ARDS and Proning: Learning in Real Time

Impact to clinicians at the bedside

As clinicians at the bedside, we need to rely on the evidence and recommendations made available to us. We also need to accept that every day we will learn something new about managing COVID-19 patients. Everyone can agree that the strength of the evidence regarding many of the recommendations for managing COVID-19 are either best practice statements or weak recommendations, and it will take time and research to discover what truly works for these patients.  

Here are the current suggestions and recommendations from the Society of Critical Care Medicine (Alhazzani et al., 2020):

• Start with supplemental oxygen if the SpO₂ is less than 90% and consider it if the SpO₂ is less than 92%.
• In acute hypoxemic respiratory failure, target oxygen therapy to keep the SpO₂ no higher than 96%.
• Use high flow nasal cannula (HFNC) over conventional oxygen therapy in acute hypoxemic respiratory failure; if this is not available and the patient doesn’t need intubation, it is suggested to try noninvasive positive-pressure ventilation (NIPPV) with close monitoring.

For mechanically ventilated patients, the Society of Critical Care Medicine recommends the following:

• Use a low tidal volume (Vt) ventilation strategy (Vt 4-8 mL/kg of predicted body weight).
• Target plateau pressures of less than 30 cm H₂O.
• In those with moderate to severe ARDS, consider a higher PEEP strategy (greater than 10 cm H₂O) and monitor closely for barotrauma.
• For patients with hypoxemia despite optimized ventilation, consider using recruitment maneuvers but do not use staircase/incremental PEEP.

In conclusion, COVID-19 is not the “typical ARDS”

COVID-19 presents an opportunity for us to learn as we practice and research the optimal approach to managing these patients. It is important to recognize that the pathophysiology of COVID-19, high compliance ARDS, is different than traditional ARDS. It begs the question, should we call what we are seeing in clinical practice high compliance ARDS or should we call it severe acute hypoxemic respiratory failure secondary to COVID-19? Regardless, understanding what we are seeing clinically is not “typical ARDS” can help us fine-tune our approach to mechanically ventilated patients in order to prevent barotrauma and optimize oxygenation, ventilation, and perfusion.
 

References:
Alhazzani, W., Moller, M., Arabi, Y., Loeb, M., Gong, M., Fan, E.,…Rhodes, A., (2020). Surviving Sepsis Campaign: Guidelines on the Management of Critically Ill Adults with Coronavirus Disease 2019 (COVID-19). Critical Care Medicine. doi: 10.1097/CCM.0000000000004363

Anesi, G., 2020. Coronavirus disease 2019: Critical care issues. UpToDate. Updated March 2020.  

ARDS Definition Task Force, Ranieri V., Rubenfeld G., Thompson B., Ferguson N., Caldwell E., Fan, E., Camporota, L., & Slutsky A. (2012). Acute respiratory distress syndrome: the Berlin Definition. JAMA,.307(23). doi: 10.1001/jama.2012.5669.

Auwaerter, P. (2020). Coronavirus 2019: COVID-19. Johns Hopkins POC-IT Guide. Unbound Medicine. Updated April 2020.

Gattinoni, L., Coppola, S., Cressoni, M., Busana, M., and Chiumello, D. (2020). COVID-19 Does Not Lead to a “Typical” Acute Respiratory Distress Syndrome. ATS Journals. doi: https://doi.org/10.1164/rccm.202003-0817LE

Siegel, M., & Hyzy, R., (2020). Ventilator management strategies for adults with acute respiratory distress syndrome. UpToDate. Updated Mar 2020.