By: Dr. Bradley Salemi
What is ARDS and what are the clinical manifestations?
Respiratory failure characterized by acute, diffuse, inflammatory lung injury resulting in increased permeability of the alveolar capillary membrane, leading to development of non-hydrostatic pulmonary edema. The acute phase is characterized by diffuse alveolar damage.Clinically, patients will have marked hypoxemia and respiratory distress, frequently progressing to respiratory failure requiring mechanical ventilation and a stay in the ICU.
What are some of the causes of ARDS?
Many conditions can cause ARDS, these include but are not limited to pneumonia, extrapulmonary sepsis or septic shock, pancreatitis, and trauma.
How is ARDS diagnosed?
Patients are diagnosed based on the Berlin Criteria, patients must have the following
New or worsening symptoms within one week of a known clinical insult
Bilateral opacities observable on AP CXR that are not due to effusions, lobar or lung collapse
Hypoxemia defined by a PaO2/FiO2 < 300mmHg and a minimum PEEP greater than or equal to 5mm Hg
ARDS can be further subdivided based on degree of hypoxemia: mild = PaO2/FiO2 200-300mmHg, moderate = 100-200 mmHg, and severe = PaO2/FiO2 < 100 mmHg
Ok cool, that’s nice, but how is ARDS treated?
Mechanical Ventilation
The cornerstone of treatment is lung protective ventilation, this is to prevent any ventilator-induced lung injury. Lower tidal volumes relative to predicted body wate and limiting plateau pressures have significantly improved mortality among patients with ARDS. Additionally, higher levels of PEEP are suggested for moderate to severe ARDS.
Prone Positioning
Prone positioning can redistribute mechanical forces in the lungs, allowing for a more event distribution of lung inflation and alveolar recruitment. This strategy is recommended for patients with a PaO2/FiO2 < 150 mmHg.
VV-ECMO
Previously considered a last resort, rescue therapy, more evidence is emerging that it can be a bridge to recovery.
Pharmacological Therapy
Steroids theoretically reduce inflammation seen in ARDS. However, hyperglycemia, hypernatremia and muscular weakness are important adverse events to consider.
Deep sedation with neuromuscular blockade can be considered for patients who require invasive mechanical ventilation to facilitate delivery of regular, low tidal volumes.
COVID and ARDS
Development of ARDS secondary to COVID continues to be common. It was initially thought that there were two different types of ARDS seen in COVID patients: type H which is consistent with severe ARDS, and type L which is consistent with mild to moderate ARDS. As more evidence has been accumulated, this characterization has fallen out of favor. So, what does that mean? It means that there is no convincing evidence COVID-associated ARDS is a distinct entity. That being said, ARDS therapies may be effective for COVID patients. High flow nasal cannula and prone positioning when awake improve hypoxemia and avoid intubation. Steroids have been shown to reduce mortality patients among COVID patients.
ARDS therapy in the future
Other motes of ventilation may be effective in managing ARDS. APRV may minimize ventilator-induced lung injury, however more clinical trials are needed. Patient self-inflicted lung injury is another area requiring more study. The risk of P-SILI can theoretically be reduced by controlling respiratory drive and through neuromuscular blockade, however like APRV, it requires more study. Finally, alternatives to VV-ECMO are being explored to reduce the risk of hypoventilation and resulting hypercapnia and acidosis.
For those wanting a refresher from last month’s EMerging insights, see the link below for information on mechanical ventilation in the ED.