44-year-old woman who was previously healthy is admitted to the emergency department in the winter with a 2-day history of fevers, upper respiratory tract symptoms, productive cough and today worsening shortness of breath. Her COVID-19 PCR nasal swab is positive for SARS-CoV-2 and she rapidly deteriorates and requires intubation. Her bedside exam is notable for diffuse rhonchi on lung auscultation, but there is no peripheral edema or notable jugular venous distention, and no S3 or S4 on cardiac exam. Her portable chest x-ray is notable for bilateral diffuse infiltrates. Her initial ventilator settings in the volume assist control (VAC) mode are an FIO2 100%, respiratory rate of 24, tidal volume of 6 mL/kg predicted body weight, and PEEP = 5 cm H2O. After 30 min on these settings, an arterial blood gas (ABG) is performed which has a pH 7.31, Paco2 44 and Pao2 145 mm Hg.
According to the Berlin criteria, what is the classification of this patient's acute hypoxemic respiratory failure?
C) Moderate ARDS
According to the Berlin criteria published in 2012 which was designed to reclassify patients with acute respiratory distress syndrome (ARDS), this patient has moderate ARDS.
The original classification for patients with ARDS was developed by an American-European Consensus Conference and published in 1994. This group proposed that patients with an acute onset of bilateral infiltrates on chest radiograph with a pulmonary capillary pressure ≤18 mm Hg would be considered to have acute lung injury (ALI) if their PaO2/FIO2 (P/F) ratio was <300 mm Hg, and the subset with a P/F ratio of <200 mm Hg would be classified as ARDS. Further division of the patients with ARDS was not specified.
At that time, the P/F criteria had not been well studied as a predictor of lung injury severity or mortality. Also, the best method to exclude cardiogenic pulmonary edema as the cause of the patient's respiratory failure had not been clarified. As a consequence, a new criteria for defining ARDS and its severity was developed.
Two results of this reclassification effort were to 1) eliminate the use of the pulmonary artery wedge pressure and 2) eliminate the category of acute lung injury (ALI). Specific guidelines for CXR interpretation were provided, and a minimal PEEP level of 5 cm H2O was required in order to measure and apply the P/F ratio.
Finally, the P/F ratio was shown to be a significant prognostic measurement for patients with ARDS. With a P/F ≤100 mg (severe ARDS), observed mortality was 45%, with P/F 100-200 mm Hg (moderate ARDS) observed mortality was 32%, and with P/F 200-300 mg Hg (mild ARDS) observed mortality was 27%
These new proposed criteria are of relevance to the clinician since they are now guiding the identification and stratification of patients enrolled in current clinical trials studying the treatment of ARDS, and they help define subgroups that might warrant additional and/or salvage therapies, such as prone position ventilation or extracorporeal support (ECMO).
