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The Truth About “Type L” COVID-19 ARDS Phenotype

December 15, 2020

Written by Clay Smith

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The ARDS “L phenotype” of low elastance (high compliance) existed before COVID-19 hit the planet. Evidence-based low tidal volume ARDSnet ventilation strategies are best.

Why does this matter?
Early in the pandemic, there was an article published in JAMA that posited different ARDS phenotypes: L and H. “Type L” was low elastance (high compliance). “Type H” was characterized by high elastance (low compliance). The thought was type L would have little benefit from PEEP and higher tidal volumes could be used, while type H would be managed with the usual ARDSnet strategies. Turns out, the whole L and H thing was just wrong, and we should have just managed them all with usual low tidal volume ventilation. One way to show that COVID-19 didn’t cause a new kind of ARDS would be to demonstrate that the type L phenotype predated COVID-19. That’s exactly what they did.

“There is nothing new under the sun.” Ecclesiastes 1:9
This was a secondary analysis of a prior ARDS study, LUNG SAFE, conducted in the winter of 2014, with 1,117 patients. In this cohort, 136 patients (12%) had preserved lung compliance (≥50 ml/cm H2O), the “type L” (low elastance) phenotype. This study was done well before COVID-19 was present on planet earth. Of these patients, 43% (58/136) had significant hypoxemia (PaO2/FiO2 <150). Also, there was no clear association between lung compliance and PaO2/FiO2 ratio. However, as compliance increased (that’s good), the adjusted odds of dying decreased (also good), and this was statistically significant. There was no clear inflection point with regard to lung compliance. As the editorialist opined, COVID-19 has not ushered in a new ARDS subtype; rather, this analysis, “should provide additional reassurance that applying evidence-based therapies developed in the pre–COVID-19 era to patients with COVID-19 remains standard of care.”

Compliance Phenotypes in Early Acute Respiratory Distress Syndrome before the COVID-19 Pandemic. Am J Respir Crit Care Med. 2020 Nov 1;202(9):1244-1252. doi: 10.1164/rccm.202005-2046OC.

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