MESSAGE #1: COVID19 Lung Injury is NOT HAPE [1]
HAPE
- Excessive and uneven hypoxic pulmonary vasoconstriction
- Increased pulmonary artery pressure in certain regions of the lung
- Increased pulmonary capillary hydrostatic pressure
- Leakage of fluid out of vascular space into alveolar space
- Treatment: Supplemental O2 alone OR descent to lower elevation (Resolves alveolar and interstitial edema within hours to days)
- Supplemental O2 helps reverse pulmonary vasoconstriction
- Medications such as nifedipine and vasodilators can also help with HAPE (NOT for COVID-19)
COVID-19 Lung Injury
- Viral mediated inflammation
- Alveolar epithelial inflammation/dysfunction
- Impaired surfactant function/alveolar fluid clearance
- Alveolar collapse and/or filling (V/Q mismatch)
- Increased pulmonary artery pressure is a consequence, NOT the cause, of alveolar edema
- Treatment: Supplemental O2 (Improves hypoxemia but does not resolve underlying inflammation or lung injury)
- In addition positive pressure (CPAP) and awake proning are treatment options as well
MESSAGE #2: COVID19 Pneumonia is ARDS…just not “typical” ARDS [2]
- Same Virus, Two Phenotypes
- Despite sharing a single etiology (SARS-CoV-2), COVID-19 pneumonia has two distinct presentations:
- This is most likely a time-related spectrum of disease:
- L Type
- Low elastance (i.e. high compliance)
- Low ventilation/perfusion ratio
- Low lung weight
- Low recruitability
- CT scan: well aerated lungs (-1000 to -700 HU)
- H Type
- High elastance (i.e. low compliance)
- High right-to-left shunt
- High lung weight
- High recruitability
- CT scan: non-aerated lungs (-300 to 100 HU)
- L Type
- If using the Berlin criteria to define ARDS, COVID-19 lung injury meets the definition. The key difference is lung compliance can often be normal/high (L type) but can progress to low compliance (H type)
- Berlin Criteria:
- Timing: Within 7 days
- Imaging: Bilateral infiltrates
- Origin: Not explained by cardiac failure/fluid overload
- Oxygenation:
- Mild: P/F <200mmHg (≤300 with PEEP/CPAP ≥5cmH20)
- Mod: P/F <100mmHg (≤200 with PEEP/CPAP ≥5cmH20)
- Severe: P/F ≤100mmHg with PEEP ≥5cmH20
- In the image below we see two CT scans of patients with two differing phenotypes however both have a P/F ratio <100mmHg
- Patients with L Type ARDS (High Compliance) need a high FiO2/Lower PEEP strategy
- Patients with H Type ARDS (Low Compliance) should follow the traditional ARDSnet protocol
Take Home Messages:
- COVID-19 lung injury is not HAPE
- COVID-19 pneumonia is ARDS, it’s just not “typical” ARDS
References:
- Luks AM et al. COVID-19 Lung Injury is Not High Altitude Pulmonary Edema. High Altitude Medicine & Biology 2020. PMID: 32281877
- Gattinoni L et al. COVID-19 Pneumonia: Different Respiratory Treatment for Different Phenotypes. Intensive Care Med 2020. [Epub Ahead of Print]
Post Peer Reviewed By: Anand Swaminathan, MD (Twitter: @EMSwami)
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