Pulmonary Disorders I

First described in 1967; several definitions have evolved over the years

Variable

Criteria

Timing

Onset within 1 wk of a known clinical insult or new or worsening respiratory symptoms

Chest imaging

Bilateral opacities not fully explained by effusions, lobar/lung collapse, or nodules

Origin of edema

Respiratory failure not fully explained by cardiac failure or fluid overload

Need objective assessment (ie, ECHO) to exclude hydrostatic edema if no risk factors present

Oxygenation

Mild

200 mm Hg < Pao2/Fio2 ≤ 300 mm Hg with PEEP or CPAP ≥ 5 cm H2O

Moderate

100 mm Hg < Pao2/Fio2 ≤ 200 mm Hg with PEEP ≥ 5 cm H2O

Severe

Pao2/Fio2 ≤ 100 mm Hg with PEEP ≥ 5 cm H2O

ARDS = acute respiratory distress syndrome; CPAP = continuous positive airway pressure; ECHO = echocardiogram; Fio2 = fraction of inspired oxygen; Pao2 = partial

pressure of oxygen; PEEP = positive end-expiratory pressure..

In 2014, ARDS represented 10.4% of all ICU admissions and 23.4% of patients requiring mechanical

Of these, mild ARDS accounted for 30%, moderate for 46.6%, and severe for 23.4%.

Hospital mortality was 34.9% for mild ARDS, 40.3% for moderate ARDS, and 46.1% for severe

ARDS.

During the COVID-19 pandemic, ARDS occurred at a rate of 50% to 80% compared with the

general ICU population (BMJ. 2024;369:e076612).

Med 2011;37:1932-41).

Direct and indirect causes of lung injury

Direct: Pneumonia, aspiration, trauma

Indirect: Sepsis, transfusion injury, pancreatitis, burn injury, trauma

The hallmark clinical manifestation of ARDS is hypoxemia from alveolar collapse and edema.

The exudative phase is characterized by increased permeability from the alveolar epithelium and

capillary endothelial complex damage, leading to diffuse alveolar edema with fluid and cellular

debris (e.g., neutrophils, cytokines, platelets). In addition, type II cells responsible for surfactant

production are damaged. The peak incidence of this phase typically occurs within the first week

after the initial insult.

The fibroproliferative phase marks either recovery or progression of ARDS. Patients may partly

or fully recover pulmonary function from drainage of the alveolar fluid, type II cellular repair,

and improvement in the integrity of the endothelium-epithelium complex. However, patients whose

conditions are progressively worsening may develop significant alveolar, interstitial, and capillary

fibrosis. This phase typically manifests later in the course of ARDS (i.e., more than 7 days after the

initial insult).