Introduction
In ARDS, Ventilator Induced Lung Injury occurs with over-distention of more compliant anterior lung segments and shear injury from repetitively opening and closing of less compliant posterior lung segments. This lung injury is exacerbated by the supine position directs ventilation more anteriorly while leaving the posterior lung atelectatic. The prone or lateral position improves the mal-distribution of tidal volume and reduces ARDS mortality.
ARDS was first described by Ashbaugh and Petty in Lancet in1967,because it was frequently encountered in wounded soldiers during the Vietnam war, it was often referred to DaNang, shock and/or wet lung.
Overview
Acute lung injury (ALI, pO2/Fio2 < 300) and Acute Respiratory Distress Syndrome (ARDS, pO2/Fio2 < 200) is defined by acute onset of; hypoxia, tachypnea and diffuse pulmonary infiltrates the latter not due to elevated left atrial pressure (CHF). Multiable etiologies include: aspiration; SIRS from trauma, burns, sepsis, shock, adverse reactions to drug, blood products and environmental factors.
Observations
Mortality of ARDS approached 50% in the 60s and has slowly declined with data in 2014, with the application of proning in ARDS, mortality was reduced to 16%. This drop has in large part been due to the recognition of ventilator induced lung injury (VILI) with over-distension of relatively normal lung units and shear forces associated with the repetitive opening and closing of diseased units with use of then normal tidal volumes of 10CC/kg. Reduction of ventilator induced lung injury (VILI) by low tidal volume strategy (6cc/kg),along with early use of paralytics and prone positioning have all shown to have reduced mortality by preventing VILI. Conservative fluid management has been shown to more rapidly improve lung injury scores and reduce ventilator and ICU length of stay but not mortality.
Goals
Ongoing trials in ARDS include use of surfactant, immunomodulation, neutrophil elastase inhibitors, corticosteroids for the fibroproliferative phase, etc. but clear benefit
to date include; conservative fluid management, low tidal volume, early paralytic use, prone positioning, the latter three are directed at further reduction of VILI. Early low dose steroids (within 3 day of onset) in some studies show benefit by reducing the late complication of lung scaring (fibroproliferative phase of ARDS).