All are seen in ARDS, Except:

Ans. is 'c' Hypercapnia (Ref. Harrison, 17/e, p 1680-1681 & 16/e, p 1592 (15/e, p. 1524)).Hypocapnia is seen in ARDS.ARDS is an acute* diffuse* alveolar injury * that is caused by variety of aetiological factors. Patients with ARDS are often seriously ill with some other diseases and the features of ARDS are superimposed on it.It is characterized by acute diffuse alveolar damage leading to necrosis and loss of type I pneumocytes*. Endothelial damage also occurs leading to exudation of protein rich fluid into alveoli and resulting in pulmonary edema and formation of hyaline membrane*. Hyaline membrane is composed of fibrin* together with coagulated cell debris*. Hyaline membrane leads to decreased compliance and stiff lung.These changes result in diffusion defects. Due to low diffusion capacity of 02 hypoxemia occurs. Hypoxemia causes increase in respiratory rate due to which C02 wash out occurs so the level of CO2 is low in ARDS.Criteria for diagnosis of ARDS are:Antecedent history of precipitating conditions.Refractory hypoxemia* - Pa02 < 60 mmHg.Radiological evidence of newly evolving bilateral pulmonary infiltrates suggestive of pulmonary edema*.A pulmonary artery occlusion pressure (PAOP) less than 15-18 mmHg in the presence of normal colloid oncotic pressure.Total thoracic compliance less than 30 ml/cm H2O.