Acute respiratory distress syndrome (ARDS) which occurs in all age groups is a common and critical condition in intensive care units. Their clinical manifestations are mainly bilateral diffuse pulmonary infiltrates and acute hypoxemic respiratory failure. The main pathogenic factor is infections. According to the Berlin Conference in 2012, ARDS is mainly classify patients into mild, moderate and severe severity based on oxygenation capacity (PaO2/FiO2).

ARDS usually occur within seven days after exposure to risk factor. The early symptoms is shortness of breath and hypoxemia due to impaired alveolar gas exchange. As it gets worse, the skin may take on a bluish tinge and the function of heart and brain may become abnormal. Additionally, wheezing and bilateral lung shadows will occur. Because of chronic hypoxia, ARDS usually develops complications of other organ failures. If it is not treated in time, severe cases can result in death.

There are many various pathologies can trigger ARDS such as pneumonia and mechanical ventilation, pulmonary contusion, sepsis, massive transfusion and so on.

The clinical evolution of ARDS can be divided into three phases:

1. Exudative early phase occurs approximately within 72 hours after the direct lung injury or host insult. This phase involves the migration of immune cells from the circulation into the alveolar following diffuse damage to the endothelial and epithelial surfaces. Changes in alveolar-capillary barrier permeability could develop pulmonary edema and affect gas exchange.
2. Proliferative phase incorporates the repair of the damaged alveolar structure with proliferation of fibroblasts. Hyaline membranes can be development by protein-rich fluid.
3. Fibrotic phase occurs approximately 1 to 3 weeks after the initial lung injury. Patients with chronic inflammation and fibrosis of the alveoli have impaired alveolar gas exchange. If it is not treated in time, chronic hypoxia might result in multiple organ dysfunction.

Source: Meridigen Biotech. Co., Ltd.

Many conditions can directly or indirectly injure the lungs and lead to ARDS which is characterized by the progressive hypoxemia and respiratory distress. Since the incidence rate is affected by the evolution of the definition, there is no complete data on the incidence of ARDS in each country.

Currently, although advances of ventilator strategies in the management of ARDS associated with outcome improvements, ARDS appears to be undertreated and mortality remains elevated up to 40%.

The current treatment of ARDS is supportive treatment and to deal with the reversible factors that potentially aggravate ARDS. The purposes of mechanical ventilation and nutrition therapy are to ensure adequate oxygen supply to the tissue and to prevent second injury that causes by mechanical ventilation. Patients with severe hypoxemia use muscle relaxants and prone position to reduce patient mortality. Extracorporeal membrane oxygenation (ECMO) is a treatment strategy for severe ARDS patients to maintain the blood oxygen. However, this treatment is no therapeutic effect on ARDS. Additionally, the timing and dosage of steroids and bronchodilators currently used are still controversial.

Recently, since the mesenchymal stem cells have strong immunomodulatory capability for activated lymphocytes and macrophages, lung cytoprotection, anti-apoptosis and other functions are widely studied, stem cell therapy is considered as a potential drug to treat ARDS.