Serum Surfactant Protein-D (SP-D) Level: Does it Correlates with Pulmonary Compliance and ARDS in Severe Cases of COVID-19?

Acute Respiratory Distress Syndrome (ARDS) happens to 67% of all severe COVID-19 cases with mortality rate of 50-94%. Main characteristics of ARDS are poor oxygenation and non-compliant lungs. The lungs are like inflatable balloons that are actively inflated by the positive pressure within and/or the negative pressure created in the pleural cavity. Compliance is the ability of the lungs to distend at a certain level of transpulmonary pressure. Surfactant lowers the surface tension of the alveoli so that end-expiratory pulmonary collapse could be prevented and pulmonary compliance could be maintained. Surfactants also play an important role in the host's defense against infection. Surfactants comprise of phospholipids, cholesterol and surfactant proteins (SP) SP-A, -B, -C and –D. Previous study showed that concentration of SP-D was significantly higher in ARDS patients. Higher serum SP-D concentrations were associated with a greater risk of death, fewer ventilator-free days, and fewer organ failure-free days. This review highlights the role of surfactant especially SP-D in pulmonary compliance and ARDS in COVID-19.