Pleural fluid collections in critically ill patients

Abstract

The inner surface of the chest wall and the surface of the lungs are covered by the parietal and visceral pleura, respectively, with a 10– 24 mm separation normally between the two surfaces. This space is usually filled with a very small amount of fluid. However, large amounts (4–5 litres in an adult) of fluid can accumulate in the pleural space under pathological conditions. The parietal pleura has sensory innervation. Both pleural surfaces are mainly supplied by systemic arterial vessels. Lymphatic vessels from the parietal pleura drain to lymph nodes along the anterior and posterior chest wall; lymphatics from the visceral surface drain to the mediastinal lymph nodes. The pleural space typically contains a small amount of a colourless alkaline fluid (0.1–0.2 ml kg, pH 7.62), which has a low amount of protein (,1.5 g dl). Approximately 90% of accumulated fluid in the pleural space is drained by the venous circulation; the other 10% is absorbed by the lymphatics. A delicate balance between the oncotic and hydrostatic pressures of the pleural space regulates filtration and drainage of pleural fluid. Net absorption of pleural fluid is slightly greater than net filtration forces. In addition, lymphatic drainage from the parietal pleura can surpass the rate of fluid filtration in the pleural space. Chest wall and diaphragmatic movements also enhance absorption of pleural fluid by the vascular and lymphatic vessels. Excessive filtration of fluid can overwhelm these efficient absorptive mechanisms and lead to the formation of pleural effusion.