PATHOPHYSIOLOGY OF PLEURAL FLUID ACCUMULATION
The pleural space is the potential space between the visceral and parietal pleura. Normally, it contains 7 to 16 mL of hypotonic ﬂuid, which acts to lubricate the membranes to allow near-frictionless movement of the two pleural surfaces against each other during breathing. A large number of disease processes may result in abnormal accumulation of ﬂuid in the pleural space.
Fluid and plasma protein movement across biologic membranes is governed by the revised Starling law describing water ﬂux (Jv) between two compartments: where Kf is the ﬁltration coefﬁcient, PH is hydraulic pressure, π is colloidosomotic pressure, and is the solute reﬂection coefﬁcient of the membrane.
Based mainly on animal studies, pleural ﬂuid is ﬁltered at the parietal level from systemic microvessels into the pleural space. Some contribution from visceral parietal ﬁltration may also be present. Pleural ﬂuid drains primarily via the parietal pleural lymphatics. Fluid drainage can increase markedly to maintain constant pleural ﬂuid volume, approaching approximately 700 mL/d in humans. However, when formation exceeds drainage, pleural ﬂuid accumulation occurs. Plain chest radiography can identify as little as 50 mL of accumulated pleural ﬂuid when examining a lateral view, revealed by blunting of the posterior costophrenic angle. Usually at least 200 mL of ﬂuid is required to be detected by blunting of the lateral costophrenic angles in the posteroanterior view. Chest ultrasonography can detect as little as 5 mL of ﬂuid, and computed tomography is even more sensitive.
Typically, one of four major mechanisms results in excessive pleural ﬂuid formation: elevated hydrostatic pressure (PH), decreased oncotic pressure (π), decreased lymphatic drainage caused by mechanical obstruction, or increased extravasation (Kf) through inﬂamed pleural membranes. The ﬁrst two mechanisms typically result in ﬂuid classiﬁed as transudative, and the latter two mechanisms usually result in exudative ﬂuid. Transudates and exudates are deﬁned by the ratio of pleural ﬂuid-to-serum levels of protein and lactate dehydrogenase (LDH) with a protein ratio of above 0.50 or LDH ratio (compared with the upper limit of normal serum LDH) of above 0.67 deﬁning an exudate.
Whereas transudates imply normal pleura and can usually be diagnosed based on other clinical characteristics, exudates often require further testing. These tests on the ﬂuid include appearance, character, odor, color, cell count, microbiology, cytology, and bio- chemical tests (e.g., pH, glucose, amylase, triglycerides). In some cases, pleural biopsy may be required to diagnose the cause of an exudative pleural effusion.