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.
