Accumulations of excessive fluid in the pleural, peritoneal, or pericardial space represent edema of these body cavities.
As such, the dynamics of fluid accumulation are governed by Starling's law. In general, pleural, peritoneal, and pericardial
effusions can result from either inflammatory or non-inflammatory causes. Non-inflammatory edema results from arterial pressure
(a rare cause in veterinary medicine), venous stasis, hypoalbuminemia (reduces plasma oncotic pressure), or lymphatic obstruction.
Inflammation causes fluid accumulation through vascular damage and leakage.
Evaluation of abnormal fluid accumulation is based upon integrated assessment of both physical and cytologic characteristics.
Collection techniques and sample management are not detailed here.
Physical assessment of effusions includes estimation of total solids (total protein) by refractometry and determination of
total nucleated cell count with either an automated particle or manual methods. On the basis of physical characteristics,
effusions are classified as transudates, modifies transudates, or exudates. As a general rule (there are exceptions which
will be discussed later), transudates are non-inflammatory in origin while exudates are the result of inflammation. Thus,
physical characteristics of effusions allow classification according to pathophysiologic mechanism of fluid accumulation but
reveal little about specific disease diagnosis.
Cytologic evaluation of effusions augments physical evaluation by allowing the diagnostician to identify specific etiologic
agents, to follow the progression of a disease process, and to monitor response to therapy. The decision of whether or not
to culture effusion fluid is based upon cytologic findings. Through cytologic examination, inflammatory reactions are classified
as to type, hyperplastic responses of the mesothelial lining can be identified, and neoplasia is diagnosed.
Physical evaluation of effusions
Transudates are defined as excessive accumulations of fluid having normal physical characteristics. Transudates therefore
have low total proteins (less than 3 g/dl) and low nucleated cell counts (less than 500 nucleated cell/μl). The single exception
to these criteria is peritoneal fluid from the horse where normal values of up to 2000 nucleated cells have been reported.
Transudative effusions result most commonly from venous stasis and less frequently from hypoalbuminemia and lymphatic obstruction.
For example, they are the typical effusion seen with congestive heart failure (venous stasis), liver failure (hypoalbuminemia,
venous stasis, and lymphatic obstruction), the nephrotic syndrome (hypoalbuminemia), and in some cases of neoplasia (venous
stasis and lymphatic obstruction). It is emphasized that pleural and peritoneal effusions due solely to hypoalbuminemia will
only occur when serum albumin levels fall below 1 g/dl. At values greater than this some other additional contributory mechanism
must also be considered.
Transudative fluids are quite clear at the time of collection. Despite the paucity of cells, however, cytologic examination
is certainly warranted. While most transudates are nonspecific cytologically, those which are caused by neoplasms may contain
malignant cells which allow specific diagnosis. To maximize the likelihood of finding such abnormal cells, sediment smears
should always be used when transudates are examined.
The accumulation of transudative fluid in one of the body cavities causes increased pressure which is irritating to the mesothelial
cells lining the space. They respond by proliferating and sloughing into the effusion.
With time the sloughed mesothelial cells die and as they do they release chemoattractants which draw small numbers of phagocytes
into the effusion to remove cellular debris. The result is a mild increase in both total protein (3.0-5.0 g/dl) and nucleated
cell count (less than 5000/μl). When this occurs, the fluid is known as a modified transudate. Thus, modified transudates
are nothing more than transudates which have been present long enough to illicit a mild inflammatory reaction. Eventually,
physical characteristics of the fluid may change enough so that the fluid is classified as an exudate.
In cases of extended duration, modified transudates can have a cloudy to almost milky gross appearance. Such fluids strongly
resemble chyle and, in fact, have been called "pseudochylous effusions" (a term which in our opinion should be dropped). The
gross appearance of these fluids is the result of a high lipid content but is in no way related to a true chylous effusion.
The phagocytes attracted to remove cellular debris from transudates are rich in enzymes which digest protein but are virtually
devoid of enzymes which will break down complex lipids. Consequently, while most of the constituents of dying cells are removed
by phagocytosis, lipid content of the cells simply accumulates in the effusion fluid. Pseudochylous effusions formed in this
manner are easily distinguished cytologically from true chylous effusions.
Exudates are defined as fluid accumulations which are abnormally high in total solids and/or nucleated cell count. Total proteins
range between 3.0 and 7.0 g/dl and total cell counts may be as high as 100,000/μl. The vast majority of exudates are caused
by inflammation; however, the common denominator of exudate formation is vascular (including lymphatic) damage. Consequently,
both hemothorax (and hemoperitoneum) and chylothorax (chyloperitoneum) are classified as exudates on the basis of physical
characteristics and pathogenesis.