Page 370 - Clinical Small Animal Internal Medicine

338 Section 4 Respiratory Disease

intestinal neoplasia can also cause significant hypopro- infrequently reported in veterinary medicine; however, if
VetBooks.ir teinemia. In general, albumin concentrations less than present, cancer is the most common underlying etiology,
followed by thoracic wall trauma, pneumothorax, con-
1.0 g/dL cause oncotic pressure to be low enough for a
transudate to develop; however, if increased hydrostatic
tions. Pyogranulomatous inflammation may indicate
pressure is also present, a pure transudate can be seen gestive heart failure, and allergic hypersensitivity condi-
with a higher albumin concentration. fungal or foreign body reaction. Cytologic slides should
Modified transudates are variable in protein content be closely evaluated for the presence of intracellular bac-
(2.5–7.5 g/dL) and cell count (1000–7000 cells/μL). teria and aerobic and anaerobic cultures with antimicro-
Grossly, they can be tan and slightly turbid, to pink, or bial susceptibilities should always be submitted when
opaque and white in appearance depending on the degenerate neutrophils are present. Nondegenerate
underlying etiology. Most of the nucleated cells are mon- neutrophils typically indicate a nonseptic process and
onuclear (macrophages, lymphocytes, or a combination may be secondary to neoplasia, chronic diaphragmatic
of both) with a lower number of mesothelial cells and hernia, lymphatic obstruction or leakage, and infectious
nondegenerate neutrophils. Modified transudates result processes. If the patient has a history of recent cholecys-
from increased vascular or lymphatic permeability with tectomy or trauma and yellow, nonseptic exudate is
or without increased hydrostatic pressure. They are collected, bilothorax should be a differential and further
modified by the addition of proteins, cells, or chyle, and diagnostics performed.
classifying a fluid as a modified transudate implies the
disease process causing fluid to accumulate is nonin- Specific Pleural Fluid Tests
flammatory. Most modified transudates are a conse- When chylothorax is suspected, triglyceride concentra-
quence of obstructed venous or lymphatic drainage tion should be analyzed in the pleural effusion and
secondary to numerous underlying diseases, with right‐ patient’s serum. Diagnosis of chylothorax is confirmed if
sided heart failure being most common. Some modified the triglyceride concentration in the pleural fluid is
transudates may become exudative effusions, especially higher than the triglyceride concentration in the patient’s
with chronicity because fluid in the pleural space is serum. Decreased glucose (<60 mg/dL) in pleural fluid is
inflammatory. Chylous effusion has been classified as due to decreased transport of glucose to the pleural fluid
both a modified transudate and an exudate for this rea- or increased utilization of glucose by mononuclear cells,
son. Initially, chylous effusions contain a predominant neutrophils, bacteria, or malignant cells. If the effusion
population of mononuclear cells (lymphocytes) and fluid appears hemorrhagic, the packed cell volume (PCV) of
accumulates due to lymphatic obstruction from nonin- the pleural effusion should be compared to the PCV of
flamed vessels. In cats, chylous effusion can result from the patient, with hemothorax present if the pleural fluid
congestive heart failure or heartworm disease. Additional PCV is at least 25% of the patient’s PCV. Bilirubin should
causes in veterinary species include lung atelectasis, be quantified in the pleural fluid and blood if bilothorax
neoplasia, diaphragmatic or peritoneal‐pericardial her- is suspected, such as those patients with recent biliary
nias, lung lobe torsion, lymphangiectasia, cranial vena surgery or known/suspected trauma. If bilirubin in the
cava thrombosis, trauma, restrictive pericarditis, peri- pleural fluid is higher than in the patient’s blood, bilotho-
cardial disease, and mediastinal disease. Often, an under- rax is diagnosed.
lying disease for chylous effusion is not identified and is
therefore idiopathic. Pleural Pressure Monitoring
Exudates have a high protein concentration (>3.0 g/dL) Normal pleural pressure in a dog or cat is negative with
and nucleated cell count (>3000 cells/μL). Exudates vary respect to the atmosphere and functions to facilitate
in gross appearance from white to amber to pink depend- lung inflation and reduce the work of breathing. Negative
ing on etiology and are typically turbid. The predominant intrapleural pressure is generated by the lymphatic sys-
cell type is the neutrophil, which may be nondegenerate tem as it drains the normal, physiologic small volume of
(aseptic) or degenerate (septic), with macrophages, lym- pleural fluid present. As pleural effusions accumulate,
phocytes, eosinophils, and mesothelial cells in lesser lung parenchyma collapses and intrathoracic pressure
quantities. Exudative effusions usually result from an increases. Dyspnea and tachypnea result due to col-
inflammatory process in the pleural cavity causing lapsed parenchymal lung tissue and pleural pressure
release of cytokines and other vasoactive mediators, becomes less negative, or positive to the atmosphere.
leading to increased capillary permeability (filtration When the fluid is removed by thoracocentesis, the
coefficient). The initial inflammatory process can be due expectation is that intrapleural pressure returns to its
to endogenous (chyle, neoplastic cells) or exogenous normal negative pressure and the lungs can reexpand but
(bacteria, fungi) mediators. Eosinophilic effusions this is not always the case, especially with chronic
determined cytologically with >10% eosinophils are effusions. The pleural surface can become fibrinous,