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34

Pleural Effusion
Ashley Allen, DVM, DACVECC and Gareth Buckley, MA, VetMB, MRCVS, DACVECC

College of Veterinary Medicine, University of Florida, Gainesville, FL, USA

Pleural effusion is the pathologic collection of fluid within transport fluid back through the lymphatic system.
the pleural space resulting from one of three primary Compared to other species, dogs and cats have a thin
mechanisms: disruption of pleural pressure homeostasis, pleural surface, approximately 15 μm thick. The medi-
diminished lymphatic drainage, or increased mesothelial astinum divides the pleural space into a right and left
and capillary endothelial permeability. Given the dynamic pleural cavity, although in dogs and cats the mediastinum
state of the pleural space, overlap can occur between the may not be complete, allowing for bilateral disease from
mechanisms of effusion, depending on the underlying a unilateral origin. The highly metabolically and immu-
disease. Dogs and cats with pleural effusion present with nologically active multipotent mesothelial cells line the
various clinical signs including lethargy, hyporexia, entire pleural cavity and play an active role in pleural
tachypnea, dyspnea, and a paradoxical breathing pattern, fluid production and reabsorption. Sialomucins are nega-
regardless of precipitating cause. History and physical tively charged glycoconjugates which coat the free meso-
examination may help to formulate a differential diagno- thelial surface, functioning to repel opposing membranes
sis for pleural effusion, but ultimately collecting a sample as well as organisms, abnormal cells, and particles.
of the effusion for cytologic analysis and additional tests In healthy animals, a small volume of fluid is present in
provides the most important information to obtain an the pleural space which facilitates smooth gliding of the
accurate diagnosis. Treatment options and prognosis are lung over the thoracic wall and transmission of forces
dependent on the underlying cause of pleural effusion. during normal respirations. Normal pleural fluid is low in
protein (<1.5 g/dL) and cellular content (1500–2500
cells/μL), consisting of mesothelial cells (9–30%), mono-
Anatomy and Physiology cytes (61–77%), lymphocytes (7–11%), and neutrophils
(<2%). Healthy dogs and cats produce approximately
The pleura lines the thoracic cavity and is a thin, serous 0.01–0.02 mL/kg/h of pleural fluid via microvascular fil-
membrane with five layers: an outer fibroelastic layer, an tration from pleural capillaries. Fluid crosses the pleural
extensively vascularized subpleural loose connective tis- capillary endothelium followed by the interstitium.
sue layer, an elastic superficial tissue layer, a loose con- Pleural capillary endothelium imparts most of the resist-
nective submesothelial tissue layer, and a mesothelial ance to flow; however, the resistive properties of the
layer. The visceral pleura line the lungs, whereas the pari- endothelium and interstitium are additive, which
etal pleura line the thoracic wall with the potential space becomes significant during some disease processes that
between the two known as the pleural space. The visceral interfere with the resistive properties of these layers. The
pleura functions to provide mechanical lung support, production and reabsorption of pleural fluid are multifac-
limit lung expansion, and contribute to elastic recoil and torial, dependent on Starling forces, solute flux, hydraulic
deflation of the lung. The parietal pleura is subdivided conductivity (the resistive properties of the two pleural
into the costal, mediastinal, and diaphragmatic parietal surfaces), total surface area, the solute reflection coeffi-
pleura and contains numerous lymphatic vessels that cient (ability of the surface to restrict larger molecules),
form stomas between mesothelial cells for lymphatic membrane diffusive permeability, pleural lymphatic
drainage. Lacunae are dilated lymphatic vessels in the drainage via stomata in the parietal pleura, and cellular
submesothelial parietal pleura where stomas form and mechanisms of transport by mesothelial cells (Table 34.1).

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical