Pleural Effusions – Diagnosis and Treatment

A small amount of transudative fluid is normally contained within the pleural space. The purpose of this fluid is to allow structures to slide freely during respiration. The production and absorption of this fluid is a continuous process controlled by Starling’s forces. Pleural effusion develops when disease processes alter normal fluid dynamics, vascular permeability, lymphatic drainage, or pleural surface area. There are seven general types of pleural and mediastinal effusions based on cytological characteristics: transudate, modified transudate, exudate, septic exudate, hemorrhagic effusion, chylous effusion, and neoplastic effusions. A retrospective study of pleural and mediastinal effusions in 81 dogs found the most common disease process to be pyothorax, although 28 distinct disease processes were identified (Mellanby et al., 2002).

Animals with pleural disease typically present with rapid, shallow respirations indicative of a restrictive respiratory pattern due to fluid, tissue, or air occupying the pleural space. Other clinical signs may include pyrexia, lethargy, anorexia, weight loss, and coughing. The duration of clinical signs varies widely as animals may present in acute distress or with more insidious signs of chronicity. Physical examination and thoracic auscultation will typically reveal muffled cardiac sounds, but increased respiratory noise. Thoracic percussion may be valuable in identifying the cause of respiratory distress. Percussion over normal lung fields results in a low-frequency vibration, while percussion over the heart, consolidated tissue, or fluid-filled pleural space causes a dulling of the vibration or hyporesonance. Systematic percussion of the thoracic cavity may identify a fluid line giving the examiner a subjective assessment of the amount of fluid within the pleural space.

Diagnostic evaluation of pleural and mediastinal disease starts with thoracic radiographs. A full radiographic examination is performed if conditions allow, which includes right and left lateral views, and a ventrodorsal or dorsoventral view. If the animal becomes stressed during the procedure or respiratory signs worsen, priority is given to the lateral views as this positioning is usually tolerated. Otherwise, if the animals can tolerate a sternal position, lateral images can be obtained with horizontal beam in addition to the dorsoventral view. If an animal is severely compromised, thoracic radiographs should be delayed in favor of therapeutic thoracocentesis.