n CME : Your SA Journal of CPD - A practical approach to diagnosing pleural effusion in southern Africa




Excessive fluid accumulating within the pleural space is a common medical affliction and invariably indicates disease. Under physiological conditions fluid enters and exits the pleural space at the same rate. The rate of production is determined by Starling's forces, i.e. hydrostatic pressure, osmotic pressure and membrane permeability, whereas the exit rate is determined by clearance through lymphatic drainage. Alterations in any of these forces can lead to the formation of a pleural effusion. While the physiological amount of pleural fluid present has various positive effects on respiratory function, e.g. assisting in creating a negative intrathoracic pressure and reducing friction between pleural membranes, excessive fluid can significantly impair normal respiratory function. Once the volume of fluid in the pleural space reaches 200 - 250ml, it is detectable on a standard PA chest radiograph (CXR). The presence of a pleural effusion can be clinically detected only after the fluid volume reaches 300 - 350ml, but in the event of small volumes the sensitivity and specificity in its detection rates remain low. Because the differential diagnosis for intrapleural fluid accumulation encompasses a wide spectrum of conditions, a systematic approach to these effusions is particularly important in their investigation.


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