A 63-year-old man who has been previously healthy is admitted to the hospital with a 2-day history of cough, rigors, fever, and right-sided pleuritic chest pain. Chest x-ray shows consolidation of the right lower lobe (RLL) and a free-flowing right pleural effusion. Thoracentesis is performed, and the pleural fluid has the following characteristics:Cell count=1110/mm3Glucose=75 mg/dL (serum glucose=85 mg/dL)Protein=4.0 g/dL (serum protein=7.0 g/dL)LDH=400 U/Lserum LDH=200 U/L, normal=100-200 U/LpH=7.35What is the pathogenesis of the pleural effusion?
High-Yield Explanation
Clinical conditions associated with either an increase in hydrostatic pressure (such as congestive heart failure) or a decrease in oncotic pressure (such as nephrotic syndrome) are associated with transudative pleural effusions. This patient's pleural fluid is exudative by all three of the Light criteria: pleural fluid/serum protein ratio is greater than 0.5, pleural fluid LDH/serum LDH ratio is greater than 0.6, and pleural fluid LDH is greater than two-thirds the upper limits of the normal serum LDH. The most likely explanation for an exudative pleural effusion in the setting of an acute pneumonia is a parapneumonic effusion. Parapneumonic effusions occur in about 40% of patients with bacterial pneumonia. Parapneumonic effusions are exudative due to the fact that there is increased permeability of the visceral pleural membrane capillaries, and interstitial fluid moves across the visceral pleura into the pleural space. Parapneumonic effusions may be simple or complicated. Simple parapneumonic effusions are sterile and free flowing. If bacteria invade the pleural space, neutrophils move into the pleural space and anaerobic metabolism of glucose results in a low pleural fluid pH (<7.20) and glucose (<60). The characteristics of this patient's pleural fluid suggest that bacterial invasion of the pleural space has not occurred, and that this is a simple parapneumonic effusion. Hemorrhage into the pleural space occurs with trauma, cancer, and pulmonary embolism, but rarely with pneumonia.