A 77-year-old man comes to the emergency department due to cough, shortness of breath, sharp chest pains, night sweats, and fatigue for 4 days. Temperature is 38.3 C (101 F), blood pressure is 110/62 mm Hg, pulse is 106/min, and respirations are 24/min. The patient appears ill. Neck veins are nondistended. There is decreased chest expansion on the right side. Auscultation reveals decreased breath sounds over the right lung base. Chest imaging shows a right lower lobe infiltrate and a small, free-flowing pleural effusion on the right side. Thoracentesis is performed. Laboratory results are as follows:
| Pleural fluid analysis | |
| Leukocytes | 620/mm3 |
| pH | normal |
| Glucose | normal |
| Pleural fluid/plasma lactate dehydrogenase ratio | 0.7 |
Which of the following processes best explains this patient's pleural fluid findings?
Parapneumonic effusions | ||
Uncomplicated | Complicated* | |
Etiology | Sterile exudate in | Bacterial invasion of |
Radiologic | Small to moderate & | Moderate to large, |
Pleural fluid |
|
|
Treatment | Antibiotics | Antibiotics + drainage |
*Empyema represents advanced progression of a complicated effusion. | ||
This patient's presentation with fever and several days of cough, shortness of breath, and sharp chest pain is consistent with community-acquired pneumonia (CAP). The right lower lobe infiltrate on chest x-ray supports the diagnosis of CAP and reveals an associated parapneumonic effusion, which develops in approximately 40% of bacterial pneumonia cases.
Parapneumonic effusions result from an inflammatory increase in vascular membrane permeability and therefore are exudative by Light criteria, which is confirmed by this patient's pleural fluid/serum lactate dehydrogenase ratio >0.6. Most parapneumonic effusions are uncomplicated, resulting from movement of sterile exudate into the pleural space and exhibiting normal pH and glucose concentration (near that of serum) as well as relatively low leukocyte count (eg, <50,000/mm3) (as in this patient). Complicated parapneumonic effusions involve inflammatory disruption of the visceral pleura membrane with bacterial seeding of the pleural space. These effusions are recognized by low pH and glucose levels (due to bacteria metabolizing glucose and producing acidic wastes) and high leukocyte count.
(Choices A and D) Changes in hydrostatic or oncotic pressure drive the development of transudative (not exudative) pleural effusions. Elevated pulmonary capillary hydrostatic pressure is the cause of pleural effusion in decompensated heart failure, and reduced pleural capillary oncotic pressure contributes to pleural effusion in conditions that cause hypoalbuminemia (eg, malnutrition, nephrotic syndrome). The pleural fluid/serum lactate dehydrogenase ratio is <0.6 with transudative effusions.
(Choice C) Fluid drains from the pleural cavity via lymphatics on the parietal surface; therefore, lymphatic obstruction (eg, due to malignant spread within the pleural space) can cause a pleural effusion. These effusions are usually exudative by Light criteria due to malignancy-associated inflammation. However, this patient's clinical presentation is more consistent with CAP.
Educational objective:
Parapneumonic effusions are exudative by Light criteria and are classified into uncomplicated and complicated. Uncomplicated parapneumonic effusions result from movement of sterile exudate into the pleural space, whereas complicated parapneumonic effusions involve inflammatory disruption of the visceral pleural membrane with bacterial translocation into the pleural space.