9.2 Pleural Effusion & Pneumothorax

Pleural pathology represents one of the most heavily assessed clinical domains in MRCP(UK) Part 1. Candidates are expected to seamlessly integrate pleural fluid biochemistry using Light's criteria, identify specific indicators for urgent chest tube drainage, and correctly apply the British Thoracic Society (BTS) management guidelines for primary versus secondary pneumothoraces.

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1. Pleural Effusion: Transudates vs Exudates

The pleural space normally contains 0.1–0.2 mL/kg (~10–15 mL) of sterile fluid that lubricates the parietal and visceral pleura. Fluid is continuously filtered across parietal pleural microvessels and absorbed by parietal stomata into the lymphatic drainage network. An effusion develops when fluid filtration overwhelms lymphatic clearance.

General Classification by Protein Concentration

• Gross Differentiation: Pleural fluid total protein <30 g/L indicates a transudate; pleural protein >30 g/L indicates an exudate.
• Indeterminate Range (25–35 g/L): In effusions where total protein falls between 25 and 35 g/L, gross protein cut-offs are unreliable. Light's criteria must be applied to definitively differentiate transudative from exudative pathology.

Etiological Distinctions

• Transudative Causes (Systemic Mechanical Imbalance):
  • Congestive cardiac failure (most common transudate; elevated pulmonary capillary hydrostatic pressure)
  • Hepatic cirrhosis with hepatic hydrothorax (peritoneal fluid translocating across diaphragmatic defects)
  • Nephrotic syndrome (severe hypoalbuminemia reducing intravascular oncotic pressure)
  • Peritoneal dialysis
  • Hypoalbuminemia / severe protein-losing enteropathy
  • Constrictive pericarditis and superior vena cava obstruction
• Exudative Causes (Local Pleural or Microvascular Pathology):
  • Parapneumonic effusions and bacterial empyema
  • Malignancy (bronchogenic carcinoma, metastatic adenocarcinoma of breast/ovary/GI tract, malignant pleural mesothelioma)
  • Pulmonary infarction secondary to pulmonary embolism
  • Autoimmune connective tissue diseases: Rheumatoid arthritis, systemic lupus erythematosus (SLE)
  • Tuberculosis (TB pleurisy)
  • Acute or chronic pancreatitis
  • Post-cardiac injury syndrome (Dressler's syndrome)
  • Benign asbestos pleural effusion
  • Drug-induced pleuritis (amiodarone, nitrofurantoin, methotrexate, dasatinib)

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2. Light's Criteria & Pleural Fluid Biochemistry

Light's criteria exhibit a sensitivity approaching 98% for identifying exudates, though specificity is approximately 83%.

Light's Criteria Table

An effusion is formally classified as an exudate if at least one of the three criteria is fulfilled. If none of the criteria are met, the effusion is a transudate.

The Pseudo-Exudate Trap: In patients with congestive cardiac failure who are actively receiving loop diuretic therapy, water is cleared from the pleural space more rapidly than macromolecules. This concentrates pleural protein and LDH, causing Light's criteria to misclassify a transudate as an exudate. In this clinical scenario, calculate the serum-to-pleural albumin gradient: $\text{Albumin Gradient} = \text{Serum Albumin} - \text{Pleural Fluid Albumin}$ If the gradient is >12 g/L, the effusion is confirmed to be a transudate, and costly investigations for malignancy or infection can be safely averted.

High-Yield Pleural Fluid Biochemical Parameters

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3. Pneumothorax: Pathophysiology & Classification

A pneumothorax occurs when gas accumulates within the pleural cavity, uncoupling the visceral and parietal pleura and collapsing the underlying lung parenchyma.

• Primary Spontaneous Pneumothorax (PSP): Occurs in individuals with no clinically apparent underlying parenchymal lung disease. Classically affects tall, slender young males (aged 15–35 years) due to increased negative intrapleural pressures at the lung apices causing formation and rupture of subpleural apical blebs or bullae. Cigarette smoking increases the risk up to 20-fold.
• Secondary Spontaneous Pneumothorax (SSP): Occurs as a complication of pre-existing lung pathology. Most frequently seen in patients >50 years with chronic obstructive pulmonary disease (COPD; rupture of emphysematous bullae), cystic fibrosis, idiopathic pulmonary fibrosis, severe asthma, Pneumocystis jirovecii pneumonia (in HIV/AIDS), thoracic endometriosis (catamenial pneumothorax), or necrotizing lung infections. Patients with SSP have markedly compromised physiological reserve and tolerate even small pneumothoraces poorly.

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4. British Thoracic Society (BTS) Management Algorithms

On an erect posterior-anterior (PA) chest radiograph, the size of a pneumothorax is measured as the distance between the lung margin (visceral pleura) and the inner chest wall at the level of the hilum:

• Small Pneumothorax: Visible rim of air <2 cm at the hilum.
• Large Pneumothorax: Visible rim of air >=2 cm at the hilum (a 2 cm rim corresponds to approximately 50% volume collapse of the hemithorax).

BTS Spontaneous Pneumothorax Management Summary

Tension Pneumothorax: Emergency Decompression

Tension pneumothorax develops when a one-way tissue valve mechanism permits air entry into the pleural cavity during inspiration but prevents egress during expiration. Intrapleural pressure becomes positive, collapsing the ipsilateral lung, shifting mediastinal structures and trachea to the contralateral side, kinking the superior and inferior vena cava, and precipitating immediate hemodynamic arrest.

• Diagnosis: Strictly a clinical emergency diagnosis—never delay management to obtain a chest radiograph!
• Clinical Signs: Severe respiratory distress, cyanosis, tracheal deviation away from the affected side, hyper-resonance, absent breath sounds on the affected side, distended neck veins, and profound hypotension.
• Immediate Intervention: Emergency decompression via a large-bore cannula (14–16G) inserted into the 2nd intercostal space in the mid-clavicular line (or 4th/5th intercostal space anterior axillary line) on the affected side. A hiss of escaping pressurized air confirms decompression. This must be followed immediately by formal intercostal chest drain insertion into the safe triangle (5th intercostal space mid-axillary line).

Aviation & Scuba Diving Advice (High Exam Yield)

• Commercial Air Travel: Patients must not fly until at least 1 to 2 weeks following complete radiographic re-expansion of the pneumothorax. Cabin pressure drops to equivalent altitudes of 8,000 feet, causing trapped air pockets to expand by ~30% (Boyle's law: $P_1V_1 = P_2V_2$), risking tension pneumothorax.
• Scuba Diving: Diving is permanently contraindicated following a spontaneous pneumothorax due to the catastrophic risk of tension pneumothorax and arterial gas embolism during ascent. The only exception is if the patient has undergone definitive bilateral surgical pleurectomy and demonstrates completely normal baseline post-operative thoracic CT imaging and full pulmonary function testing.