MVS Pathophysiology: Pneumothorax

community acquired pneumonia | pulmonary embolism | pneumothorax | asthma

A. Definition

Pneumothorax is an accumulation of gas, commonly air, in the pleural space. This creates positive intrapleural pressure and prevents proper lung inflation. If this accumulation is of a sufficient size, the lung parenchyma may collapse.

B. Etiology

Spontaneous (Simple) Pneumothorax
Spontaneous rupture of subpleural blebs at the lung apex can release gas from the lungs into the pleural space. This most commonly occurs in healthy, tall males between the ages of 20 to 40.

Secondary (Complicated) Pneumothorax
These occur as a result of trauma or pre-existing pulmonary disease (eg TB, malignancy, emphysema, histiocytosis X, insterstitial fibrosis). Trauma can allow gas into the pleural space via penetration of the visceral pleura, chest wall, diaphragm, mediastinum or esophagus. Iatrogenic pneumothorax as a result of CVP lines, thoracentesis or mechanical ventilation is not uncommon. However, widespread emphysema is the most common cause of secondary pneumothorax. Other causes of pneumothorax such as asthma, certain interstitial lung diseases, lung carcinoma or abscess are less common. An uncommon cause of pneumothorax is from the accumulation of gas produced by microorganisms in an empyema.

C. Clinical Features

Presenting symptoms in a patient with pneumothorax include acute onset dyspnea, pleuritic chest pain, and cough.

Common physical findings include diminished breath sounds, reduced lung expansion, decreased tactile and vocal fremitus and a hyperresonant percussion note on the side of the pneumothorax. The trachea often deviates away from the side of the pneumothorax.

However, a small pneumothorax may be asymptomatic.

D. Diagnosis

A chest x-ray is usually sufficient to diagnose pneumothorax . The affected lung in a large pneumothorax will appear more dense and take up less volume than normal. In a small pneumothorax, separation of the visceral and parietal pleura may be seen at the lung apex as a fine cresecentic line. Small amounts of fluid may sometimes be present.

E. Treatment

Small pneumothoraces, affecting less than 20% of the hemithorax, will usually resolve spontaneously in 1 to 2 weeks, and observation may be sufficient for these cases. Larger or secondary pneumothoraces may require air aspiration orplacement of an intercostal chest tube to maintain negative intrapleural pressure. A chest tube should be left 2 to 4 days until the leak seals. Chemical pleurodesis or surgery may be appropriate for patients with recurrent episodes (3 or more pneumothoraces on one side). Surgery involves open thoracotomy and abrasion of pleural surfaces which seals the parietal and visceral pleurae together.

F. Complications

Pneumothorax is most commonly a benign condition, but occasionally severe or life-threatening complications may ensue.

Bilateral Simultaneous Pneumothorax

This is a rare complication but can cause immediate death.

Hemopneumothorax

If a pneumothorax is accompanied by bleeding into the pleural space.

Tension Pneumothorax

A tension pneumothroax is an life-threatening emergency that must be treated immediately. This condition is produced by an opening in the pleura which allows gas to enter but not to escape via a ball-valve mechanism at the site of the leak, causing a gradual increase in intrapleural pressure. The building pressure will slowly compress the ipsilateral lung and eventually the opposite lung through a mediastinal shift. Increasing intrathoracic pressure will reduce venous return, cardiac output and blood pressure. Death can result from decreased respiratory and circulatory function.

Patients undergoing positive pressure mechanical ventilation have a significantly increased risk of tension pneumothorax. Patients with this condition will likely have cyanosis, distended neck veins, decreased blood pressure and rapid, shallow breathing. A hyperresonant percussion note along with absence of breath sounds on the side of the pneumothorax will also be present. The trachea or heart may show signs of a shift away from the affected lung. Treatment involves immediate depressurization of the intrapleural space followed by insertion of a chest tube.

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