Background
Pneumothorax is defined as the presence of air or gas in the pleural cavity. Primary spontaneous pneumothoraces (PSPs) occur in people without underlying lung disease or trauma to the thorax. Many patients whose condition is labeled as primary spontaneous pneumothorax have an unrecognized lung disease. Secondary spontaneous pneumothoraces occur in people with underlying parenchymal lung disease. Traumatic pneumothoraces result from injury, often secondary to medical intervention (ie, iatrogenic pneumothorax). When air is trapped in the pleural cavity, resulting in positive pressure, a tension pneumothorax develops.
Pathophysiology
Primary spontaneous pneumothoraces result from apical pleural blebs lying under the visceral pleura. Primary spontaneous pneumothoraces are typically observed in tall young people without parenchymal lung disease because of increased shear forces in the apex. While patients do not have overt parenchymal disease, they commonly are smokers. More than 90% of patients with primary spontaneous pneumothorax are smokers. The relative risk of primary spontaneous pneumothorax increases as the number of cigarettes smoked per day increases. This incremental risk with increasing number of cigarettes smoked per day is much more pronounced in female smokers. Eighty percent of patients have emphysemalike changes on CT scan, particularly in upper long zones.
The body of evidence that genetic factors may be important in the pathogenesis of many cases of primary spontaneous pneumothorax is growing. Familial clustering of this condition has been reported. Several genetic disorders have been linked to primary spontaneous pneumothorax. Marfan syndrome, homocystinuria, and Birt-Hogg-Dube syndrome are among such disorders. Birt-Hogg-Dube syndrome is an autosomal dominant disorder that is characterized by benign skin tumors (hair follicle hamartomas), renal and colon cancer, and spontaneous pneumothorax. The spontaneous pneumothorax occurs in about 22% of patients with this syndrome. The gene responsible for this syndrome has been identified and is located on chromosome 17. Genetic testing is now available for Birt-Hogg-Dube syndrome.
Secondary spontaneous pneumothoraces (SSP) occur in the presence of lung disease, primarily in the presence of chronic obstructive pulmonary disease (COPD). Other diseases that may be present when secondary spontaneous pneumothoraces occur include tuberculosis, sarcoidosis, cystic fibrosis, malignancy, and idiopathic pulmonary fibrosis.
Pneumocystis jiroveci pneumonia (previously known as Pneumocystis carinii pneumonia [PCP]) was a common cause of secondary spontaneous pneumothorax in patients with AIDS during the last decade. With the advent of highly active antiretroviral therapy (HAART) and widespread use of trimethoprim-sulfamethoxazole prophylaxis, the incidence of PCP and associated SSP has significantly declined. This disease is now primarily seen in patients who are noncompliant with HIV therapy or trimethoprim-sulfamethoxazole prophylaxis or those taking inhaled pentamidine for PCP prophylaxis. PCP in other immunocompromised patients is seen only when trimethoprim-sulfamethoxazole prophylaxis is withdrawn prematurely. For practical purposes, if the immunocompromised patient has been taking trimethoprim-sulfamethoxazole prophylaxis reliably, PCP is reasonably excluded from the differential diagnoses.
Traumatic pneumothoraces can result from both penetrating and nonpenetrating lung injuries. Complications include hemopneumothorax and bronchopleural fistula. Traumatic pneumothoraces can create a 1-way valve in the pleural space, only letting in air without escape, and can lead to a tension pneumothorax.
Iatrogenic pneumothorax is a complication of medical or surgical procedures. It most commonly results from transthoracic needle aspiration. Other procedures commonly causing iatrogenic pneumothorax are therapeutic thoracentesis, pleural biopsy, central venous catheter insertion, transbronchial biopsy, positive pressure mechanical ventilation, and inadvertent intubation of the right mainstem bronchus. Therapeutic thoracentesis is complicated by pneumothorax 30% of the time when performed by inexperienced operators in contrast to only 4% of the time when performed by experienced clinicians. The routine use of ultrasonography during diagnostic thoracentesis is associated with lower rates of pneumothorax (4.9% vs 10.3%) and need for tube thoracostomy (0.7% vs 4.1%). Similarly, in patients who are mechanically ventilated, thoracentesis guided by bedside ultrasonography without radiology support results in a relatively lower rate of pneumothorax.
Tension pneumothorax typically occurs in the intensive care setting in patients who are ventilated. With air trapping in the pleural space, positive pressure rises. This pressure compresses the mediastinum, decreasing venous return to the heart and reducing cardiac output. In addition, owing to ipsilateral lung collapse and contralateral lung compression, gas exchange is compromised, leading to hypoxemia.
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