Occasionally, gas enters the pleural space during the inspiratory phase and is prevented from escaping during expiration, presumably because an airway or tissue flap acts as a one-way valve. This situation is referred to as tension pneumothorax. Tension pneumothorax occurs in only one to two percent of the cases of idiopathic spontaneous pneumothorax. However, it is a more common manifestation of the barotrauma that may occur as a complication of positive pressure mechanical ventilation. Tension pneumothorax usually presents as a medical emergency.
Patients are often dyspneic at rest and gasping for breath.
Cyanosis and hypotension are common. The diagnosis cannot be established by a plain chest radiograph, although a marked contralateral shift of the mediastinum and depression or inversion of the ipsilateral hemidiaphragm are suggestive. Usually, the patient's acute distress makes ascertainment of whether the pneumothorax is under tension of academic interest. Immediate action must be taken to remove gas from the pleural space. In patients with tension pneumothorax, the gas that is under pressure will rush out of the chest through the open catheter.