Tension pneumothorax: The accumulation of air under pressure in the pleural space. An imminent danger is that the lung will collapse under the pressure. In tension pneumothorax, what happens is that air enters the pleural cavity and is trapped there during expiration so the air pressure within the thorax mounts higher than atmospheric pressure, compresses the lung, may displace the mediastinum and its structures (including the lung) toward the opposite side, and cause cardiopulmonary impairment. Also called pressure pneumothorax,
