Tension pneumothorax is characterized by dyspnea, hypotension, decreased venous return to the heart, midline shift of the trachea and mediastinal structures away from the side of tension, a hyperresonant percussion note, and tachycardia. Untreated tension pneumothorax can quickly deteriorate to cardiac arrest and pulseless electrical activity. Although sometimes clinically indistinguishable from tamponade, a tension pneumothorax is the more common occurrence and a clinical diagnosis is usually possible. Treatment is needle decompression followed by tube thoracostomy.