The diagnosis of tension pneumothorax has typically been taught as the presence of hemodynamic compromise with an expanding intrapleural space air mass. This may occur quickly or gradually, depending on the degree of lung injury and respiratory state of the patient. Experimentally, tension pneumothorax is a multifactorial event that manifests a state of central hypoxemia, compensatory mechanisms, and mechanical compression on intrathoracic structures. Studies using animal models suggest that over hypotension is a delayed finding that immediately precedes cardiorespiratory collapse. Recognition of early signs and symptoms associated with tension pneumothorax, e.g., progressive hypoxemia, tachycardia, and respiratory distress, can alert medical personnel to the need for rapid decompression before physiologic decompensation.