Which of the following is not a sign of tension pneumothorax?

In the context of tension pneumothorax, increased lung capacity is not a sign associated with this condition. Tension pneumothorax occurs when air enters the pleural space and cannot escape, leading to increased pressure within the thoracic cavity. This accumulation of air compresses the lung on the affected side, causing it to collapse and ultimately leading to decreased lung capacity, not an increase.

The other signs associated with tension pneumothorax, such as rapid heartbeat, decreased breath sounds on the injured side, and jugular vein distention, stem from the body’s physiological response to the worsening condition.

A rapid heartbeat often indicates the body is attempting to compensate for reduced oxygen exchange due to lung collapse. Decreased breath sounds on the affected side occur as the lung is not distending properly due to the increased pressure in the chest. Jugular vein distention can show increased venous pressure, indicating that the heart's ability to pump blood is compromised, which is often a result of mediastinal shift due to pressure in the thorax. Thus, increased lung capacity does not align with the signs of tension pneumothorax, making it the correct choice.