Which of the following is a reversible cause included among Hs and Ts?

• A. Tension pneumothorax
• B. Hypermagnesemia
• C. Thyrotoxicosis
• D. Thrombosis

Answer: (A)

In ACLS, reversible causes of cardiac arrest are grouped as Hs and Ts, guiding rapid assessment and treatment during resuscitation. Tension pneumothorax is a quintessential part of the Ts because the air trapped under pressure in the chest collapses the lung and shifts the mediastinum, severely reducing venous return to the heart and stopping effective circulation. The fix is immediate decompression—needle thoracostomy or chest tube placement—to relieve pressure and restore cardiac output, often reversing the arrest quickly.

Hypermagnesemia and thyrotoxicosis aren’t listed as standard Hs or Ts to be addressed in the immediate code sequence. Thrombosis is a known Ts cause as well, but tension pneumothorax stands out as a classic, rapidly reversible problem you can treat right away to improve outcomes in the moment.