Which of the following is a reversible H (human physiological cause) of cardiac arrest?

• A. Hypovolemia
• B. Tamponade
• C. Hypoxia
• D. Hypothermia

Answer: (A)

During cardiac arrest, recognizing reversible human physiological problems (the Hs) helps you intervene to restore circulation. Hypovolemia means there isn’t enough circulating blood volume, often from severe bleeding or major fluid loss. With very low preload, the heart can’t generate adequate output. The fix is rapid restoration of intravascular volume—administering isotonic fluids or blood products and controlling the source of loss. Replacing volume directly tackles the root problem and can quickly improve coronary and cerebral perfusion, which is why this cause is the classic reversible H.

Other items in this set involve different mechanisms: tamponade is a mechanical issue around the heart that’s relieved by draining the pericardial sac; hypoxia improves with adequate oxygen delivery; hypothermia requires warming. These are reversible too, but they stem from different processes, whereas restoring volume addresses the immediate preload deficit that leads to arrest in many patients.