Which step is used to prevent hypoxia?

• A. Proper purge
• B. Raising the oxygen mix during the dive
• C. Rapid ascents to the surface
• D. Ignoring partial pressure limits

Answer: (A)

Maintaining a safe oxygen level in the breathing loop is what prevents hypoxia. A proper purge is the step that directly prepares the system to deliver exactly the gas you expect. By purging, you clear out any stale or mixed gas that may have lingered in the loop, remove moisture or contaminants, and ensure the gas path from the supply to your lungs is filled with fresh gas at the intended oxygen fraction. This helps guarantee that the oxygen partial pressure you’re breathing matches the plan and that the oxygen sensor readings reflect the actual gas in the loop. In other words, purging sets up the system so you’re not inhaling gas that could be too low in oxygen, which is how hypoxia would develop.

Context: closed-circuit systems recycle breathing gas, so even small amounts of old or improperly mixed gas can creep in. A thorough purge helps confirm the system is delivering the correct mix and that monitoring equipment is reading accurately before and during the dive.

Raising the oxygen mix during the dive is a controlled adjustment to manage ppO2, but it relies on a correctly purged, functioning system to be effective. Rapid ascent and ignoring partial pressure limits are unsafe and do not address the root cause of hypoxia.