Which mechanism is described as a cause of hypoxia in a closed-circuit diving context?

• A. Hyperventilation before breath holds during purging of the breathing loop
• B. Gradual acclimatization to depth
• C. Adequate rest and hydration before dive
• D. Using a trusted gas mix at all times

Answer: (A)

In closed-circuit diving, the drive to breathe is largely controlled by CO2 levels. Hyperventilating before a breath-hold purge lowers CO2 in the breathing loop, which suppresses the ventilatory drive. When you then purge and the loop continues to use oxygen, the partial pressure of oxygen can fall without the diver feeling the urge to breathe. This masking of hypoxia means you can reach dangerous low O2 levels before noticing, making hypoxia a real risk in this scenario. The other ideas don’t describe how hypoxia would be driven in a CCR setup: acclimatization, rest and hydration, or simply using a trusted gas mix don’t explain a mechanism that would cause hypoxia to develop and go unnoticed.