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Hypercapnia is a condition where there is too much carbon dioxide (CO2) in the body. Carbon dioxide is a gaseous product of the body's metabolism, which is normally expelled through the lungs.
Hypercapnia is generally caused by hypoventilation, lung disease, or diminished consciousness. It may also be caused by exposure to environments containing abnormally high concentrations of CO2 (usually due to volcanic or geothermal causes), or by rebreathing exhaled carbon dioxide.
The symptoms of hypercapnia are disorientation, panic, hyperventilation, unconsciousness, and eventually death. In rebreather diving, this effect is sometimes called shallow-water blackout.
In closed circuit SCUBA (rebreather) diving, exhaled carbon dioxide must be removed from the breathing system, usually by a "scrubber" containing a solid chemical compound with a high affinity for CO2. If not removed from the system, it may be re-inhaled, causing an increase in the inhaled concentration.
Carbon dioxide poisoning during diving
There are a variety of reasons for CO2 retention where carbon dioxide is not being expelled completely when the diver exhales:
- The diver is exhaling into a vessel with inadequate ventilation, such as a long snorkel, full face diving mask, or diving helmet, and then re-inhaling from that vessel.
- The scrubber in the rebreather the diver is failing to remove sufficient carbon dioxide from the loop.
- The diver is overexerted, producing excess CO2 due to elevated metabolic activity.
- The density of the breathing gas is higher at depth, so the effort required to fully inhale and exhale has increased, making breathing more difficult and less efficient.
- The diver is deliberately hypoventilating, or "skip breathing," a technique which conserves breathing gas with open-circuit scuba but should not be done with a rebreather. (On top of the risk of burst lung from holding the breath while ascending.)
See Also
hypocapnia, decreased level of carbon dioxide
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