Hypercapnea

Hypercapnea is generally caused by hypoventilation or lung disease. 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 inhaled carbon dioxide.

The symptoms are disorientation, panic, hyperventilation, unconsciousness and death. Carbon dioxide gas is a product of the body's metabolism, which is normally expelled through the lungs.

In closed-ciruit SCUBA diving, or general anesthesia, 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.

Hypercapnea during diving

There are a variety of reasons the carbon dioxide is not being expelled completely when the diver exhales:

• the lungs are not fully emptying due to tight fitting clothing around the chest
• the diver is exhaling into a vessel 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 using has failed and is not removing carbon dioxide from the loop
• the diver is working hard and therefore producing more carbon dioxide, breathing quicker and possibly not fully exhaling
• 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 to conserve breathing gas