Hypercapnia (from the Greek hyper = "above" and kapnos = "smoke") is a condition where there is too much carbon dioxide (CO₂) in the blood. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs.

Causes

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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 carbon dioxide (usually due to volcanic or geothermal causes), or by rebreathing exhaled carbon dioxide.

Symptoms

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Symptoms of early hypercapnia (i.e. where arterial carbon dioxide pressure, PaCO₂, is elevated but not extremely so) include flushed skin, full pulse, extrasystoles, muscle twitches, hand flaps, and possibly a raised blood pressure. In severe hypercapnia (generally PaCO₂ greater than 10kPa or 75mmHg), symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.

During diving

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Reasons

There are a variety of reasons for carbon dioxide 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 carbon dioxide scrubber in the diver's rebreather is failing to remove sufficient carbon dioxide from the loop.
• The diver is overexerted, producing excess carbon dioxide 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,".

Skip breathing

Skip breathing is a technique which conserves breathing gas when using open-circuit scuba but leads to CO₂ not being exhaled efficiently. Skip breathing is counter productive with a rebreather where the act of breathing pumps the gas around the "loop" pushing carbon dioxide through the scrubber and mixing freshly injected oxygen. With skip breathing there is also an increased risk of burst lung from holding the breath while ascending.

Rebreathers

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, such as soda lime. If not removed from the system, it may be re-inhaled, causing an increase in the inhaled concentration.

See also

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• Permissive hypercapnia
• hypocapnia, decreased level of carbon dioxide
  
• Repiratory Physiology

References

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• CO₂ Effects on Humans

diving medicine | pulmonology

Hyperkapnie | Hiperkapnio | Ipercapnia | Hiperkapnia | Гиперкапния