Hypercapnia

Overview
Hypercapnia or hypercapnea (from the Greek hyper = "above" and kapnos = "smoke"), also known as hypercarbia, 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.

Hypercapnia is the opposite of hypocapnia.

Causes
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. It can also be an initial effect of administering supplemental oxygen on a patient with sleep apnea. In this situation the hypercapnia can also be accompanied by respiratory acidosis.

Symptoms
Symptoms of early hypercapnia, where arterial carbon dioxide pressure, PaCO2, is elevated but not extremely so, include flushed skin, full pulse, extrasystoles, muscle twitches, hand flaps, reduced neural activity, and possibly a raised blood pressure. In severe hypercapnia (generally PaCO2 greater than 100 hPa or 75 mmHg), symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.

Laboratory Values
Hypercapnia is generally defined as a blood gas carbon dioxide level over 45 mmHg. Since carbon dioxide is in equilibrium with bicarbonate in the blood, hypercapnia can also result in a high serum bicarbonate (HCO3-) concentration. Normal bicarbonate concentrations vary from 22 to 28 milligrams per deciliter.

During diving
Normal respiration in divers results in alveolar hypoventilation resulting in inadequate CO2 elimination or hypercapnia. Lanphier's work at the US Naval Experimental Diving Unit answered the question "why don't divers breathe enough?":
 * Higher Inspired Oxygen (PiO2) at 4 ata (404 kPa) accounted for not more than 25% of the elevation in End Tidal CO2 (etCO2) above values found at the same work rate when breathing air just below the surface.
 * Increased Work of Breathing accounted for most of the elevation of PACO2 (alveolar gas equation) in exposures above 1 ata (101 kPa), as indicated by the results when helium was substituted for nitrogen at 4 ata (404 kPa).
 * Inadequate ventilatory response to exertion was indicated by the fact that, despite resting values in the normal range, PetCO2 rose markedly with exertion even when the divers breathed air at a depth of only a few feet.

Additional Sources of CO2 in diving
There are a variety of reasons for carbon dioxide not being expelled completely when the diver exhales:
 * The diver is exhaling into a vessel that does not allow all the CO2 to escape to the environment, such as a long snorkel, full face diving mask, or diving helmet. The diver then re-inhales from that vessel (Increased deadspace).
 * The carbon dioxide scrubber in the diver's rebreather is failing to remove sufficient carbon dioxide from the loop (Higher inspired CO2).
 * The diver is over-exercising, 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 (Work of breathing). The higher gas density also causes gas mixing within the lung to be less efficient, thus increasing the deadspace (wasted breathing).
 * The diver is deliberately hypoventilating, known as "skip breathing" (see below).

Skip breathing
Skip breathing is a controversial technique to conserve breathing gas when using open-circuit scuba, which consists of briefly holding one's breath between inhalation and exhalation (i.e. "skipping" a breath). It leads to CO2 not being exhaled efficiently. There is also an increased risk of burst lung from holding the breath while ascending.

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.

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.