Hypercapnia

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 normally triggers a reflex which increases breathing and access to oxygen, such as arousal and turning the head during sleep. A failure of this reflex can be fatal, as in sudden infant death syndrome.^[1]

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.^[2]

Symptoms and signs

Symptoms and signs of early hypercapnia include flushed skin, full pulse, tachypnea, dyspnoea, extrasystoles, muscle twitches, hand flaps, reduced neural activity, and possibly a raised blood pressure. According to other sources, symptoms of mild hypercapnia might include headache, confusion and lethargy. Hypercapnia can induce increased cardiac output, an elevation in arterial blood pressure, and a propensity toward arrhythmias.^[5][6] In severe hypercapnia (generally PaCO₂ greater than 10 kPa or 75 mmHg), symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.^[7][8]

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 Navy Experimental Diving Unit answered the question "why don't divers breathe enough?":^[9]

• Higher Inspired Oxygen (PiO₂) at 4 atm (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.^{[10][11][12][13]}
• Increased Work of Breathing accounted for most of the elevation of PACO₂ (alveolar gas equation) in exposures above 1 atm (101 kPa), as indicated by the results when helium was substituted for nitrogen at 4 atm (404 kPa).^{[10][11][12][13]}
• 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.^{[10][11][12][13]}

Additional sources of carbon dioxide in diving

There is 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, and the diver then re-inhales from that vessel, causing increased deadspace.^[13]
• 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).^[9] The higher gas density also causes gas mixing within the lung to be less efficient, thus increasing the deadspace (wasted breathing).^[13]
• 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.^[14] There is also an increased risk of burst lung from holding the breath while ascending. It is counterproductive 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.^[15] If not removed from the system, it may be re-inhaled, causing an increase in the inhaled concentration.

See also

• Hypocapnia, decreased level of carbon dioxide
• Ocean acidification
• Permissive hypercapnia
• Respiratory Physiology
• Sleep apnea
• Animal euthanasia
• Lake Nyos

References