Abstract

Hypercapnia, also known as hypercarbia and CO retention, is a condition of abnormally elevated carbon dioxide (CO) levels 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 (O), such as arousal and turning the head during sleep. A failure of this reflex can be fatal, for example as a contributory factor in sudden infant death syndrome.

Hypercapnia is the opposite of hypocapnia, the state of having abnormally reduced levels of carbon dioxide in the blood. Hypercapnia is from the Greek "hyper" = "above" or "too much" and "kapnos" = "smoke".

Signs and symptoms

Symptoms and signs of early hypercapnia include flushed skin, full pulse, tachypnea, dyspnea, 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. Hypercapnia may increase pulmonary capillary resistance. In severe hypercapnia (generally PaCO greater than 10 kPa or 75 mmHg), symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.

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, such as from volcanic or geothermal activity, 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.

Diagnosis | Laboratory values

Hypercapnia is generally defined as a blood gas carbon dioxide level over 45 mmHg. Since carbon dioxide is in equilibrium with carbonic acid in the blood, hypercapnia can drive serum pH down, resulting in a respiratory acidosis. Clinically, the effect of hypercapnia on pH is estimated using the ratio of the arterial pressure of carbon dioxide to the concentration of bicarbonate ion, PCO/[HCO].

During diving

Normal respiration in divers results in alveolar hypoventilation resulting in inadequate CO elimination or hypercapnia. Lanphier's work at the US Navy Experimental Diving Unit answered the question, "Why don't divers breathe enough?":

- Higher inspired oxygen (PiO) at accounted for not more than 25% of the elevation in end tidal CO (etCO) 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 PACO (alveolar gas equation) in exposures above , as indicated by the results when helium was substituted for nitrogen at .

- Inadequate ventilatory response to exertion was indicated by the fact that, despite resting values in the normal range, PetCO rose markedly with exertion even when the divers breathed air at a depth of only a few feet.

During diving | Failure to adequately eliminate CO

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 CO 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 dead space.

- The carbon dioxide scrubber in the diver's rebreather is failing to remove sufficient carbon dioxide from the loop (Higher inspired CO).

- 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 (high work of breathing). The higher gas density also causes gas mixing within the lung to be less efficient, thus increasing the dead space .

- The diver is deliberately hypoventilating, known as "skip breathing" ("see below").

During diving | Failure to adequately eliminate CO | 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 CO not being exhaled efficiently. There is also an increased risk of burst lung (pulmonary barotrauma of ascent) if the breath is held while ascending. It is particularly 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.

During diving | Failure to adequately eliminate CO | 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 CO, such as soda lime. If not removed from the system, it may be re-inhaled, causing an increase in the inhaled concentration.

MRI

The hypercapnic state is routinely used to calibrate blood-oxygen-level dependent functional magnetic resonance imaging (BOLD fMRI), a modality that is sensitive to changes in blood oxygenation. However, this calibration crucially relies on the assumption that hypercapnia has no effect on neuronal function, which is a matter of debate.

Names

The spelling "hypercapnea" is occasionally seen in published medical articles (44 results in a PubMed search in 2016), but it is not entered in major dictionaries and is not tied to any etymology that involves the "" suffix. It is most likely a misspelling by writers who misapprehend that the word "hypercapnia" ends in the same suffix that "apnea" does.