Hypercapnia or hypercapnea (from the Greekhyper = "above" or "too much" and kapnos = "smoke"), also known as hypercarbia, is a condition of abnormally elevated carbon dioxide (CO2) levels in the blood. Carbon dioxide is a gaseous product of the body'smetabolism 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.
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 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. In severe hypercapnia (generally PaCO2 greater than 10 kPa or 75 mmHg), symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.
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, PaCO2/[HCO3-].
Tolerance to increased atmospheric CO2 concentration
%CO2 in inspired air
Expected tolerance for useful activity on continued exposure to elevated CO2
no detectable limitations
> 1 month
mild respiratory stimulation
> 1 month
> 1 month
> 1 month
moderate respiratory stimulation
> 1 week
> 1 week
moderate respiratory stimulation, exaggerated respiratory response to exercise
> 8 hours
> 4 hours
prominent respiratory stimulus, exaggerated respiratory response to exercise
> 1 hours
> 0.5 hours
prominent respiratory stimulus, exaggerated respiratory response to exercise, beginnings of mental confusion
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.
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).
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 carbon dioxide in diving
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 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. 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.
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.