Hypercapnia or hypercapnea, also known as hypercarbia, is a condition of abnormally
elevated carbon dioxide 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,
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 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, 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
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, 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 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 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?": Higher Inspired Oxygen at 4 atm accounted
for not more than 25% of the elevation in End Tidal CO2 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 in exposures
above 1 atm, as indicated by the results when helium was substituted for nitrogen at 4 atm.
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 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.
The carbon dioxide scrubber in the diver's rebreather is failing to remove sufficient
carbon dioxide from the loop.
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.
The higher gas density also causes gas mixing within the lung to be less efficient, thus
increasing the deadspace.
The diver is deliberately hypoventilating, known as "skip breathing".
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.
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.
Rebreathers In closed circuit SCUBA 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
Animal euthanasia Lake Nyos
Respiratory Physiology Hypocapnia, decreased level of carbon dioxide
Ocean acidification Permissive hypercapnia
Sleep apnea Water boarding
References