Hypercapnia

Hypercapnia
Carbon dioxide
Classification and external resources
ICD-10	R06.8
ICD-9	786.09
DiseasesDB	95
MeSH	D006935

Hypercapnia or hypercapnea (from the Greek hyper = "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'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, for example as a contributory factor in sudden infant death syndrome.^[1]

Hypercapnia is the opposite of hypocapnia, the state of having abnormally reduced levels of carbon dioxide in the blood.

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

Main symptoms of Carbon dioxide toxicity, by increasing volume percent in air.^[3][4]

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.^[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 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, P_aCO₂/[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?":^[9]

• Higher inspired oxygen (PiO₂) at 4 atm (400 kPa) accounted for not more than 25% of the elevation in end tidal CO₂ (etCO₂)^[10] above values found at the same work rate when breathing air just below the surface.^{[11][12][13][14]}
• Increased work of breathing accounted for most of the elevation of PACO₂ (alveolar gas equation) in exposures above 1 atm (100 kPa), as indicated by the results when helium was substituted for nitrogen at 4 atm (400 kPa).^{[11][12][13][14]}
• 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.^{[11][12][13][14]}

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.^[14]
• 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).^[14]
• 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.^[15] 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.^[16] 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.^[17]

See also

• Inert gas asphyxiation
• Lake Nyos
• Respiratory Physiology
• Hypocapnia, decreased level of carbon dioxide
• Ocean acidification
• Permissive hypercapnia
• Sleep apnea
• Water boarding

References

Underwater diving Modes Freediving Saturation diving Scuba diving Snorkelling Surface-supplied diving Recreational specialities Altitude diving Cave diving Deep diving Ice diving Mixed gas diving Muck diving Open-water diving Rebreather diving Solo diving Side mount diving Technical diving Wreck diving Underwater sports Equipment Backplate and wing Buoyancy control device Breathing gas Burst disc Distance line/dive reel Dive computer Diving air compressor Diving bell Diving chamber Diving cylinder Diving equipment Diving helmet Dive light Diver's pump Diving regulator Diver propulsion vehicle Diving shot Diving suit Diving weighting system Fins Full-face mask Integrated Diver Display Mask Lifting bag Line marker Mask Moon pool Rash vest Rebreather Scuba set Snoopy loop Snorkel Surface marker buoy Towboard Vintage scuba Wetsuit Applications Clearance diver Commercial offshore diving Frogman List of military diving units Police diving Professional diving Public safety diving Recreational diving Scientific diving Underwater archaeology Underwater photography Underwater videography Hazards Barotrauma Decompression sickness Deep water blackout Drowning Dysbaric osteonecrosis High pressure nervous syndrome Human factors in diving safety Hypercapnia Hypothermia Isobaric counterdiffusion List of diving hazards and precautions List of signs and symptoms of diving disorders Nitrogen narcosis Oxygen toxicity Seasickness Shallow water blackout Silt out Task loading Procedures Ascending and descending (diving) Buddy diving Controlled buoyant lift Controlled emergency swimming ascent Decompression (diving) Dive planning Diver communications Diver rescue Diver trim Emergency ascent Night diving Penetration diving Recreational diver training Rule of thirds (diving) Scuba skills Scuba gas planning Underwater navigation Underwater searches Underwater search and recovery History Ancient divers History of decompression research and development Timeline of diving technology List of researchers in underwater diving Categories: Diving equipment Professional diving Recreational diving Underwater diving history Underwater diving procedures Underwater diving safety Commons Glossary Portal

Diving medicine, physiology, physics and environment Diving medicine: Injuries and Disorders Pressure Oxygen Deep water blackout Hyperoxia Oxygen toxicity Shallow water blackout Inert gases Atrial septal defect Avascular necrosis Decompression sickness Dysbaric osteonecrosis High-pressure nervous syndrome Hydrogen narcosis Isobaric counterdiffusion Nitrogen narcosis Taravana Uncontrolled decompression Carbon dioxide Hypercapnia Aerosinusitis Air embolism Alternobaric vertigo Barodontalgia Barostriction Barotrauma Compression arthralgia Decompression illness Dental barotrauma Dysbarism Ear clearing Frenzel maneuver Valsalva maneuver Immersion Asphyxia Drowning Hypothermia Hypoxia (medical) Immersion diuresis Instinctive drowning response Salt water aspiration syndrome Swimming-induced pulmonary edema List of signs and symptoms of diving disorders Diving disorders Seasickness Cramps Treatments Diving medicine Hyperbaric medicine In-water recompression Oxygen therapy Diving chamber Diving physiology Artificial gills (human) Blood–air barrier CO₂ retention Cold shock response Blood shift Decompression (diving) Decompression theory Gas exchange History of decompression research and development Lipid Mammalian diving reflex Maximum operating depth Metabolism Normocapnia Oxygen window in technical diving Perfusion Pulmonary circulation Respiratory exchange ratio Respiratory quotient Systemic circulation Tissue (biology) Diving physics Ambient pressure Anti fog Amontons' law Archimedes' principle Atmospheric pressure Boyle's law Breathing performance of regulators Buoyancy Charles' law Combined gas law Dalton's law Diffusion Diving physics Force Gay-Lussac's law Henry's law Hydrophobe Hydrostatic pressure Ideal gas law Molecular diffusion Neutral buoyancy Oxygen fraction Partial pressure Permeation Pressure Psychrometric constant Snell's law Solution Solubility Supersaturation Surfactant Surface tension Torricellian chamber Underwater vision Weight Diving environment Algal bloom Breaking wave Current (ocean) Current (stream) Ekman transport Halocline Longshore current Rip current Stratification Surf Surge (wave action) Thermocline Tides Turbidity Undertow (water waves) Upwelling Researchers in diving medicine, physiology and physics Arthur J. Bachrach Albert R. Behnke Paul Bert George F. Bond Sir Robert Boyle Albert A. Bühlmann John R Clarke William Paul Fife John Scott Haldane Robert William Hamilton, Jr. Leonard Erskine Hill Felix Hoppe-Seyler Christian J. Lambertsen Simon Mitchell Charles Momsen John Rawlins (Royal Navy officer) Charles Wesley Shilling Jules Triger Edward D. Thalmann Diving medical research organisations Aerospace Medical Association Divers Alert Network Diving Diseases Research Centre Fitness to dive List of diving hazards and precautions National Board of Diving and Hyperbaric Medical Technology Naval Submarine Medical Research Laboratory Royal Australian Navy School of Underwater Medicine Rubicon Foundation South Pacific Underwater Medicine Society Undersea and Hyperbaric Medical Society United States Navy Experimental Diving Unit Categories: Diving medicine Underwater diving physiology Underwater diving physics Physical oceanography Glossary Portal