Hypercapnia (from the
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
''hyper'' = "above" or "too much" and ''kapnos'' = "
smoke
Smoke is a suspension of airborne particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product ...
"), also known as hypercarbia and CO
2 retention, is a condition of abnormally elevated
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
(CO
2) levels in the blood. Carbon dioxide is a
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
eous product of the
body's metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
and is normally expelled through the
lung
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
s. Carbon dioxide may accumulate in any condition that causes
hypoventilation
Hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (''hypo'' meaning "below") to perform needed respiratory gas exchange. By definition it causes an increased concentration of carbon dioxide (hypercapnia ...
, a reduction of
alveolar ventilation
Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen.
All aerobic creatures need oxygen for cellular ...
(the clearance of air from the small sacs of the lung where
gas exchange
Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liqui ...
takes place) as well as resulting from inhalation of CO
2. Inability of the lungs to clear carbon dioxide, or inhalation of elevated levels of CO
2, leads to
respiratory acidosis
Respiratory acidosis is a state in which decreased ventilation (hypoventilation) increases the concentration of carbon dioxide in the blood and decreases the blood's pH (a condition generally called acidosis).
Carbon dioxide is produced continuou ...
. Eventually the body compensates for the raised acidity by retaining alkali in the kidneys, a process known as "metabolic compensation".
Acute hypercapnia is called acute hypercapnic respiratory failure (AHRF) and is a medical emergency as it generally occurs in the context of acute illness. Chronic hypercapnia, where metabolic compensation is usually present, may cause symptoms but is not generally an emergency. Depending on the scenario both forms of hypercapnia may be treated with medication, with mask-based
non-invasive ventilation
Non-invasive ventilation (NIV) is the use of breathing support administered through a face mask, nasal mask, or a helmet. Air, usually with added oxygen, is given through the mask under positive pressure; generally the amount of pressure is alter ...
or with
mechanical ventilation
Mechanical ventilation, assisted ventilation or intermittent mandatory ventilation (IMV), is the medical term for using a machine called a ventilator to fully or partially provide artificial ventilation. Mechanical ventilation helps move air ...
.
Hypercapnia is a hazard of underwater diving associated with breath-hold diving, scuba diving, particularly on rebreathers, and deep diving where it is associated with increased breathing gas density due to the high ambient pressure.
Signs and symptoms
Hypercapnia may happen in the context of an underlying health condition, and symptoms may relate to this condition or directly to the hypercapnia. Specific symptoms attributable to early hypercapnia are dyspnea
Shortness of breath (SOB), also medically known as dyspnea (in AmE) or dyspnoea (in BrE), is an uncomfortable feeling of not being able to breathing, breathe well enough. The American Thoracic Society defines it as "a subjective experience of brea ...
(breathlessness), headache, confusion and lethargy. Clinical signs include flushed skin, full pulse
In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...
(bounding pulse), rapid breathing
Tachypnea, also spelt tachypnoea, is a respiratory rate greater than normal, resulting in abnormally rapid and shallow breathing.
In adult humans at rest, any respiratory rate of 1220 per minute is considered clinically normal, with tachypnea b ...
, premature heart beats, muscle twitches, and hand flaps ( asterixis). The risk of dangerous irregularities of the heart beat is increased. Hypercapnia also occurs when the breathing gas is contaminated with carbon dioxide, or respiratory gas exchange cannot keep up with the metabolic production of carbon dioxide, which can occur when gas density limits ventilation at high ambient pressures.
In severe hypercapnia (generally greater than 10 kPa
KPA may refer to:
* Keele Postgraduate Association, Keele University, UK, formerly Keele Research Association (KRA)
* Kensington (Olympia) station, London, England, National Rail station code
* Kenya Ports Authority
* ''Kiln phosphoric acid'', a ...
or 75 mmHg
A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly pascals. It is denoted mmHg or mm Hg.
Although not an SI ...
), symptomatology progresses to disorientation, panic
Panic is a sudden sensation of fear, which is so strong as to dominate or prevent reason and logical thinking, replacing it with overwhelming feelings of anxiety and frantic agitation consistent with an animalistic fight-or-flight reactio ...
, hyperventilation
Hyperventilation is irregular breathing that occurs when the rate or tidal volume of breathing eliminates more carbon dioxide than the body can produce. This leads to hypocapnia, a reduced concentration of carbon dioxide dissolved in the blood. ...
, convulsions
A convulsion is a medical condition where the body muscles contract and relax rapidly and repeatedly, resulting in uncontrolled shaking. Because epileptic seizures typically include convulsions, the term ''convulsion'' is sometimes used as a s ...
, unconsciousness
Unconsciousness is a state in which a living individual exhibits a complete, or near-complete, inability to maintain an consciousness, awareness of self and environment or to respond to any human or environmental Stimulus (physiology), stimulus. ...
, and eventually death
Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...
.
Causes
Carbon dioxide is a normal metabolic product but it accumulates in the body if it is produced faster than it is cleared. During strenuous exercise the production rate of carbon dioxide can increase more than tenfold over the production rate during rest. Carbon dioxide is dissolved in the blood and elimination is by gas exchange in the lungs during breathing. Hypercapnia is generally caused by hypoventilation
Hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (''hypo'' meaning "below") to perform needed respiratory gas exchange. By definition it causes an increased concentration of carbon dioxide (hypercapnia ...
, lung disease
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
, or diminished consciousness
Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...
. 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
A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is ...
exhaled carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
. In this situation the hypercapnia can also be accompanied by respiratory acidosis.
Acute hypercapnic respiratory failure may occur in acute illness caused by chronic obstructive pulmonary disease
Chronic obstructive pulmonary disease (COPD) is a type of progressive lung disease characterized by long-term respiratory symptoms and airflow limitation. The main symptoms include shortness of breath and a cough, which may or may not produce ...
(COPD), chest wall deformity, some forms of neuromuscular disease
A neuromuscular disease is any disease affecting the peripheral nervous system (PNS), the neuromuscular junction, or skeletal muscle, all of which are components of the motor unit. Damage to any of these structures can cause muscle atrophy and w ...
(such as myasthenia gravis
Myasthenia gravis (MG) is a long-term neuromuscular junction disease that leads to varying degrees of skeletal muscle weakness. The most commonly affected muscles are those of the eyes, face, and swallowing. It can result in double vision, dro ...
, and obesity hypoventilation syndrome
Obesity hypoventilation syndrome (OHS) is a condition in which severely overweight people fail to breathe rapidly or deeply enough, resulting in low oxygen levels and high blood carbon dioxide (CO2) levels. The syndrome is often associated with ...
. AHRF may also develop in any form of respiratory failure where the breathing muscles become exhausted, such as severe pneumonia
Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severity ...
and acute severe asthma
Acute severe asthma, also known as status asthmaticus, is an acute exacerbation of asthma that does not respond to standard treatments of bronchodilators (inhalers) and corticosteroids. Asthma is caused by multiple genes, some having protective ef ...
. It can also be a consequence of profound suppression of consciousness such as opioid overdose
An opioid overdose is toxicity due to excessive consumption of opioids, such as morphine, codeine, heroin, fentanyl, tramadol, and methadone. This preventable pathology can be fatal if it leads to respiratory depression, a lethal condition that ca ...
.
During diving
Normal respiration in divers results in alveolar Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* ...
hypoventilation
Hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (''hypo'' meaning "below") to perform needed respiratory gas exchange. By definition it causes an increased concentration of carbon dioxide (hypercapnia ...
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 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 (alveolar gas equation The alveolar gas equation is the method for calculating partial pressure of alveolar oxygen (PAO2). The equation is used in assessing if the lungs are properly transferring oxygen into the blood. The alveolar air equation is not widely used in clin ...
) in exposures above , as indicated by the results when helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
was substituted for nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
at .[
* Inadequate ventilatory response to exertion was indicated by the fact that, despite resting values in the normal range, rose markedly with exertion even when the divers breathed air at a depth of only a few feet.][
A variety of reasons exists 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
A diving helmet is a rigid head enclosure with a breathing gas supply used in underwater diving. They are worn mainly by professional divers engaged in surface-supplied diving, though some models can be used with scuba equipment. The upper part ...
, and the diver then reinhales from that vessel, causing increased dead space
''Dead Space'' is a science fiction/horror fiction, horror media franchise created by Glen Schofield and Michael Condrey, developed by Visceral Games, and published and owned by Electronic Arts. The franchise's chronology is not presented in a lin ...
.[
* The ]carbon dioxide scrubber
A carbon dioxide scrubber is a piece of equipment that absorbs carbon dioxide (CO2). It is used to treat exhaust gases from industrial plants or from exhaled air in life support systems such as rebreathers or in spacecraft, submersible craft or a ...
in the diver's rebreather
A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's breathing, exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. ...
is failing to remove sufficient carbon dioxide from the loop (higher inspired CO2), or the breathing gas is contaminated with CO2.
* The diver is overexercising, producing excess carbon dioxide due to elevated metabolic activity and respiratory gas exchange cannot keep up with the metabolic production of carbon dioxide.
* Gas density limits ventilation at high ambient pressures. The density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of the breathing gas
A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed h ...
is higher at depth, so the effort required to fully inhale and exhale increases, making breathing more difficult and less efficient (high work of breathing
Work of breathing (WOB) is the energy expended to inhale and exhale a breathing gas. It is usually expressed as work per unit volume, for example, joules/litre, or as a work rate (power), such as joules/min or equivalent units, as it is not partic ...
). Higher gas density also causes gas mixing within the lung to be less efficient, thus increasing the effective dead space.
* The diver is deliberately hypoventilating, known as "skip breathing".
Skip breathing
Skip breathing is a controversial technique to conserve breathing gas
A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed h ...
when using open-circuit scuba
A scuba set, originally just scuba, is any breathing apparatus that is entirely carried by an underwater diver and provides the diver with breathing gas at the ambient pressure. ''Scuba'' is an anacronym for self-contained underwater breathing ...
, which consists of briefly holding one's breath between inhalation and exhalation (i.e., "skipping" a breath). It can lead to CO2 not being exhaled efficiently. The risk of burst lung (pulmonary barotrauma
Barotrauma is physical damage to body tissues caused by a difference in pressure between a gas space inside, or contact with, the body and the surrounding gas or liquid. The initial damage is usually due to over-stretching the tissues in tensio ...
of ascent) is increased if the breath is held while ascending. It is particularly counterproductive with a rebreather
A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's breathing, exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. ...
, 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 rebreather diving
Rebreather diving is underwater diving using diving rebreathers, a class of underwater breathing apparatus which recirculate the breathing gas exhaled by the diver after replacing the oxygen used and removing the carbon dioxide metabolic produc ...
, exhaled carbon dioxide must be removed from the breathing system, usually by a scrubber
Scrubber systems (e.g. chemical scrubbers, gas scrubbers) are a diverse group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams. An early application of a carbon dioxide scr ...
containing a solid chemical compound with a high affinity for CO2, such as soda lime
Soda lime is a mixture of NaOH and CaO chemicals, used in granular form in closed breathing environments, such as general anaesthesia, submarines, rebreathers and recompression chambers, to remove carbon dioxide from breathing gases to prevent ...
. If not removed from the system, it may be reinhaled, causing an increase in the inhaled concentration.
Under hyperbaric conditions, hypercapnia contributes to nitrogen narcosis
Narcosis while diving (also known as nitrogen narcosis, inert gas narcosis, raptures of the deep, Martini effect) is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain gas ...
and oxygen toxicity
Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen () at increased partial pressures. Severe cases can result in cell damage and death, with effects most often seen in the central nervous system, lu ...
by causing cerebral vasodilation which increases the dosage of oxygen to the brain.
Mechanism
Hypercapnia normally triggers a reflex which increases breathing and access to oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
(O2), 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
Sudden infant death syndrome (SIDS) is the sudden unexplained death of a child of less than one year of age. Diagnosis requires that the death remain unexplained even after a thorough autopsy and detailed death scene investigation. SIDS usuall ...
.
Hypercapnia can induce increased cardiac output, an elevation in arterial blood pressure (higher levels of carbon dioxide stimulate aortic and carotid chemoreceptor
A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...
s with afferents -CN IX and X- to medulla oblongata with following chrono- and ino-tropic effects), and a propensity toward cardiac arrhythmia
Arrhythmias, also known as cardiac arrhythmias, heart arrhythmias, or dysrhythmias, are irregularities in the heartbeat, including when it is too fast or too slow. A resting heart rate that is too fast – above 100 beats per minute in adults ...
s. Hypercapnia may increase pulmonary capillary resistance.
Physiological effects
A high arterial partial pressure of carbon dioxide () causes changes in brain activity that adversely affect both fine muscular control and reasoning. EEG
Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...
changes denoting minor narcotic effects can be detected for expired gas end tidal partial pressure of carbon dioxide () increase from to approximately . The diver does not necessarily notice these effects.
Higher levels of have a stronger narcotic effect: Confusion and irrational behaviour may occur around , and loss of consciousness around . High triggers the fight or flight response, affects hormone levels and can cause anxiety, irritability and inappropriate or panic responses, which can be beyond the control of the subject, sometimes with little or no warning. Vasodilation is another effect, notably in the skin, where feelings of unpleasant heat are reported, and in the brain, where blood flow can increase by 50% at a of , Intracranial pressure may rise, with a throbbing headache. If associated with a high the high delivery of oxygen to the brain may increase the risk of CNS oxygen toxicity at partial pressures usually considered acceptable.
In many people a high causes a feeling of shortness of breath, but the lack of this symptom is no guarantee that the other effects are not occurring. A significant percentage of rebreather deaths have been associated with CO2 retention. The effects of high can take several minutes to hours to resolve once the cause has been removed.
Diagnosis
Blood gas test
A blood gas test or blood gas analysis tests blood to measure blood gas tension values, it also measures blood pH, and the level and base excess of bicarbonate. The source of the blood is reflected in the name of each test; arterial blood gases ...
s may be performed, typically by radial artery puncture
Radial artery puncture is a medical procedure performed to obtain a sample of arterial blood for gas analysis. A needle is inserted into the radial artery
In human anatomy, the radial artery is the main artery of the lateral aspect of the f ...
, in the setting of acute breathing problems or other acute medical illness. Hypercapnia is generally defined as an arterial blood carbon dioxide level over 45 mmHg (6 kPa). Since carbon dioxide is in equilibrium with carbonic acid in the blood, hypercapnia drives serum pH down, resulting in 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, .
Tolerance
CO2 toxicity in animal models
Tests performed on mongrel dogs showed the physiological effect of carbon dioxide on the body of the animal: after inhalation of a 50% CO2 and 50% air mixture, respiratory movement increased for about 2 minutes, and then, it decreased for 30 to 90 minutes. Hill and Flack showed that CO2 concentrations up to 35% have an exciting effect upon both circulation and respiration, but those beyond 35% are depressant upon them. The blood pressure (BP) decreased transiently during the increased respiratory movement and then rose again and maintained the original level for a while. The heart rate slowed slightly just after the gas mixture inhalation. It is believed that the initial BP depression with the decreased heart rate is due to the direct depressant effect of CO2 upon the heart and that the return of blood pressure to its original level was due to the rapid rise of . After 30–90 min, the respiratory center was depressed, and hypotension occurred gradually or suddenly from reduced cardiac output, leading to an apnea and eventually to circulatory arrest.
At higher concentrations of CO2, unconsciousness occurred almost instantaneously and respiratory movement ceased in 1 minute. After a few minutes of apnea, circulatory arrest was seen. These findings imply that the cause of death in breathing high concentrations of CO2 is not the hypoxia but the intoxication of carbon dioxide.
Treatment
The treatment for acute hypercapnic respiratory failure depends on the underlying cause, but may include medications and mechanical respiratory support. In those without contraindications, non-invasive ventilation
Non-invasive ventilation (NIV) is the use of breathing support administered through a face mask, nasal mask, or a helmet. Air, usually with added oxygen, is given through the mask under positive pressure; generally the amount of pressure is alter ...
(NIV) is often used in preference to invasive mechanical ventilation. In the past, the drug doxapram
Doxapram hydrochloride (marketed as Dopram, Stimulex or Respiram) is a respiratory stimulant. Administered intravenously, doxapram stimulates an increase in tidal volume, and respiratory rate.
Mechanism of action
Doxapram stimulates chemorecep ...
(a respiratory stimulant), was used for hypercapnia in acute exacerbation of chronic obstructive pulmonary disease
An acute exacerbation of chronic obstructive pulmonary disease, or acute exacerbations of chronic bronchitis (AECB), is a sudden worsening of chronic obstructive pulmonary disease (COPD) symptoms including shortness of breath, quantity and color o ...
but there is little evidence to support its use compared to NIV, and it does not feature in recent professional guidelines.
Very severe respiratory failure, in which hypercapnia may also be present, is often treated with extracorporeal membrane oxygenation
Extracorporeal membrane oxygenation (ECMO), also known as extracorporeal life support (ECLS), is an extracorporeal technique of providing prolonged cardiac and respiratory support to persons whose heart and lungs are unable to provide an adequat ...
(ECMO), in which oxygen is added to and carbon dioxide removed directly from the blood.
A relatively novel modality is extracorporeal carbon dioxide removal
Extracorporeal carbon dioxide removal (ECCO2R) is the removal of carbon dioxide (CO2) from the bloodstream in people who have elevated levels of carbon dioxide as a result of respiratory failure.
The use of extracorporeal carbon dioxide removal ...
(ECCO2R). This technique removes CO2 from the bloodstream and may reduce the time mechanical ventilation is required for those with AHRF; it requires smaller volumes of blood flow compared to ECMO.
Terminology
Hypercapnia is the opposite of hypocapnia
Hypocapnia (from the Greek words υπό meaning ''below normal'' and καπνός ''kapnós'' meaning ''smoke''), also known as hypocarbia, sometimes incorrectly called acapnia, is a state of reduced carbon dioxide in the blood. Hypocapnia usually ...
, the state of having abnormally reduced levels of carbon dioxide in the blood.
See also
* , decreased level of carbon dioxide
*
*
*
*
*
References
External links
{{Authority control
Diving medicine
Medical emergencies
Respiratory diseases