Hypercapnia
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Hypercapnia (from the Greek hyper, "above" or "too much" and kapnos, "smoke"), also known as hypercarbia and CO 2 retention, is a condition of abnormally elevated carbon dioxide (CO 2 ) levels in the blood. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs. Carbon dioxide may accumulate in any condition that causes hypoventilation, a reduction of alveolar ventilation (the clearance of air from the small sacs of the lung where gas exchange 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. 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 or with mechanical ventilation.…
Hypercapnia (from the Greek hyper, "above" or "too much" and kapnos, "smoke"), also known as hypercarbia and CO 2 retention, is a condition of abnormally elevated carbon dioxide (CO 2 ) levels in the blood. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs. Carbon dioxide may accumulate in any condition that causes hypoventilation, a reduction of alveolar ventilation (the clearance of air from the small sacs of the lung where gas exchange 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. 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 or with mechanical ventilation. 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 high work of breathing caused by increased breathing gas density due to the high ambient pressure.
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 can lead to CO 2 not being exhaled efficiently. The risk of burst lung (pulmonary barotrauma of ascent) is increased 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. In closed-circuit 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 2 , such as soda lime. 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 and oxygen toxicity 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 (O 2 ), 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 can induce increased cardiac output, an elevation in arterial blood pressure (higher levels of carbon dioxide stimulate aortic and carotid chemoreceptors with afferents -CN IX and X- to medulla oblongata with following chrono- and ino-tropic effects), and a propensity toward cardiac arrhythmias. Hypercapnia may increase pulmonary capillary resistance.
Physiological effects
A high arterial partial pressure of carbon dioxide ( {P_{a_{CO_2}}} ) causes changes in brain activity that adversely affect both fine muscular control and reasoning. EEG changes denoting minor narcotic effects can be detected for expired gas end tidal partial pressure of carbon dioxide ( {P_{ET_{CO_2}}} ) increase from to approximately . The diver does not necessarily notice these effects. Higher levels of {P_{a_{CO_2}}} have a stronger narcotic effect: Confusion and irrational behaviour may occur around , and loss of consciousness around . High {P_{a_{CO_2}}} 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 {P_{ET_{CO_2}}} of , Intracranial pressure may rise, with a throbbing headache. If associated with a high {P_{a_{CO_2}}} 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 {P_{a_{CO_2}}} 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 CO 2 retention. The effects of high {P_{a_{CO_2}}} can take several minutes to hours to resolve once the cause has been removed.
Diagnosis
Blood gas tests may be performed, typically by radial artery puncture, 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, {P_{a_{CO_2/HCO_3^-}}} .
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 (NIV) is often used in preference to invasive mechanical ventilation. In the past, the drug doxapram (a respiratory stimulant), was used for hypercapnia in acute exacerbation of chronic obstructive pulmonary disease 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 (ECMO), in which oxygen is added to and carbon dioxide removed directly from the blood. A relatively novel modality is extracorporeal carbon dioxide removal (ECCO 2 R). This technique removes CO 2 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, the state of having abnormally reduced levels of carbon dioxide in the blood.
Text derived from https://en.wikipedia.org/w/index.php?oldid=1347464907 under CC BY-SA 4.0; modifications have been made.