Hypercapnia
Hypercapnia, also known as hypercarbia and CO2 retention, 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. Carbon dioxide may accumulate in any condition that causes hypoventilation, a reduction of alveolar ventilation. Inability of the lungs to clear carbon dioxide 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 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 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, headache, confusion and lethargy. Clinical signs include flushed skin, full pulse, rapid breathing, premature heart beats, muscle twitches, and hand flaps. 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, symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.
Causes
Carbon dioxide is a normal metabolic product but it accumulates in the body if it is produced faster than it is cleared. The production rate can increase more than tenfold from resting to strenuous exercise. Carbon dioxide is dissolved in the blood and elimination is by gas exchange in the lungs during breathing. 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, such as from volcanic or geothermal activity, or by rebreathing exhaled carbon dioxide. 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, chest wall deformity, some forms of neuromuscular disease (such as myasthenia gravis, and obesity hypoventilation syndrome. AHRF may also develop in any form of respiratory failure where the breathing muscles become exhausted, such as severe pneumonia and acute severe asthma. It can also be a consequence of profound suppression of consciousness such as opioid overdose.
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 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 in exposures above, as indicated by the results when helium was substituted for nitrogen 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.
- 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 reinhales from that vessel, causing increased dead space.
- The carbon dioxide scrubber in the diver's rebreather is failing to remove sufficient carbon dioxide from the loop, 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 of the breathing gas is higher at depth, so the effort required to fully inhale and exhale increases, making breathing more difficult and less efficient. 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".
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 CO2, such as soda lime. If not removed from the system, it may be reinhaled, causing an increase in the inhaled concentration.
Mechanism
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.Hypercapnia can induce increased cardiac output, an elevation in arterial blood pressure, and a propensity toward cardiac arrhythmias. 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 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
s 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. 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
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 is often used in preference to invasive mechanical ventilation. In the past, the drug doxapram, 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, in which oxygen is added to and carbon dioxide removed directly from the blood.
A relatively novel modality is extracorporeal carbon dioxide removal. 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.