Hypercapnia

While providing the body with oxygen, the respiratory system simultaneously removes the metabolic product – carbon dioxide (carbon dioxide, CO2), which the blood brings from the tissues to the alveoli of the lungs, and thanks to alveolar ventilation it is removed from the blood. So, hypercapnia means an abnormally elevated level of carbon dioxide in the blood.

Epidemiology

According to foreign statistics, with obesity with a BMI of 30-35, hypoventilation syndrome develops in 10% of cases, and with a body mass index of 40 and above - in 30-50%.

Among patients with severe hypercapnia, mortality due to respiratory failure averages 65%.

Causes hypercapnia

Pulmonologists cite the following reasons for increased levels of carbon dioxide (its partial pressure – PaCO2) in the blood:

• chronic obstructive bronchitis and COPD;
• exacerbation of asthma and prolonged reduction in airway patency (obstruction), leading to asthmatic status;
• pulmonary hypertension, which may be associated with damage to the alveoli in chronic obliterating bronchiolitis, pulmonary emphysema, pneumonitis, as well as silicosis and other pneumonic diseases - occupational diseases of the respiratory tract;
• adult respiratory distress syndrome;
• reduction of respiratory volume, including in cases of pneumosclerosis (often caused by chronic bronchitis); pulmonary atelectasis and chronic bronchiectasis; heart problems and some systemic diseases;
• sleep apnea, which negatively affects the balance of O2 and CO2 in the blood due to shallow and intermittent breathing; [ 1 ]
• decreased tone and/or elasticity of the diaphragm and intercostal respiratory muscles in myopathies of a dystrophic or neurological nature, for example, myasthenia, amyotrophic lateral sclerosis, Guillain-Barré syndrome.

Hypercapnia and stroke, traumatic brain injury and brain tumors may be etiologically related due to cerebral circulatory disorders and damage to the respiratory center of the medulla oblongata.

In addition, metabolic hypercapnia is also observed, caused by electrolyte imbalance (acid-base imbalance) during fever, hormonal disorders (hypercorticism, thyrotoxicosis), nephrological diseases (renal failure), metabolic alkalosis, and the development of sepsis. [ 2 ]

Hypercapnia in children can be caused by:

• congenital malformations of the bronchopulmonary system;
• respiratory distress syndrome in newborns;
• aspiration of the respiratory tract of newborns with amniotic fluid and meconium;
• persistent pulmonary hypertension of the newborn.

In premature infants, a lack of oxygen in the blood - hypoxemia and hypercapnia develop with bronchopulmonary dysplasia associated with prolonged artificial respiratory support (ALS). [ 3 ]

Risk factors

In addition to frequent infectious lung lesions – bronchopneumonia and pneumonia, as well as all chronic bronchopulmonary diseases, the risk of hypercapnia increases with:

• smoking;
• high degree of obesity (with excess weight with a BMI of more than 30-35, the breathing process becomes difficult);
• lung damage caused by inhalation of toxic substances or inhalation of air containing abnormally high concentrations of CO2;
• hypothermia (hypothermia);
• lung cancer;
• large doses of alcohol, overdose of opium derivatives (depressing central respiration);
• deformations of the chest, in particular, with curvature of the spine;
• autoimmune pathologies with systemic fibrosis (rheumatoid arthritis, cystic fibrosis, etc.);
• the presence of genetic abnormalities - congenital central hypoventilation or Ondine's curse syndrome.

Pathogenesis

During the process of cell metabolism, carbon dioxide is produced in their mitochondria, which then diffuses into the cytoplasm, intercellular space and capillaries – dissolving in the blood, that is, binding to the hemoglobin of erythrocytes. And the removal of CO2 occurs during respiration by gas exchange in the alveoli – diffusion of gas through the alveolar-capillary membranes. [ 4 ]

Normally (at rest), the respiratory volume is 500-600 ml; lung ventilation is 5-8 l/min, and the minute volume of alveolar ventilation is 4200-4500 ml.

Often equating hypercapnia, hypoxia and respiratory acidosis, physiologists associate the pathogenesis of increased partial pressure of carbon dioxide (PaCO2) in the blood with impaired ventilation – alveolar hypoventilation, the result of which is hypercapnia.

By the way, hypercapnia and acidosis are interconnected, since respiratory acidosis with a decrease in arterial blood pH is a violation of the acid-base balance with an increase in the content of carbon dioxide in the blood, which is caused by hypoventilation. It is respiratory acidosis that explains headaches, daytime sleepiness, tremors and convulsions, memory problems. [ 5 ]

But a decrease in the level of CO2 in the blood - hypocapnia and hypercapnia (i.e. its increase) - are diametrically opposed conditions. In this case, hypocapnia occurs with hyperventilation of the lungs. [ 6 ]

But let's return to the mechanism of hypercapnia development. During pulmonary ventilation, not all exhaled air (about a third) is freed from carbon dioxide, since some of it remains in the so-called physiological dead space of the respiratory system - the volume of air in its various segments, which is not immediately subject to gas exchange. [ 7 ]

Bronchopulmonary diseases and other factors cause disturbances in the pulmonary capillary bed and the structure of the alveolar tissue, reduce the diffusion surface and decrease alveolar perfusion, and also increase the volume of dead space, where the O2 level is low and the CO2 content is very high. And with the next respiratory cycle (inhalation-exhalation), carbon dioxide is not completely eliminated, but remains in the blood. [ 8 ]

For example, with chronic obstructive bronchitis, due to decreased alveolar ventilation, hypoxemia and hypercapnia are observed, that is, the level of oxygen in the blood decreases and the content of carbon dioxide increases.

Chronic hypercapnia with low O2 levels in the blood can occur in the absence of obvious causes, primarily from the respiratory system. In such cases, alveolar hypoventilation is associated with a disorder (most likely genetically determined) in the function of the central CO2 chemoreceptors in the medulla oblongata or chemoreceptors in the carotid bodies of the outer wall of the carotid artery. [ 9 ]

Symptoms hypercapnia

Slowly developing hypercapnia syndrome, or more precisely, alveolar hypoventilation syndrome, may be asymptomatic, and its first signs – headaches, dizziness, feeling of fatigue – are nonspecific.

Symptoms of hypercapnia may also include: drowsiness, flushing of the skin of the face and neck, tachypnea (rapid breathing), abnormal heart rate with arrhythmia, increased blood pressure, muscle spasms and asterix (oscillatory tremor of the hands), and fainting.

Dyspnea (shortness of breath) is observed quite often, although hypercapnia and shortness of breath may be indirectly related, since shallow but frequent breathing is observed in bronchopulmonary diseases (leading to deterioration of alveolar ventilation).

The clinical picture of severe hypercapnia is characterized by irregular heartbeat, seizures, confusion and loss of consciousness, disorientation, panic attacks. At the same time, if the brain and heart do not receive enough oxygen, there is a high risk of coma or cardiac arrest.

The emergency condition is acute hypercapnia or acute hypoxemic pulmonary failure.

Permissive hypercapnia refers to elevated partial pressure of CO2 due to hypoventilation in mechanically ventilated patients with lung injury due to acute respiratory distress syndrome or exacerbation of bronchial asthma. [ 10 ]

Complications and consequences

Moderate to severe hypercapnia can cause significant complications and negative consequences.

Hypercapnia and hypoxia lead to oxygen starvation of the body.

In addition, high levels of carbon dioxide in the blood cause an increase in cardiac output with a sharp increase in arterial and intracranial pressure; hypertrophy of the right ventricle of the heart (pulmonary heart); changes in the hormonal system, brain and central nervous system - with certain mental reactions and states of irritability, anxiety and panic.

And, of course, respiratory failure can suddenly occur, which can lead to death. [ 11 ]

Diagnostics hypercapnia

Since alveolar ventilation disorders have many causes, the patient's examination, medical history and complaints are supplemented by studies of the respiratory organs, the state of the respiratory muscles and cerebral circulation, the identification of hormonal and metabolic disorders, kidney pathologies, etc. Therefore, diagnostics may require the involvement of relevant narrow-profile specialists.

Blood tests are required for gas composition, pH level, plasma bicarbonate content, etc.

Instrumental diagnostics are performed: pulmonary spirometry, capnometry and capnography (determining the partial pressure of CO2 in arterial blood), X-ray examination of pulmonary function, EEC; if necessary - ultrasound and CT of other systems and organs

Differential diagnosis is aimed at determining the etiology of hypercapnia. [ 12 ]

Treatment hypercapnia

When the cause of hypercapnia is precisely known, treatment is aimed at the underlying bronchopulmonary disease and appropriate medications are prescribed.

First of all, these are bronchodilators: Alupent (Orciprenaline), Atrovent, Isadrin, Aerophyllin, Hexaprenaline, etc.

Physiotherapy is also widely used for obstructive bronchitis and COPD; for more details, see Physiotherapy for Chronic Obstructive Pulmonary Disease.

Benzomopine, Azamolin, Oliphen and other antihypoxants are prescribed for oxygen deficiency. Thus, the drug Oliphen (tablets and injection solution) is contraindicated for patients with cerebral circulation disorders, and its side effects are limited to allergic urticaria and moderate arterial hypotension. [ 13 ], [ 14 ]

Mechanical ventilation in hypercapnia (with endotracheal intubation) is necessary in cases of acute respiratory failure. And to improve gas exchange and prevent breathing problems and hypoxemia, non-invasive positive pressure ventilation (in which oxygen is supplied through a face mask) is used. [ 15 ]

Prevention

To avoid hypercapnia it is necessary:

• quit smoking and limit alcohol consumption;
• get rid of extra pounds;
• treat bronchopulmonary diseases in a timely manner, preventing them from becoming chronic, and also monitor the condition in the presence of systemic and autoimmune pathologies;
• Avoid inhaling toxic gases
• maintain muscle tone (through regular exercise and, if possible, sports activities).

Forecast

Hypercapnia has a variable prognosis, which depends on its etiology. And the younger the patient, the better. [ 16 ]

And with severe hypercapnia, dysfunction of the respiratory system, cessation of cardiac activity and death of brain cells from lack of oxygen are a very real threat.