In the most severe and untreated cases, hypercapnia can lead to the death of the patient.

The term ‘hypercapnia’ is derived from the Greek hyper (over) and kapnos (smoke).

About hypercapnia: normal and pathological values of capnia

Normal values of capnia, i.e. the partial pressure of carbon dioxide in arterial blood (PCO2), are between 35 and 45 mmHg.

Hypercapnia occurs when PCO2 exceeds 45 mmHg.

  • mild hypercapnia: PCO2 between 45 and 60 mmHg
  • moderate hypercapnia: PCO2 between 60 and 90 mmHg
  • severe hypercapnia: PCO2 above 90 mmHg.

When PCO2 exceeds 100 mmHg, coma may occur and, above 120 mmHg, death.

PCO2 is measured by haemogas analysis.

Since carbon dioxide in the blood is in equilibrium with bicarbonate, hypercapnia can also cause a high plasma bicarbonate concentration (HCO3-).

Hypercapnia, hypoxaemia and hypoxia

Hypercapnia may also be associated with hypoxaemia (decreased oxygen content in the blood).

Hypoxemia occurs when the partial pressure of oxygen in arterial blood (PaO2) is below 55-60 mmHg and/or the oxygen saturation of haemoglobin (SpO2) is below 90%.

It should be remembered that oxygen saturation normally ranges between 97% and 99% in healthy subjects, whereas it may be physiologically lower in the elderly (around 95%) and severely lower (at or below 90%) in subjects with pulmonary and/or circulatory diseases.

Hypoxemia can lead to hypoxia (a decrease in tissue oxygen).

Hypercapnia and hypocapnia

Hypocapnia (or ‘akapnia’) refers to a reduced concentration of carbon dioxide in the blood and is the opposite of hypercapnia.

Hypocapnia occurs when PCO2 is below 35 mmHg.


Carbon dioxide is a waste product of cellular metabolic processes.

In body fluids, it dissolves and forms carbonic acid, which is eliminated from the lungs as carbon dioxide during exhalation.

If this mechanism becomes defective, the carbonic acid causes respiratory acidosis by accumulating in the blood.

An increase in partial pressure of carbon dioxide in the blood generally indicates respiratory insufficiency due to inadequate alveolar ventilation and is associated with hypoxia.

Causes and risk factors

Hypercapnia can be caused or promoted by various diseases and conditions, almost always related to pulmonary, cardiac and/or environmental causes, including:

  • hypoventilation
  • respiratory distress syndrome (ARDS);
  • pulmonary oedema;
  • pulmonary embolism;
  • pulmonary emphysema;
  • cerebral stroke;
  • acute myocardial infarction;
  • respiratory failure;
  • heart failure (decompensation);
  • unstable angina;
  • angina pectoris;
  • asthma;
  • aspergillosis;
  • pneumonia;
  • sepsis;
  • trauma;
  • head injuries;
  • bone fractures;
  • intoxication by drugs that suppress the respiratory centre;
  • diseases that cause weakness of the respiratory muscles (e.g. Guillain-Barré syndrome, myasthenia gravis and botulism);
  • febrile states;
  • Pickwick syndrome;
  • severe burns;
  • decreased state of consciousness;
  • chronic obstructive pulmonary disease (COPD);
  • cigarette smoking;
  • bronchiectasis;
  • bronchitis;
  • croup;
  • pulmonary heart
  • malignant hyperthermia;
  • hyperthyroidism;
  • severe obesity;
  • sleep apnoea;
  • drowning;
  • exposure to environments containing abnormally high concentrations of carbon dioxide or its exhalations.

Hypercapnia from oxygen therapy

Excessive oxygen administration (oxygen therapy) in some patients (e.g. those with COPD) can lead to oxygen therapy hypercapnia and hypercapnic respiratory failure, often associated with respiratory acidosis.

In a chronic COPD patient, hypoxemia is in fact a positive stimulus for his respiratory centres: administering oxygen at high flows for long periods of time can inhibit respiratory drive.

Patients with COPD and more severe hypoxaemia are therefore at high risk of CO2 retention following uncontrolled O2 administration.

The same phenomenon has been described in severe asthma, pneumonia, hypoventilation syndrome of the severely obese patient, however, all patients with chronic respiratory failure may be at risk of hypercapnia from oxygen therapy.

In these patients, an SpO2 target of 88-92% must be maintained to avoid hypercapnia.

Symptoms and signs

Symptoms and signs of hypercapnia generally become evident when PCO2 exceeds 60-70 mmHg.

Such symptoms and signs are:

  • reddening of the skin (cherry colour);
  • tachycardia (increased heart rate);
  • tachypnoea (increased respiratory rate) or bradypnoea (decreased respiratory rate);
  • dyspnoea (difficulty breathing);
  • arrhythmias;
  • extrasystoles;
  • muscle spasms;
  • reduced brain activity;
  • arterial hypertension;
  • increased cerebral blood flow;
  • headaches;
  • confusional state;
  • lethargy;
  • increased cardiac output.

Depending on the pathology or condition that caused the hypercapnia, other symptoms may be present.

If, along with hypercapnia, hypoxaemia also occurs, the following may also appear

  • cyanosis (bluish skin);
  • general malaise;
  • Cheyne-Stokes respiration;
  • apnoea;
  • ventricular fibrillation;
  • cardiac arrest;
  • coughing;
  • haemoptysis (emission of blood from the respiratory tract);
  • sweating;
  • asthenia (lack of strength);
  • hippocratic (drumstick) fingers.

Risks of severe hypercapnia

In the case of severe hypercapnia (due to breathing in air with a CO2 partial pressure of more than 10 kPa or 75 mmHg), symptoms progress to:

  • disorientation
  • panic;
  • hyperventilation;
  • convulsions;
  • loss of consciousness;
  • irreversible tissue damage;
  • coma and death in the most severe and untreated cases (with PCO2 above 100 – 120 mmHg).


The treatment of hypercapnia is dependent on the specific upstream cause that led to it.

Since the evolution of the condition can lead to life-threatening complications, in the event of manifestations suggesting that you or a loved one is suffering from hypercapnia, it is a good idea to head immediately for the emergency room or contact the emergency number, avoiding further waste of time and ‘do-it-yourself’ remedies that could even make the situation worse.

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