Felis ISSN 2398-2950

Hypercapnia

Synonym(s): Hypercarbia (elevated plasma bicarb (>26mEq/L

Contributor(s): Graham Bilbrough, Rondeau Mark

Introduction

  • Hypercapnia is an elevation above normal of the arterial pressure of carbon dioxide (CO2) ( PaCO2 >45 mm Hg).
  • Most cases of hypercapnia are a result of hypoventilation. Sequelae of prolonged hypercapnia include respiratory acidosis Acid base imbalance  and increased intracranial pressure Intracranial pressure measurement.  Mechanical ventilation may be required to correct hypercapnia until the underlying condition can be corrected.

Pathogenesis

Etiology

  • Hypercapnia is almost exclusively a result of hypoventilation, which can be a result of numerous underlying disease states.

Predisposing factors

General

  • Chronic pulmonary disease.
  • CNS or neuromuscular disease.
  • Prolonged hypoxemia which may result in respiratory fatigue.
  • Respiratory depressant drugs.

Pathophysiology

  • Carbon dioxide (CO2) metabolism:
    • Produced by cellular metabolism.
    • In the bloodstream CO2 combines with water to form carbonic acid, which dissociates to form hydrogen and bicarbonate ions.  While the hydrogen ions combine with intracellular buffers such as Hb, the bicarbonate moves into the extracellular space.  Metabolically produced CO2 is carried in the bloodstream primarily as bicarbonate while effecting little change on the extracellular pH.
    • The reverse process results in CO2 excretion from the RBC when Hb is oxygenated, thus ventilation allows for removal of CO2 from the bloodstream.
  • Control of Ventilation:
    •  Main stimuli to respiration are decreased arterial PO2 (hypoxemia) and increased arterial PCO2 (hypercapnia).
    • CO2 is the major stimulus via effects on chemoreceptors of the respiratory center in the medulla which respond to changes in CSF pH:
      • Ventilation will increase by 1-4 L/min for every 1 mm Hg rise in PCO2.
    • O2 plays a lesser role via effects on the chemoreceptors located in the carotid bodies:
      • Hypoxemia does not substantially promote ventilation until the PaO2 is less than 50-60 mm Hg.
  • Development of hypercapnia:
    • Decreased ventilation:
      • Most common reason for development of hypercapnia by far.
    • Increased CO2 production:
      • Increased metabolic CO2 production may occur with hyperthermia.
      • Increased inspired CO2 is usually an anesthetic accident.

Diagnosis

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Treatment

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Outcomes

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Further Reading

Publications

Refereed papers

Other sources of information

  • King L G (2004) Textbook of Respiratory Disease in Dogs and Cats. Saunders, St. Louis.
  • West J B  (2000) Respiratory Physiology The Essentials. 6th edn. Lippincott Williams & Wilkins, Baltimore.
  • Rose B D (1994) Clinical Physiology of Acid-Base and Electrolyte Disorders. 4th edn.  McGraw-Hill, Inc., New York.


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