ABSTRACT
Ventilation refers to the rate at which air is breathed in and out of the lungs. Alveolar ventilation is a parameter of fundamental importance, as it is the primary determinant of arterial CO2 concentration. Although each breath contains around 500 mL, approximately 150 mL do not reach the alveoli but remain within the dead space, which is comprised of the trachea and major airways. Because of the very large volume of CO2 held buffered in the body, it takes 20-30 minutes for a new steady state to be reached after a change in ventilation. There is increasing interest in use of venous blood gases for monitoring of PCO2 and pH in patients with acute respiratory failure, as these values correlate somewhat to the respective arterial values. Hypoventilation of gas-exchanging alveoli is the commonest mechanism of hypercapnia. Although chronic obstructive pulmonary disease is the commonest cause of hypercapnia, familiarity with this condition frequently leads doctors to miss other causes of hypercapnia.
