Blood gases - arterial blood

1 mL arterial blood collected anaerobically in a heparinised syringe and transported rapidly to the laboratory in a capped syringe with the needle removed.

An appropriately collected 'arterialised' capillary blood specimen may have values close to those of arterial blood.

Fractional concentration of inspired O₂  [FiO₂] should be specified on request form.

FiO₂ and patient’s temperature must be known.

Decreased pO₂ is seen with hypo­ventilation, ventilation/perfusion mismatch, alveolar-capillary block and right to left shunts.

Increased pO₂ may be seen with hyperventilation or oxygen therapy.

Decreased pCO₂ (respiratory alkalosis) is usually a compensatory phenomenon in metabolic acidosis, but may be a primary abnormality; in both situations it is due to hyperventilation.

Increased pCO₂ (respiratory acidosis) occurs in respiratory failure, but is also seen as a compensatory phenomenon, caused by hypoventilation, in metabolic alkalosis.

Decreased pH indicates a net acidaemia and increased pH indicates a net alkalaemia. The acid-base balance component (viz metabolic or respiratory) that is in the same direction as the pH is the primary abnormality in acid-base imbalance.

Base excess is decreased in metabolic acidosis and compensated respiratory alkalosis.

It is increased in metabolic alkalosis or compensated respiratory acidosis.

Alveolar-arterial pO₂ difference is elevated in all causes of hypoxia except hypoventilation. The reference interval is defined only for room air.