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 change 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.