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Normal values for both the non‐pregnant and the pregnant state are given in Table 5A.1. To interpret a blood gas, review the following:

 Check PaO2 (normal values 11–13 kPa ON AIR): if it is low, then the patient is hypoxaemic

 Check the pH value: to determine the direction of primary change (normal, acidosis or alkalosis); compensation is always incomplete

 Check PaCO2, which is determined by breathing (alveolar ventilation): a low PaCO2 (hyperventilation) indicates a respiratory alkalosis or respiratory compensation for a metabolic acidosis; a raised PaCO2 (hypoventilation) indicates respiratory acidosis – note that PaCO2 does not rise to compensate for a metabolic alkalosis

 Check standard bicarbonate (the bicarbonate value adjusted to what it would have been if the PaCO2 were normal): if the standard bicarbonate is raised then there is either a metabolic alkalosis or metabolic compensation for a respiratory acidosis; if the standard bicarbonate is low then there is either a metabolic acidosis or metabolic compensation for a respiratory alkalosis

 Check base excess: if it is negative then there is a metabolic acidosis; if it is positive then there is a metabolic alkalosis

Table 5A.1 Blood gases in non‐pregnant and pregnant women

	pH	PaCO2	Standard bicarbonate	Base excess
Normal values	7.34–7.44	4.7–6.0 kPa	21–27 mmol/l	–2 to +2 mmol/l
Values in pregnancy	7.40–7.46	3.7–4.2 kPa	18–21 mmol/l	No change
	Increased	Decreased	Decreased
Respiratory acidosis	↓	↑	↑	+ve	Hypoventilation leading eventually to compensatory renal retention of bicarbonate
Respiratory alkalosis	↑	↓	↓	–ve	Hyperventilation leading to renal excretion of bicarbonate
Metabolic acidosis	↓	↓	↓	–ve	Excess metabolic acid leading to respiratory hyperventilation to compensate Raised lactate in most types of shock
Metabolic alkalosis	↑		↑	+ve	Excess metabolic alkali but no respiratory compensation compensation