1 Determine the arterial pH statuspH < 7.40 is acidemic, pH > 7.44 is alkalemic But a normal pH does not rule out an acid-base disorder | |||

2 If the arterial pH is abnormal, determine whether the primary process is respiratory, metabolic, or both | |||

pH | Pco_{2} | Bicarbonate | |

Respiratory acidosis | Low | High | — |

Metabolic acidosis | Low | — | Low |

Mixed respiratory and metabolic acidosis | Low | High | Low |

Respiratory alkalosis | High | Low | — |

Metabolic alkalosis | High | — | High |

Mixed respiratory and metabolic alkalosis | High | Low | High |

3 Calculate the anion gap | |||

Anion gap = sodium – (chloride + bicarbonate) | |||

If serum albumin is low, add 2.5 mmol/L to the anion gap for every 1 g the serum albumin is below normal | |||

An anion gap > 10 mmol/L is elevated | |||

4 Check the degree of compensation (respiratory or metabolic) Pco_{2} and bicarbonate should move in the same direction | |||

Nominal normal levels: bicarbonate 25 mmol/L and Pco_{2} 40 mm Hg | |||

In respiratory acidosis, for every 10-mm Hg increase in Pco_{2}, bicarbonate should increase by 1 mmol/L in the first 48 hours and 4 mmol/L afterward | |||

In metabolic acidosis, for every 1-mmol/L decrease in bicarbonate, Pco_{2} should decrease by 1.3 mm Hg | |||

In respiratory alkalosis, for every 10-mm Hg decrease in Pco_{2}, bicarbonate should decrease by 2 mmol/L in the first 48 hours and by 5 mmol/L afterward | |||

In metabolic alkalosis, for every 1-mmol/L increase in bicarbonate, Pco_{2} may increase by 0.6 mm Hg | |||

5 If the patient has metabolic acidosis with an elevated anion gap, check whether the bicarbonate level has decreased as much as the anion gap has increased | |||

In metabolic acidosis, the anion gap should increase by the same amount that bicarbonate decreases; a difference in these two changes is called a delta gap |

Pco

_{2}= partial pressure of carbon dioxideBased on information in reference 1