Altered mental status and an acid-base disturbance January 2017

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TABLE 2

‘Rules of 5’ for acid-base problem-solving

1 Determine the arterial pH status pH < 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	Pco₂	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) Pco₂ and bicarbonate should move in the same direction
Nominal normal levels: bicarbonate 25 mmol/L and Pco₂ 40 mm Hg
In respiratory acidosis, for every 10-mm Hg increase in Pco₂, 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, Pco₂ should decrease by 1.3 mm Hg
In respiratory alkalosis, for every 10-mm Hg decrease in Pco₂, 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, Pco₂ 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