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🌱 來自: Huppert’s Notes

Acid–Base Disorders

First, a reminder of the normal values:

pH = 7.4 ± 0.05

PaCO2= 40 ± 5 mmHg

HCO3- = 24 mEq/L (typically normal range 22–30 mEq/L)

Normal Anion Gap = Albumin x 2.5 (so typically 8–12 is normal)

Example Case:

pH 7.19, PaCO2 26, HCO3– 9, anion gap 17

Follow these steps:

•   Check pH: Acidemia pH <7.4, Alkalemia pH >7.4

-   Example: 7.19 = Acidemia

•   Check PaCO2: If the same direction as pH (either both high or both low) then primary metabolic process is present; if they are going in the opposite directions, then primary respiratory process is present

-   Example: PaCO2 = 26, so both pH and PaCO2 are low, indicating a primary metabolic acidosis

•   Check compensation: Respiratory compensation occurs in hours, whereas metabolic compensation takes days

-   Example: Since we have a primary metabolic acidosis, use Winter’s formula to check compensation. Expected PaCO2 = 1.5(HCO3–) + 8 ± 2 … so expected PaCO2 = 1.5(9) + 8 = 21… but in this case the actual PaCO2 was 26. If the actual PaCO2 is higher than calculated PaCO2 then the patient has a combined metabolic acidosis + respiratory acidosis.

•   What is the anion gap? Anion gap = Na+ – (Cl- + HCO3–). The expected anion gap = 2.5 × albumin.

-   Example: Our patient has an anion gap of 17, which is elevated, so there is a metabolic anion gap acidosis here

•   Check the “gap-gap” (delta anion gap/delta HCO3–)? Compare change in anion gap (calculated anion gap – expected anion gap) to the change in HCO3– (24 – measured HCO3–). Anion gap change/bicarb change should be between 1 and 2 if only an anion-gap metabolic acidosis is present.

-   If anion gap change/bicarb change <1 (bicarb decreased more than expected) → There is also a normal anion gap acidosis lowering the bicarb

-   If anion gap change/bicarb change >2 (bicarb decreased less than expected) → There is also metabolic alkalosis raising the bicarb

-   Example: Here, the delta-delta was: (calculated AG 17 – expected AG of 10) / (24 – HCO3– of 9) = 7/15 = 0.45. Therefore, the ratio is < 1, indicating a concurrent normal anion gap acidosis.

-   Thus: We have a mixed gap/nongap metabolic acidosis and respiratory acidosis (triple acid–base problem)