EVALUATION OF ACID-BASE STATUS

Condition

pH_a < 7.37 => Acidemia (or Acidosis)

pH_a> 7.43 => Alkalemia (or Alkalosis)

Respiratory Contribution

P_aCO₂ > 43 mmHg => contribute Acidosis

P_aCO₂< 37 mmHg => contribute Alkalosis

Metabolic Contribution

[HCO₃^- ] < 21 mEq/L => contribute to Acidosis
[HCO₃^- ] > 27 mEq/L => contribute to Alkalosis

Note:  A more accurate index of the metabolic contribution would be Base Excess

Base Excess < -3 (negative BE) => contribute to Acidosis

Base Excess > +11 => contribute to Alkalosis

Base Excess can be calculated from the following:

H⁺ = ΔMetabolic + ΔBicarbonate + ΔBuffer

and

ΔBicarbonate = [ HCO₃^- ]_a - 24 meq/liter

ΔBuffer = Buffer capacity x (pH - 7.40)

Note that H⁺ very small compared to the others, so

–ΔMetabolic = ΔBicarbonate + ΔBuffer

but

–ΔMetabolic = Base Excess (BE)

so

Base Excess = [ HCO₃^- ]_a – 24 + Buffer capacity x (pH - 7.40)

For acute cases, most buffering is interstital and blood, for which buffer capacity is relatively low, so

Base Excess = [ HCO₃^- ]_a – 24 (approximate)

which gives a measure of the metabolic contribution to the acidosis