Bicarbonate & Base Excess

All calculations should be independently verified prior to clinical use. These calculators are intended to supplement, not replace, clinical judgment.

Calculate the bicarbonate concentration (HCO₃⁻) and base excess (B.E.) from arterial pH and P_aCO₂.

P_aCO₂ torr

HCO₃⁻ mEq/L

Base excess mEq/L

The calculations are based on the Henderson-Hasselbalch equation:

pH = pK + log ( [HCO₃⁻] / [CO₂] )

Substituting pK = 6.1 and [CO₂] = 0.03 × pCO₂, and solving for bicarbonate:

HCO₃⁻ = 0.03 × pCO₂ × 10^{(pH − 6.1)}

Base excess is then derived from the Siggaard-Andersen equation:

B.E. = 0.9287 × ( HCO₃⁻ − 24.4 + 14.83 × (pH − 7.4) )

≡ 0.9287 × HCO₃⁻ + 13.77 × pH − 124.58

Combining these two, the base excess can be expressed directly in terms of pH and pCO₂:

B.E. = 0.02786 × pCO₂ × 10^{(pH − 6.1)} + 13.77 × pH − 124.58

For a comprehensive tutorial on acid-base balance, see the online resource by Dr. Alan W. Grogono, MB, BS, MD, FRCA (Retired Professor and former Chair, Department of Anesthesiology, Tulane University): acid-base.com.

References

Siggaard-Andersen O. An acid-base chart for arterial blood with normal and pathophysiological reference areas. Scand J Clin Lab Invest. 1971;27(3):239–245.
Hasselbalch KA. Die Berechnung der Wasserstoffzahl des Blutes aus der freien und gebundenen Kohlensäure desselben, und die Sauerstoffbindung des Blutes als Funktion der Wasserstoffzahl. Biochem Z. 1917;78:112–144.
Grogono AW. Acid-base balance [online tutorial]. Available at: http://www.acid-base.com/. Tulane University School of Medicine.