Standard Bicarbonate and Base Excess (Buffer Base)

1. By Mohammadi M.
Master of Science (Medical Physics)
Department of Medical Physics, KUMS

2.  Derived from pH and pCO2 using the Henderson-Hasselbach
equation.
 Normal range 21 - 26 mmol.
 H-H equation: pH = 6.1 + log [HCO3
-]/ 0.03 pCO2
 6.1 is the pKa (negative logarithm of the acid dissociation
constant) for carbonic acid (H2CO3)
 0.03 is the factor which relates pCO2 to the amount of CO2
dissolved in plasma.

3.  The calculated concentration of HCO3
- if the sample it were
at 'standard conditions‘.
 At a temperature of 37°C and a pCO2 of 5.3 kPa (40mmHg;
the middle of the pCO2 reference range).
 By assuming a pCO2 of 5.3kPa, the standard bicarbonate
compensates for any abnormality in HCO3
- caused by a
respiratory cause.

4.  Base excess and base deficit refer to an excess or deficit,
respectively, in the amount of base present in the blood.
 Usually reported as a concentration in units of mEq/L
(milliequivalents of solute per litre of solvent).
 Positive numbers indicating an excess of base and negative
a deficit.
 A typical reference range for base excess is −2 to +2 mEq/L.

5.  Base excess can be estimated from the serum bicarbonate
concentration ([HCO3
−]) and pH by the equation:
 The same can be alternatively expressed as:

6.  Base excess beyond the reference range indicates:
 metabolic alkalosis if too high (more than +2 mEq/L)
 metabolic acidosis if too low (less than −2 mEq/L)