base excess and bicarbonate

base excess is the central term in the Siggaard-Andersen approach to acid-base analysis, and bicarbonate plays the main character within the "physiological approach".

you can read more about these written by their own protagonists on:
the Siggaard-Andersen homepage
and an excellent website about the BE approach (event though very critical of the Stewart approach .... ): Alan W. Grogono's homepage.
the bicarbonate approach, also called the "physiological approach", is well described by Adrogué, even though he has little good to say about our favourite theory, either:
a recent article by Adrogué
Adrogué HJ, Gennari FJ, Galla JH, Madias NE; Assessing acid-base disorders; Kidney Int 2009 Oct 7

quoting James Figge from page 224 in "Stewart's Textbook of Acid-Base", advertised on this website:
"Base Excess is defined by Siggaard-Andersen as the negative value of the concentration of titrable hydrogen in blood or plasma." the formula is given as:
BE = [HCO3-] - [HCO3-]0 + ßpl*(pH-pH0)

[HCO3-]0 and pH0 are the reference values at 37oC, i.e. 24.5mEq/l and 7.4.
ßpl is the buffering capacity of non-bicarbonate buffers in plasma, 7.7mEq/l.

the crucial point with this is, that it assumes normal values for albumin and phosphate. as long as this condition is met, the Stewart approach gives results completely equivalent to the Siggaard-Andersen method.

not many of our ICU-patients do meet this condition -
BUT: the base excess approach is the only one taking into account the modifying ("buffering") effects of both the red blood cell intracellular fluid and the interstitial fluid, giving you an idea about how much you have to change the SID to normalise the pH (if you elect to do so) - with the same caveat, though, that it assumes normal values for the weak acids and for the volume ratios between these different fluid spaces, too.
not all is dismal disagreement, though!
on the Siggaard-Andersen homepage you will find the complete version of the "van Slyke equation":
the "van Slyke equation"

this actually includes a way of calculating albumin's contribution to the acid-base behaviour of plasma:
quoting from that page:
"If the albumin concentration (cAlb) is known, the buffer value of non-bicarbonate buffers in plasma may be expressed as a function of cAlb: ßP = ßPo + ßmAlb ยท (cAlb - cAlbo)"

the same page even offers a way of unifying the concepts of "buffer base" and SID:

"Buffer base (BBˉ) is defined as the concentration of buffer anions minus the concentration of buffer cations (the latter being virtually zero at physiological pH). In plasma or whole blood, buffer base therefore is the sum of HCO3ˉ, net albumin anion, and phosphate.

Strong ion difference (SID) is defined as the concentration of non-buffer cations minus the concentration of non-buffer anions. SID of plasma therefore is the sum of the concentrations of  Na+, K+, Ca2+, Mg2+ minus the sum of the concentrations of Clˉ, SO2+, and certain organic anions, which also represent non-buffer anions at physiological pH.

BBˉ and SID are obviously numerically equal, because the sum of all cations must equal the sum of all anions (law of electroneutrality)."

further reading

you can find more information about BE and other acid-base related matters at Radiometer's website (obviously, not a website dedicated to Stewart's physiochemical approach .... ).

consult the glossary for other aspects of acid-base equilibria and the rules and mathematics behind our website:     Glossary