“Metabolic indices” of blood acid-base status, such as base excess (BE) and buffer base (
BB
), are usually estimated by Siggaard-Andersen's nomographic method based on the measured pH of blood under preset pCO
2. Although many attempts at automating the nomogram reading have been made by numerically simulating the BE curve, the theoretical background of the hyperbolic functions of BE and
BB
had not been clearly presented.
On the assumption of two-compartmental model of blood in which both compartments, red. blood cells (RBC) and plasma, have linear buffer slopes of acid-base balance and the RBC membranes are selective in permeating HCO
3- and H
+, the relation of [HCO
3-] and pH of plasma Phase in blood have been derived :
[HCO
3-] =- ((1-Ht) k
p+Htk
c′) (pH-7.4) +24 (Ht (a-1-7.4b) +1)+ [BE] / (1-Ht) +Ht (a-bpH)
where Ht is hematocrit, k
p, k
c', linear buffer slopes of RBC and plasma; a, b, partition coefficients of [HCO
3-] between plasma and RBC. Explicit hyperbolic functions of BE and
BB
curves have been derived analytically from this equation. The predicted and the observed curves fit fairly well using the parameter values estimated on the least square principle.
The relatively simple formulae of the blood “metabolic indices” which are directly derived from this equation can be as accurate as any of the complex computer algorithms of simulating the nomographic reading.
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