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Department of Veterinary Clinical Medicine, College of Veterinary Medicine, University of Illinois, Urbana, Illinois 61802
The strong ion approach
provides a quantitative physicochemical method for describing the
mechanism for an acid-base disturbance. The approach requires
species-specific values for the total concentration of plasma
nonvolatile buffers (Atot) and the effective dissociation constant for plasma nonvolatile buffers (Ka),
but these values have not been determined for human plasma.
Accordingly, the purpose of this study was to calculate accurate
Atot and Ka values using data
obtained from in vitro strong ion titration and CO2
tonometry. The calculated values for Atot (24.1 mmol/l) and
Ka (1.05 × 10
7) were
significantly (P < 0.05) different from the
experimentally determined values for horse plasma and differed from the
empirically assumed values for human plasma (Atot = 19.0 meq/l and Ka = 3.0 × 107). The derivatives of pH with respect to the
three independent variables [strong ion difference (SID),
PCO2, and Atot] of the strong ion
approach were calculated as follows: ![dpH/dSID<SUP>+</SUP> = [1 + 10<SUP>(p<IT>K</IT><SUB>a</SUB> − pH)</SUP>]<SUP>2</SUP>/(2.303 ×:{SP<SC>co</SC><SUB>2</SUB>10<SUP>(pH−p<IT>K</IT>′<SUB>1</SUB>)</SUP>[1 + 10<SUP>(p<IT>K</IT><SUB>a</SUB>−pH</SUP>]<SUP>2</SUP> + A<SUB>tot</SUB>10<SUP>(p<IT>K</IT><SUB>a</SUB>−pH)</SUP>});](/content/vol91/issue3/fulltext/1364/img001.gif)
![dpH/dP<SC>co</SC><SUB>2</SUB> = S10<SUP>−p<IT>K</IT>′<SUB>1</SUB></SUP>/{2.303[A<SUB>tot</SUB>10<SUP>pH</SUP>(10<SUP>pH</SUP> + 10<SUP>p<IT>K</IT><SUB>a</SUB></SUP>)<SUP>−2</SUP> − SID<SUP>+</SUP>10<SUP>−pH</SUP>]}](/content/vol91/issue3/fulltext/1364/img002.gif)
![dpH/dA<SUB>tot</SUB> = −1/{2.303[SP<SC>co</SC><SUB>2</SUB>10<SUP>(pH−p<IT>K</IT>′<SUB>1</SUB>)</SUP> + SID<SUP>+</SUP>10<SUP>(p<IT>K</IT><SUB>a</SUB>−pH)</SUP>]},](/content/vol91/issue3/fulltext/1364/img003.gif)
buffer value; plasma pH; strong ion difference; strong ion gap; anion gap
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