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Calculation of the total plasma concentration of nonvolatile weak acids and the effective dissociation constant of nonvolatile buffers in plasma for use in the strong ion approach to acid-base balance in cats

Sheila M. McCullough MS, DVM1 and Peter D. Constable BVSc, PhD2
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  • 1 Department of Veterinary Clinical Medicine, College of Veterinary Medicine, University of Illinois, 1008 W Hazelwood Dr, Urbana, IL 61802.
  • | 2 Department of Veterinary Clinical Medicine, College of Veterinary Medicine, University of Illinois, 1008 W Hazelwood Dr, Urbana, IL 61802.

Abstract

Objective—To determine values for the total concentration of nonvolatile weak acids (Atot) and effective dissociation constant of nonvolatile weak acids (K a) in plasma of cats.

Sample Population—Convenience plasma samples of 5 male and 5 female healthy adult cats.

Procedure—Cats were sedated, and 20 mL of blood was obtained from the jugular vein. Plasma was tonometered at 37oC to systematically vary PCO2 from 8 to 156 mm Hg, thereby altering plasma pH from 6.90 to 7.97. Plasma pH, PCO2, and concentrations of quantitatively important strong cations (Na+, K+, and Ca2+), strong anions (Cl, lactate), and buffer ions (total protein, albumin, and phosphate) were determined. Strong ion difference was estimated from the measured strong ion concentrations and nonlinear regression used to calculate Atot and Ka from the measured pH and PCO2 and estimated strong ion difference.

Results—Mean (± SD) values were as follows: Atot = 24.3 ± 4.6 mmol/L (equivalent to 0.35 mmol/g of protein or 0.76 mmol/g of albumin); Ka = 0.67 ± 0.40 × 10–7; and the negative logarithm (base 10) of Ka (pKa) = 7.17. At 37oC, pH of 7.35, and a partial pressure of CO2 (PCO2) of 30 mm Hg, the calculated venous strong ion difference was 30 mEq/L.

Conclusions and Clinical Relevance—These results indicate that at a plasma pH of 7.35, a 1 mEq/L decrease in strong ion difference will decrease pH by 0.020, a 1 mm Hg decrease in PCO2 will increase plasma pH by 0.011, and a 1 g/dL decrease in albumin concentration will increase plasma pH by 0.093. (Am J Vet Res 2003;64:1047–1051)

Abstract

Objective—To determine values for the total concentration of nonvolatile weak acids (Atot) and effective dissociation constant of nonvolatile weak acids (K a) in plasma of cats.

Sample Population—Convenience plasma samples of 5 male and 5 female healthy adult cats.

Procedure—Cats were sedated, and 20 mL of blood was obtained from the jugular vein. Plasma was tonometered at 37oC to systematically vary PCO2 from 8 to 156 mm Hg, thereby altering plasma pH from 6.90 to 7.97. Plasma pH, PCO2, and concentrations of quantitatively important strong cations (Na+, K+, and Ca2+), strong anions (Cl, lactate), and buffer ions (total protein, albumin, and phosphate) were determined. Strong ion difference was estimated from the measured strong ion concentrations and nonlinear regression used to calculate Atot and Ka from the measured pH and PCO2 and estimated strong ion difference.

Results—Mean (± SD) values were as follows: Atot = 24.3 ± 4.6 mmol/L (equivalent to 0.35 mmol/g of protein or 0.76 mmol/g of albumin); Ka = 0.67 ± 0.40 × 10–7; and the negative logarithm (base 10) of Ka (pKa) = 7.17. At 37oC, pH of 7.35, and a partial pressure of CO2 (PCO2) of 30 mm Hg, the calculated venous strong ion difference was 30 mEq/L.

Conclusions and Clinical Relevance—These results indicate that at a plasma pH of 7.35, a 1 mEq/L decrease in strong ion difference will decrease pH by 0.020, a 1 mm Hg decrease in PCO2 will increase plasma pH by 0.011, and a 1 g/dL decrease in albumin concentration will increase plasma pH by 0.093. (Am J Vet Res 2003;64:1047–1051)