Experimental Determination of the Apparent Value for the First Dissociation Constant (<scp>pK</scp> 1 ′) of the Plasma Carbonic Acid – Bicarbonate Buffer System in 115 Trained Standardbreds

• Published in: Equine Veterinary Journal
• Main Authors: Constable, P, Tinkler, S, Demaree, A, Couetil, L
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2014
• Subjects: General Medicine; Carbonic acid
• Online Access: http://dx.doi.org/10.1111/evj.12267_62; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fevj.12267_62; http://onlinelibrary.wiley.com/wol1/doi/10.1111/evj.12267_62/fullpdf

Introduction Calculating the total plasma concentration of carbon dioxide ( ctCO 2 ) from the results of blood gas and pH analysis assumes that the value for pK 1 ′ is constant and approximates 6.095. The objective of this study was to verify both assumptions in equine plasma. Methods Duplicate jugular blood samples were collected pre‐race from 115 trained Standardbreds into 3 mL partially evacuated plastic tubes containing lithium heparin. Blood pH ( pHb ) and partial pressure of CO 2 ( pCO 2 ) were measured in one tube using a blood gas and pH analyser (Radiometer ABL ‐700). The second tube was anaerobically centrifuged and ctCO 2 was measured (Beckman EL ‐ ISE analyser). Plasma pK 1 ′ was calculated for each horse using the Henderson‐Hasselbalch equation, measured values for pHb , pCO 2 , and plasma ctCO 2 , and the value for the solubility of CO 2 in plasma at 37C ( S = 0.0307 {mmol/ L }/mmHg) whereby: pK 1 ′ = ( pHb + 0.01)–log10({ ctCO 2 ‐ S × pCO 2 }/{ S × pCO 2 }), with the term ( pHb + 0.01) representing the pH in plasma. The calculated value for pK 1 ′ was standardized to pHb = 7.40 using the Severinghaus equation such that d pK 1 ′/ dpH = −0.044. Non‐parametric statistical procedures were used to determine the median value and 95% confidence interval ( CI ) for pK 1 ′. Results The median value for pK 1 ′ at pHb = 7.40 was 6.097, and the 95% CI for pK 1 ′ was 6.063–6.123. Conclusions The 95% CI for pK 1 ′ in equine plasma from trained Standardbreds is narrow and includes the assumed fixed value of 6.095. Our findings support the use of blood gas and pH analysis to calculate plasma ctCO 2 in order to detect the pre‐race administration of alkalinizing agents in horses. Ethical Animal Research The study was approved by the Institutional Animal Care and Use Committee at Purdue University. Sources of funding: Supported, in part, by a grant from the Indiana Horse Racing Commission. Competing interests: none.