Use of a Quantitative Strong Ion Approach to Determine the Mechanism for Acid—Base Abnormalities in Sick Calves with or without Diarrhea

摘要

Acid-base abnormalities are frequently present in sick calves. The mechanism for an acid-base disturbance can be characterized using the strong ion approach, which requires accurate values for the total concentration of plasma nonvolatile buffers ( A _tot ) and the effective dissociation constant for plasma weak acids ( K _a ). The aims of this study were to experimentally determine A _tot , K _a , and net protein charge values for calf plasma and to apply these values quantitatively to data from sick calves to determine underlying mechanisms for the observed acid-base disturbance. Plasma was harvested from 9 healthy Holstein-Friesian calves and concentrations of quantitatively important strong ions (Na+, K+, Ca ²⁺ , Mg ²⁺ , Cl ^- , L-lactate) and nonvolatile buffer ions (total protein, albumin, phosphate) were determined. Plasma was tonometered with CO ₂ at 37°C, and plasma PCO ₂ and pH measured over a range of 15–159 mm Hg and 6.93-7.79, respectively. Strong ion difference (SID) was calculated from the measured strong ion concentrations, and nonlinear regression was used to estimate values for A _tot and K _a from the measured pH and PCO ₂ and calculated SID. The estimated A _tot and K _a values were then validated using data from 2 in vivo studies. Mean (± SD) values for calf plasma were A _tot = 0.343 mmol/g of total protein or 0.622 mmol/g of albumin; K _a = (0.84 ± 0.41) × 10 ^-7 ; pK _a = 7.08. The net protein charge of calf plasma was 10.5 mEq/L, equivalent to 0.19 mEq/g of total protein or 0.34 mEq/g of albumin. Application of the strong ion approach to acid-base disturbances in 231 sick calves with or without diarrhea indicated that acidemia was due predominantly to a strong ion acidosis in response to hyponatremia accompanied by normochloremia or hyperchloremia and the presence of unidentified strong anions. These results confirm current recommendations that treatment of acidemia in sick calves with or without diarrhea should focus on intravenous or PO administration of a fluid containing sodium and a high effective SID.