Interpreting the acid-base status in clinical healthy and ill horses with fluid therapy: the diagnostic potential of the Stewart variables

Objectives: Comparison of the acid-base status with the Henderson-Hasselbalch-(HH) equation and Stewart-variables (PCO2, [strong ion difference = SID] and [Acid total = A(tot)]). Material and methods: Arterial and venous blood of 38 healthy horses was taken to establish the reference values of the Stewart variables. Two clinical cases are presented additionally. Patient 1, a 16-year-old gelding with chronic respiratory illness was treated with isotonic NaCl-solution. Patient 2, a 9-year-old gelding with colon obstruction and signs of a hypovolemic shock received Ringer's solution. Before, during and after the infusion blood was taken and all acid-base-parameters were determined within 30 min. Results: Reference intervals ((x) over bar +/- 1.96.s) for the Stewart variables for the healthy horses were: PvCO(2) = 5.0-7.3 kPa, serum-[SID3] = 38.5-45.1 mmol/l, -[SID4] = 37.7-44.2 mmol/l, -[A(tot1)] = 11.7-17.3 mmol/l, -[A(tot2)] = 11.5-15.4 mmol/l. The pH calculated with the Stewarts variables had an alcalotic difference of 0.06 compared to the measured blood-pH. Patient 1 showed no change in acid-base status. In the 3-hour period following the begin of hyperinfusion with 30 L isotonic NaCl-solution venous blood parameters changed: down arrow pH, down arrow[HCO3-], down arrow[BE] (= HH) and down arrow[SID3,4], down arrow[A(tot1,2)] (= Stewart). The transient mild acidosis can be explained with the low Stewart variables of the unphysiologically 0.9% NaCl-solution with [SID3,4] = 0 mmol/l and [A(tot1,2)] = 0 mmol/l. Before receiving treatment, the horse suffering from a colic (patient 2) displayed a normal pH with down arrow[HCO3-], down arrow[BE], down arrow PCO2, down arrow[SID4] = metabolic acidosis with respiratory compensation. Within an hour after infusion of 12 L Ringer's solution ([SID3,4] = -4.5 mmol/l, [A(tot1,2)] = 0 mmol/l), both [SID3,4] and [A(tot1,2)] were decreased, representing acidotic and alkalotic reactions, respectively. Clinical relevance: Disturbances of acid-base status can be diagnosed acurately on the bases of HH parameters with regard to acidemia or alkalemia as well as respiratory imbalances. In order to determine the cause of metabolic acidosis and alkalosis the Stewart parameters [SID] and [A(tot)] are essential indicators and allow discrimination between a disturbance caused by ions or protein concentrations and allows therefore a more informed choice from effective correction solutions with regard to fluid therapy.