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

摘要

A comparison of the acid-base status with the Henderson-Hasselbalch (HH) equation and Stewart variables (PCO2, [strong ion difference=SID] and [acid total=Atot]) was made. Arterial and venous blood samples were taken from 38 healthy horses to establish the reference values of the Stewart variables. Two clinical cases were additionally presented. Patient I, a 16-year-old gelding with chronic respiratory illness, was treated with isotonic NaCl solution. Patient II, a 9-year-old gelding with colon obstruction and signs of a hypovolaemic shock, received Ringer's solution. Before, during and after the infusion, blood was taken and all acid-base parameters were determined within 30 min. Reference intervals (x=+or-1.96 s) for the Stewart variables in the healthy horses were the following: PvCO₂=5.0-7.3 kPa; serum-[SID3]=38.5-45.1 mmol/litre; [SID4]=37.7-44.2 mmol/litre; [Atot1]=11.7-17.3 mmol/litre; and [Atot2]=11.5-15.4 mmol/litre. The pH calculated with the Stewart variables had an alcalotic difference of 0.06 compared to the measured blood pH. Patient I showed no change in acid-base status. In the 3-h period following the beginning of hyperinfusion with 30 litres isotonic NaCl solution, the venous blood parameters changed: decreased pH; decreased [HCO3-]; decreased [BE] (=HH); decreased [SID3, 4]; and decreased [Atot1, 2] (=Stewart). The transient mild acidosis could be explained with the low Stewart variables of the 0.9% NaCl solution with [SID3, 4]=0 mmol/litre and [Atot1, 2]=0 mmol/litre. Before receiving treatment, the horse suffering from a colic (patient II) displayed a normal pH with decreased [HCO3-], [BE], PCO₂ and [SID4] and metabolic acidosis with respiratory compensation. Within an hour after infusion of 12 litres Ringer's solution ([SID3, 4] =-4.5 mmol/litre, [Atot1, 2]=0 mmol/litre), both [SID3, 4] and [Atot1, 2] were decreased, representing acidic and alkaline reactions, respectively. Disturbances of acid-base status could be diagnosed accurately on the basis of HH parameters with regard to acidaemia or alkalaemia as well as respiratory imbalances. In order to determine the cause of metabolic acidosis and alkalosis, the Stewart parameters [SID] and [Atot] were essential indicators and allowed discrimination between a disturbance caused by ions or protein concentrations and allowed therefore a more informed choice from effective correction solutions with regard to fluid therapy.