Determination of reference intervals for equine arterial blood-gas, acid-base and electrolyte analysis

To establish reference intervals for arterial blood-gas (ABG), acid-base and electrolyte values from a healthy equine population.

Retrospective clinical study.

A total of 139 client-owned, systemically healthy horses, 1 year of age and older, presented for elective surgical procedures.

Blood samples were collected anaerobically from the transverse facial or common carotid artery of horses breathing room air, prior to administration of preanaesthetic medication. Samples were analysed immediately, without correction for body temperature, using an automated bench-top analyser. Variables analysed included pH, arterial partial pressure of carbon dioxide (PaCO₂) and arterial partial pressure of oxygen (PaO₂) and plasma concentrations of sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺) and chloride (Cl^-). Actual and standardized plasma bicarbonate concentration [HCO₃^- (P) and HCO₃^- (P, st)], blood and extracellular fluid base excess [base (B) and base (ECF)] and anion gap (AG) were calculated by the machine from preprogrammed algorithms. Methods used for determination of PaCO₂, PaO₂, HCO₃^- (P), HCO₃^- (P, st), base (B) and base (ECF) met the guidelines of the Clinical and Laboratory Standards Institute. Reference intervals were determined with the nonparametric or the standard parametric method dependent on data distribution.

Reference intervals were determined for pH, 7.37-7.49; PaCO₂, 4.84-7.20 kPa (36.3-54.0 mmHg); PaO₂, 11.01-14.97 kPa (82.6-112.3 mmHg); Na⁺, 133-141 mmol L^-1; K⁺, 3.05-4.65 mmol L^-1; Ca²⁺, 1.34-1.72 mmol L^-1; Cl^-, 100-110 mmol L^-1; HCO₃^- (P), 23.55-33.90 mmol L^-1; HCO₃^- (P, st), 23.87-32.45 mmol L^-1; base (B), 0.51-8.80 mmol L^-1; base (ECF), -0.53 to 9.39 mmol L^-1 and AG, 1.5-11.5 mEq L^-1.

These data were derived from the largest group of horses reported in a single study and may aid in interpretation of ABG, acid-base and electrolyte measurements in clinical practice.