• horse;
  • electrolytes;
  • TCO2;
  • adrenaline;
  • rebreathing;
  • hyperpnoea


Pre-race testing for plasma bicarbonate is used in Australia to detect administration of large doses of bicarbonate or other alkalising agents. The rules specify upper limits for plasma TCO2 from 35 to 37 mmol/l, depending on State. There has been considerable speculation about physiological factors that may increase plasma total CO2 and lead to infringements of this rule. In order to address these purported influences, we measured body temperature (BT), heart rate (HR), haemoglobin concentration (Hb), packed cell volume (PCV), TCO2, potassium [K], sodium [Na], chloride [Cl] and breathing frequency (Fb) in 6 horses under the following conditions: rebreathing into a 3 litre bag for 90 s following respiratory stimulation with doxapram (0.5 mg/kg bwt i.v.); following excitement and during sweating induced by adrenaline (2.5 mg/kg bwt i.v.).

We also evaluated whether significant diurnal variation existed in plasma electrolytes. There was no diurnal variation in sodium but potassium and chloride peaked in the evening and were lowest at early morning. TCO2 changes were also significant with peak values occurring before noon. Rebreathing produced transient marginal increases in TCO2 and potassium, declines in chloride and no change in sodium. The rise in TCO2 was from 29.6 ± 0.8 to 30.9 ± 1.4 mmol/l. Animals became markedly agitated by 90 s of rebreathing. Doxapram caused a 2-fold increase in Fb, a small (<1 mmol/l) increase in TCO2, marginal increases in sodium and potassium, but no change in chloride. Adrenaline caused marked sweating and anxiety in all horses and led to increased HR, sodium, chloride, PCV and Hb, caused decreased TCO2 (<2 mmol/l), decreased chloride and had no consistent effect on BT.

From these studies, we conclude that rebreathing, hyperpnoea induced by doxapram and sweating and excitement induced by adrenaline, in otherwise healthy horses, exerts only marginal effects on TCO2. Differences were significant for all measured electrolytes between horses, with magnitudes exceeding changes induced by experimental protocols. Mean TCO2 for individual horses ranged from 26.5 ± 0.3 to 33.8 ± 0.2 mmol/l in the adrenaline trial with similar ranges in other studies.