The effect of serial administration of bicarbonate on plasma total CO₂ concentrations in horses

The administration of alkalinising agents including bicarbonate is of concern to racing authorities because resultant alkalosis may enhance performance and interfere with the detection of drugs in post-race urine. A threshold for total carbon dioxide (TCO₂ ) of 36.0 mmol/L in plasma (with action limit of 37.0 mmol/L) has been set. Serial dosing of sodium bicarbonate has gained popularity in human athletes but has not been studied in horses previously. Sodium bicarbonate (200 g per horse) and 60 g of an electrolyte-vitamin complex was administered in 2-L water via nasogastric intubation to five Standardbred horses for three consecutive days (total dose bicarbonate 0.42 ± 0.02 g/kg). Serial blood samples were taken over Days 1-5, with the final day (5) intended to simulate a 'clear day', and TCO₂ was analysed. Following the first bicarbonate administration, plasma TCO₂ peaked at 6 h (34.8 ± 1.3 mmol/L), returning to baseline by 23 h. On Day 2, four out of the five horses showed a peak greater than 36.0 mmol/L (mean 37.0 ± 2.1 mmol/L). With daily repeated dosing, plasma TCO₂ peaked progressively earlier, and by Day 3, the peak occurred at 2 h and concentrations declined more rapidly. On Days 4 and 5, TCO₂ levels remained low (<32.1 mmol/L on Day 4 and between 27.0-31.2 mmol/L on Day 5). These studies demonstrate that serial dosing of a 'split dose' of sodium bicarbonate on three consecutive days does not result in the accumulation or carry-over of plasma TCO₂ levels beyond the levels observed following a single dose.