South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
addition of HCO3- to the base fluid may be necessary. In general, supplemental HCO3- is not
needed unless serum tCO2 concentration (95% of which is HCO3-) falls below 15 mEq/L
(compared to a normal value of 25-30 mEq/L). To estimate the amount of HCO3- to add to the
base fluid, the base deficit (BD), a calculation made during blood gas analysis that provides a
measure of the decrease in HCO3- concentration, is multiplied by body mass and a fraction
representing the portion of body mass that is composed of fluid containing HCO3-.
Recommendations for the latter fraction range from 0.3-0.6 and the author uses a value of 0.5 to
represent the entire ECF as well as a portion of the ICF. As an example, the HCO3- deficit in a
500 kg horse with profuse diarrhoea and a BD of 14 mEq/L (serum tCO2 concentration of 13
mEq/L in comparison to a normal value of 27 mEq/L) would be estimated as (assuming that 1 kg
1 litre):
HCO3- deficit = 14 mEq/L (BD) x 500 kg x 0.5 = 3500 mE q HCO3- deficit.
This deficit can be corrected, over 6-12 hours, by addition of HCO3- to the base intravenous fluid
or by enteral administration of HCO3- via a nasogastric tube. It is important to remember that
HCO3- must be coupled with a cation, typically Na+; consequently, it is not possible to add HCO3without also adding Na+. Further, HCO3- should not be added to fluids containing Ca++ because
there is a slight risk of forming CaCO3 crystals that could precipitate in the fluids. Thus, NaHCO3,
available as either a 5% solution (1 ml = 0.6 mEq of HCO3-) or an 8.4% solution (1 ml = 1.0 mEq
of HCO3-), is generally added to a base fluid of 0.9% NaCl. Assuming the same fluid therapy plan
used above for K+ supplementation (35 litres of intravenous fluid over the subsequent 12 hours),
100 mEq of NaHCO3 would need to be added to each litre to correct the 3500 mEq deficit
estimated above. As mentioned above, this would also result in addition of 100 mEq of Na+ to
each litre of fluids and would increase the Na+ concentration of the fluid to 254 mEq/litre because
the Na+ concentration of 0.9% NaCl is already 154 mEq/litre. Because Na+ continues to be lost in
diarrhoea, administration of fluids with this high of a Na+ concentration is generally safe in horses
with diarrhoea. However, addition of NaHCO3 to a base fluid of 0.9% NaCl would have to be
approached cautiously in horses with an increased plasma Na+ concentration or with acidosis
from other causes. In these instances, addition of NaHCO3 to a base fluid of 0.45% NaCl (half
strength saline) would be a safer choice.
Another option to replace the HCO3- deficit would be to administer NaHCO3 as a separate enteral
solution that ranges from 300-600 mOsm/kg in tonicity (concentration). Because 1 g of NaHCO3
has 12 mEq of both Na+ and HCO3- (for a total of 24 mOsm), the 3500 mEq HCO3- deficit could
be addressed by administering three ~100 g doses of NaHCO3 (baking soda) in 4-8 litres of water
at 4 hour intervals. Another option would be to administer NaHCO3 as an oral paste (by mixing
30 g of NaHCO3 with water or corn oil in a 60 ml dosing syringe and administering two syringes
every 2 hours). Unfortunately, the Na+ load accompanying NaHCO3 administration by any route
will usually lead to increased urine output and will exacerbate the obligate loss of K+ in urine.
Thus, attention to K+ balance, by adding KCl to the base intravenous fluid or enteral solutions
being administered or by giving KCl as an oral paste merits consideration in horses receiving
supplemental HCO3-. Finally, a less commonly pursued but logical alternative would be to use
supplemental KHCO3, rather than NaHCO3, for oral supplementation. KHCO3 has a molecular
weight of ~100 g/mole such that 1 g provides 10 mEq of both K+ and HCO3-. Administration of
KHCO3 (three ~100 g doses of in 4-8 liters of water at 4 hour intervals by nasogastric tube or as
oral pastes more frequently) would provide ~3000 mEq of both K+ and HCO3- deficit and address
both the K+ and HCO3- deficits.
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