In foals that are not refluxing, and in particular those that do not require rapid
administration of large volumes of fluid, enteral fluids may be appropriate. In
neonatal foals that are not able or permitted to nurse on their own, enteral milk
administration via an indwelling nasogastric tube is often a component of their fluid
therapy. In these cases, it is critical to take into account the volume of enterally
administered fluids/milk when calculating the patient’s total daily fluid needs to avoid
fluid overload. Large volume enteral fluid therapy is rarely used in neonatal foals.
In foals requiring fluid therapy in which the above routes are not usable, intraosseus
fluids may be administered. However, it is difficult to maintain intraosseus catheters
for long periods of time, and rapid or large volume fluid administration through this
route can be challenging. Subcutaneous fluids are not well tolerated in horses and
foals and should be avoided. Similarly, the risks associated with intraperitoneal fluid
administration in foals (e.g. gastrointestinal puncture, peritonitis) almost always
precludes the use of this method.
The key to successful fluid therapy in foals is to monitor and calculate everything that
goes in and (where possible) comes out.
Daily maintenance sodium administration should be kept below 4 mEq/kg/day in a
foal with appropriate renal function to avoid sodium overload (e.g. < ~200 mEq/day
for a 50 kg foal). Balanced polyionic replacement fluids (e.g. Plasmavet, Lactated
Ringers solution) and equine plasma all contain substantial amounts of sodium
(~140-160 mEq/L), and thus sole use of these fluids for maintenance needs in
neonatal foals can quickly produce sodium overload. The inclusion of lower-sodium
fluids such as 5% dextrose in water or 0.45% saline with 2.5% dextrose to provide
volume with less sodium can be beneficial components of a maintenance fluid plan
for a neonatal foal.
Diarrhea or renal disease may however cause excessive sodium loss. These foals
become hyponatraemic on the lower sodium fluids discussed above and require
sodium administration at a rate higher than 4 mEq/kg/day. In these cases, close
attention to plasma sodium concentration (e.g. measurement q. 8-12 hours) can help
guide fluid choices. In some cases, calculation of daily renal sodium excretion after
24-hour urine collection is necessary to determine appropriate sodium administration
rates.
Foals that are not consuming milk will rapidly develop electrolyte derangements.
Potassium in particular will rapidly become depleted in anorectic foals. Hypokalaemia
is exacerbated when potassium is driven intracellularly by the administration of
intravenous dextrose and the correction of acidaemia. Potassium support (usually
potassium chloride) should be considered in any foal on parenteral nutrition or IV
dextrose support. Potassium should not be administered intravenously faster than
0.5 mEq/kg/hr. Intravenous fluids supplemented with potassium at 10-40 mEq/L and
administered at maintenance rates are generally safe, but should never be bolussed.
Calcium and magnesium should be monitored and supplemented as needed.
Acid base and electrolyte balance
The traditional approach for clinically assessing acid-base status uses the
Henderson-Hasselbalch equation to categorize 4 primary acid-base disturbances:
respiratory acidosis (increased PCO2), respiratory alkalosis (decreased PCO2),
metabolic
acidosis
(decreased extracellular
base excess
or
actual
−
HCO3 concentration), and metabolic alkalosis (increased extracellular base excess
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
212