South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
When adding multiple supplements (e.g., KCl, NaHCO3, and dextrose) to a base fluid, the tonicity
of the final product should be considered. A general rule is that the total osmolality provided by
electrolytes should not exceed more than twice the value of plasma osmolality. Since plasma
osmolality is normally about 280 mOsm/kg, a final osmolality of about 600 mOsm/kg would be a
reasonable limit. The osmotic contribution from dextrose can largely be ignored because this
additive is metabolized and contributes little to the net number of osmoles added to body fluids.
Using the example of a base fluid of 0.9% NaCl to which 40 mEq of KCl and 100 mEq of
NaHCO3 have been added to each litre, the final osmolality would be 588 mOsm/kg (154 mEq of
both Na+ and Cl- from the base fluid; 40 mEq of both K+ and Cl- from added KCl, and 100 mEq
of both Na+ and HCO3- from added NaHCO3).
Administering fluids that are hypertonic (greater osmolality than plasma) may be advantageous in
some dehydrated horses. Specifically, initial fluid replacement in most dehydrated horses is in the
form of voluntarily water drinking. However, ingestion of plain water dilutes the electrolytes
remaining in body fluids. Because an increase in plasma osmolality is the most potent stimulus
for thirst, dilution of remaining plasma electrolytes by water drinking can abolish thirst. As a
result, horses that lose considerable amounts of electrolytes with water via prolonged sweating or
in diarrhoea can remain moderately to severely dehydrated, yet may have little thirst. Thus,
providing fluids that are hypertonic, especially due to a higher Na+ content, may stimulate thirst in
dehydrated horses. Increasing voluntary water intake is of benefit to the fluid therapy plan because
it may reduce the total volume (and cost) of fluids that need to be administered. Clearly, this
benefit is limited to horses that have lost both water and electrolytes in the dehydration process
and use of hypertonic fluids must be approached cautiously in horses that have become
dehydrated by water restriction and in all dehydrated foals.
Although not a specific additive to a base fluid, 1-2 litres of hypertonic saline (7.5% NaCl
solution) is sometimes administered intravenously concurrent with isotonic crystalloid fluids
during initial rehydration. Hypertonic saline has Na+ and Cl- concentrations of 1285.2 mEq/L or
an osmolality of 2565 mOsm/kg, a value ~9-fold greater than plasma. The goal of administering
hypertonic saline is to draw water by osmotic forces into the plasma space from the interstitial
space and ICF. This treatment is ideally suited for horses experiencing rapid fluid loss from the
vascular space resulting in shock (i.e., haemorrhagic shock) but can also be useful in patients that
have hypovolemic shock as a result of other ty pes of fluid loss. In both types of shock,
administration of hypertonic saline may stimulate drinking but close patient monitoring is
warranted because critically ill patients may not respond as expected. A further use for hypertonic
saline is in patients that have moderate to severe electrolyte depletion in the face of only mild to
moderate dehydration. Examples include horses that have performed long distance exercise (with
substantial sweat fluid loss) or that have had diarrhoea for several days. In both instances,
voluntary drinking may have replaced 50% or more of water loss yet this form of fluid
replacement is not accompanied by replacement of electrolytes. Judicious use of hypertonic saline
in these patients, along with replacement of K+ deficits via enteral fluids or administration of oral
pastes can be a reasonable alternative to administration of large volumes of isotonic fluids.
Electrolyte administration as oral pastes or in drinking water:
Use of oral electrolyte pastes has been mentioned several times as an adjunct to intravenous and
enteral fluids. Oral pastes have the distinct advantage of replacing needed electrolytes without
placing additives in the base intravenous fluid (increasing osmolality and cost) and limiting the
number of times that a nasogastric tube has to be passed. However, when they are used to
complement the fluid therapy plan, it is important to ensure that horses also drink water in the
hours after electrolyte administration. In fact, for some horses with mild to moderate dehydration
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