Page 716 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Hemodialysis and Extracorporeal Blood Purification 703

which the predialysis serum potassium is greater than clearance of the toxin or its metabolites, or there is no
6.0 mmol/L, a dialysate containing 0 mmol/L of potas- specific antidote for the toxicant. Hemodialysis can be
sium has been recommended. 34,38,59 Transfer of potas- used to eliminate toxins from the body before they pro-
sium from secluded intracellular pools may lag behind mote cellular damage or before they are converted to
its rate of removal from the extracellular compartment more toxic metabolites. The dialytic removal of exoge-
by the dialyzer, causing transient hypokalemia at the nous toxins is governed by the same molecular
end of dialysis sessions. 136 A rebound hyperkalemia characteristics that define dialytic clearance of endoge-
occurs following the delayed transfer from intracellular nous toxins. Molecular size, concentration in plasma
pools within hours of ending dialysis that extends to water, distribution volume, degree of protein binding,
the next dialysis treatment. Daily dialysis may be required and lipid solubility significantly influence the potential
until the bulk of the potassium burden is corrected. for a toxin’s elimination. 14,161,185 Toxins or drugs with
In contrast to medical treatments for hyperkalemia, low-molecular-weights (<1500 Da), small volumes of
which merely shift extracellular potassium to intracellular distribution, and minimal protein binding are excellent
pools or antagonize its neuromuscular toxicity, hemodi- candidates for diffusive and convective clearance. A small
alysis eliminates excessive potassium loads from both volume of distribution (<1.0 L/kg) predicts the toxin is
extracellular and intracellular pools. 134 Additional protein-bound or restricted to the extracellular space and
guidelines for the dialytic management of hyperkalemia accessible for extracorporeal clearance. A toxin with a
were discussed previously under the Hemodialysis Pre- large distribution volume (>1.0 L/kg) is likely to be
scription for Acute Kidney Injury section. concentrated in tissues and will have minimal transference
The dialysate sodium concentration can be propor- or availability in plasma water for removal. Only the free
tioned to concentrations ranging from 125 to fraction of protein-bound toxins can be dialyzed readily,
160 mmol/L. It also can be programmed (or profiled) and toxins or drugs that are highly protein bound may
to change in user-defined patterns throughout the dialy- not be good candidates for dialytic removal.
sis session to achieve specific treatment goals or to correct Ethylene glycol has a molecular weight of 62 Da,
predialysis dysnatremias. Hyponatremia caused by negligible protein binding, and a volume of distribution
sodium losses from excessive vomiting, diarrhea, diuretic equivalent to total body water (0.5 to 0.8 L/kg) and con-
administration, parenteral sodium-free fluid administra- sequently is an excellent candidate for dialytic removal.
tion, or oral water can be corrected by programming With timely dialysis, ethylene glycol can be removed from
the dialysate sodium concentration to increase in stepped the body before its enzymatic oxidation to more toxic
increments or continuous gradients to the desired metabolites, including glycoaldehyde, glycolate,
postdialysis concentration. Hypernatremia caused by glyoxylate,andoxalate. 14,20,34,137,161 Toxinsthatarehighly
excessive bicarbonate or hypertonic saline administration bound to serum proteins, including diazepam, salicylates,
may be difficult or inappropriate to correct with addi- nonsteroidal antiinflammatory drugs (NSAIDs), and tricy-
tional fluid administration but can be resolved easily by clic antidepressants, are dialyzed less effectively, but dialysis
adjusting the dialysate sodium concentration in progres- may still be a therapeutic option. Redistribution (rebound)
sive or incremental steps until the desired sodium concen- of a toxin or drug from peripheral tissues or cellular
tration is reached. The rate of correction can be regulated compartments to plasma may limit the efficacy of dialysis
precisely without overcorrection. Excessive isonatremic to resolve the poisoning. If redistribution of the toxin from
loads of sodium can be eliminated by ultrafiltration alone extravascular pools is much slower than its dialytic removal,
without simultaneous changes in serum sodium concen- the animal may become reintoxicated within hours after
tration. With the exception of minor Gibbs-Donnan completing dialysis. For these sequestered toxins, the
effects, the ultrafiltrate is formed with the same sodium length and frequency of dialysis may need to be increased
concentration present in plasma water. Consequently, to facilitate their whole-body elimination.
large sodium loads can be eliminated without Hemoperfusion is an adsorptive extracorporeal therapy
perturbations in sodium concentration or the risk of used to manage endogenous and exogenous intoxications
inducing sodium disequilibrium, which may trigger thatarenotclearedefficientlybyhemodialysis.Adsorption
redistribution of fluid and electrolytes from intracellular is the principle of molecular attachment of a solute to a
stores. 45,135 material surface. In contrast to the physical separation
between blood and dialysate that occurs during hemodial-
USE OF HEMODIALYSIS IN ACUTE ysis, during hemoperfusion blood is exposed directly
INTOXICATIONS to an adsorbent with the capacity to selectively or

Elimination of toxins and support for the consequences nonselectively bind toxins of defined chemical composi-
of the intoxication are important but overshadowed tion within the blood path. Hemoperfusion is a small
applications of hemodialysis. 20,79,175 This use of hemodi- but defined niche in medical therapeutics, which should
alysis is especially important if there has been a delay in be incorporated more broadly into extracorporeal
medical management, there is limited endogenous therapies in veterinary medicine. 20,79,152 Hemoperfusion