Page 391 - Clinical Small Animal Internal Medicine

36 Fluid Therapy 359

in response to the profound osmotic gradient created. Some solutions are also available in balanced crystalloid
VetBooks.ir However, like other crystalloid solutions, the intravascu- solutions such as LRS.
The use of synthetic colloids is somewhat controver-
lar volume expansion will not remain for long if adminis-
tered alone. For this reason, hypertonic saline is often
have shown impaired coagulation and decreased platelet
combined with a synthetic colloid to extend the duration sial. Studies in both the human and veterinary literature
of effect. Because the action of hypertonic saline is clump strength as determined by thromboelastography.
dependent on movement of volume from the interstitial The higher the rate of substitution, the greater effect on
space to the intravascular space, it is relatively contrain- coagulation. For this reason, HES solutions with a lower
dicated in dehydrated patients. Hypertonic saline is a rate of substitution such as the tetrastarches are consid-
popular choice for small‐volume resuscitation and is the ered theoretically safer as their effect on coagulation is
preferred choice for the resuscitation of the hypotensive less.
head trauma patient. In addition, it may also be used in Overzealous administration of any of the HES solutions
patients with intracranial hypertension with, or in lieu can still result in a dilutional coagulopathy. In addition to
of, mannitol. effect on coagulation, synthetic colloids can impact
serum total protein and urine specific gravity (USG)
measurements. The administration of HES (670/0.75)
Synthetic Colloids
has been previously shown to elevate the urine specific
Colloids are molecules of high molecular weight, which gravity. In one study, the largest increase in USG occurred
exert osmotic activity and thus promote the movement 150 minutes following administration with a mean USG
of fluid from the interstitium into the vascular space. of 1.070 +/− 0.021.
Colloids can first be described as natural or synthetic. An Both hydroxyethyl starch and dextran 70 have been
example of a natural colloid is the albumin molecule. shown to affect total solids measured by refractometry.
Synthetic colloids can further be divided into three When evaluated using a refractometer, HES will yield a
classes: gelatins, dextrans, and hydroxyethyl starches reading of 4.5 g/dL, therefore when administered in vivo,
(HES). The most commonly used class of synthetic col- the refractometric measurement of total solids will trend
loid is the hydroxyethyl starches, which are plant‐derived to 4.5 g/dL. For this reason, total solids should be inter-
proteins that have undergone modification to prevent preted with caution in patients receiving synthetic
rapid degradation. colloids. In addition, serum amylase may be elevated
When describing hydroxyethyl starches, a number of 200–250% due to binding with hydroxyethyl starch
parameters are used. First is the molecular weight, which molecules, leading to a decrease in clearance.
affects the degree of “oncotic pull” exerted by the solu- Ideally, the efficacy of synthetic colloids is monitored
tion, with lower molecular weight solutions generally through serial measurement of the COP using a mem-
exerting a greater effect. Molecules in HES formulations brane osmometer. In practice, this technology is not
tend to be polydisperse and can range in size from a few widely available and the response to therapy is used
thousand to a few million kilodaltons. The distribution instead to guide treatment.
within a solution follows a bell‐shaped curve. By conven- Finally, in human medicine, in particular in the subset
tion, the molecular weight used to describe the solution of patients with refractory sepsis, concerns have been
is the average molecular weight. raised about HES administration and renal injury based
The second parameter used in description of HES on an increase in the need for renal replacement therapy
solutions is the degree of substitution, determined by in patients receiving hydroxyethyl starches. At this time,
measuring the number of substituted glucose molecules this concern appears to be related to the cumulative dose
and dividing by the total number of glucose molecules. of HES administered. There have been some studies
HES solutions with a higher rate of substitution resist examining the issue in small animal medicine but the
hydrolysis more effectively than molecules with lower topic remains controversial.
substitution rates and thus persist in the body for a Dextrans are neutral glucopolysaccharides based on
longer period of time, extending duration of action. The glucose monomers. Dextrans are described based on
rate of substitution is used to describe the fluid and while their average mean molecular weight as well as the tonic-
it is common in veterinary medicine to refer to all ity of the fluid. Examples of dextrans include products
hydroxyethyl starches as “hetastarch,” this is a misnomer; such as dextran 40, dextran 60, and dextran 70. In
in fact, a substitution rate of 0.6 is termed a hetastarch, humans, dextran administration has been associated
0.5 is a pentastarch, and 0.4 is a tetrastarch. with anaphylactoid reactions as well as being implicated
The final parameter that can be considered when eval- in the development of renal failure in critically ill
uating HES solutions is the carrier solution. Classically, patients. This has not been documented in veterinary
the majority of HES solutions are provided in 0.9% NaCl. patients.