Page 578 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

566 FLUID THERAPY

TABLE 23-4 Isotonic Crystalloid Compositions
þ
þ

Fluid Osmolarity [Na ] [K ] [Cl ] [Mg þþ ] [Ca þþ ] Lactate Acetate Gluconate
Type (mOsm/L) (mEq/L) (mEq/L) (mEq/L) (mEq/L) (mEq/L) (mEq/L) (mEq/L) (mEq/L)
0.9% NaCl 308 154 154
Lactated 273 130 4 109 3 28
Ringer’s
solution
Plasmalyte 295 140 5 98 3 27 23
148
Normosol-R 295 140 5 98 3 27 23

Modified with permission from Silverstein DC. Daily intravenous fluid therapy. In: Silverstein DC, Hopper K, editors. Small animal critical care. St Louis:
Saunders Elsevier, 2009.

crystalloid base solution. These fluids are hyperoncotic to preparations contain high polymeric glucose compounds
the normal animal and therefore cause the movement of that are manufactured by modification of the highly
fluid from the extravascular to the intravascular space. branched starch, amylopectin. Replacement of hydroxyl
Intravascular oncotic pressure is primarily regulated by groups with hydroxyethyl groups at the C2, C3, or C6
albumin (69,000 Da), and the normal colloid osmotic carbon position of the constituent glucose molecules
pressure (COP) in most small animal patients is approxi­ prevents rapid degradation by amylase. The ratio of sub­
mately 20 mm Hg. Synthetic colloids lead to an increase stitution at the C2 versus C6 position (known as the C2:
in blood volume that is greater than that of the infused C6 ratio) also alters the half-life of the solution, with a
fluid volume and also aid in the retention of this fluid in higher ratio corresponding to a longer half-life. The
the vascular space (assuming normal capillary permeabil­ degree of substitution (DS) refers to the number of
ity). 131 Although there is no definitive evidence to support hydroxyethyl groups per molecule of glucose and the
the use of colloids over crystalloids for the treatment of higher the number of substitutions, the slower the break­
shock, they may have a longer intravascular effect, require down and elimination of the molecule. However, a higher
smaller volumes to achieve similar intravascular volume degree of substitution also means greater potential effects
expansion, and prove less likely to cause interstitial edema on coagulation. 153 Hetastarch solutions have a rather
due to their hyperoncotic characteristics. However, their high DS (0.6 to 0.7), while pentastarches and
use is also associated with coagulation impairment, higher tetrastarches have a DS of 0.5 and 0.4, respectively.
costs, and possible side effects (e.g., allergic reactions or HES solutions are further characterized by their MW
renal impairment, both primarily reported in humans). (low MW 70 kDa, medium MW 130 to 270 kDa, and
The primary synthetic colloid solutions available con­ high MW 450 kDa), their concentration (3%, 6%, or
tain either dextrans, gelatins, hemoglobin-based oxygen 10%), and their degree of substitution (0.4, 0.5, 0.6, or
carriers (HBOCs), or hydroxyethyl starches. Dextrans 0.7). It is important to note whether the MW is expressed
are composed of naturally occurring glucose polymers, as the number average molecular weight (MW n , most
but the most commonly used and studied dextran, dextran reflective of oncotic pressure) or the weight average
70, is not currently commercially available. Gelatins are molecular weight (MW w , exaggerated by larger particles).
made following the hydrolysis of bovine collagen and The MW w is determined by light scattering and is not as
subsequent succinylation or linkage to urea. The available accurate a measure of the size of the colloid as MW n ,
gelatin, oxypolygelatin, has numerous side effects and a which is the arithmetic mean of the range of molecular
short duration of action, making it a less desirable synthetic weights in the solution. The MW w is larger than the
colloid that is unlikely to gain widespread use. HBOCs MW n , and as the molecular weight distribution of the col­
contain stroma-free, ultrapurified hemoglobin glutamers loid becomes narrower, MW w approaches and eventually
that are highly polymerized to prolong their effect in the equals MW n . In addition, the ability of synthetic colloids to
circulation. Hydroxyethyl starches (HES) are made from modulate inflammationisrelated to their size and DS; those
a wide size range of amylopectin polymers with variable with a lower MW (<200 kDa) and DS (<0.4) may help to
chemical modifications that influence their pharmacoki­ decrease capillary permeability, down-regulate the expres­
netics and metabolism. These are the most commonly used sion of adhesion molecules, inhibit neutrophil recruitment,
synthetic colloids and will therefore be reviewed in detail. and minimize cytokine production. 46,84,145,146,159
However, the characteristics of most available synthetic Synthetic colloids are typically used in combination
colloid solutions are displayed in Table 23-5. with isotonic crystalloids to maintain adequate plasma
Examples of HES solutions include hetastarch, volume expansion with lower interstitial fluid volume
pentastarch, and tetrastarch (e.g., Voluven). HES expansion. Smaller total fluid volumes are needed and