Page 581 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

568 FLUID THERAPY

can therefore be administered more rapidly with fewer cause mild peripheral and intestinal vasodilatation, modu­
side effects. Measurements of COP are recommended late inflammation, improve intestinal edema and function,
with prolonged colloid administration. Animals with a and decrease intracranial pressure. 72,73,104,115,116,117,160
colloid osmotic pressure less than 16 mm Hg may benefit Hypertonic saline is especially useful for the treatment
from synthetic colloid administration and therapy should of head trauma or cardiovascular shock in animals greater
be adjusted to maintain values above this level. Animals than 30 kg that require large amounts of fluid for resusci­
with chronic hypoproteinemia may not need a COP tation (and time is of the essence [e.g., gastric dilatation-
greater than 16 mm Hg because the ratio of protein in volvulus patients). Due to the osmotic diuresis and rapid
the IV to interstitial space may not be as deranged as in redistribution of sodium cations that ensue following
those patients with acute IV losses. Total protein refrac­ the administration of hypertonic saline, the IV volume
tometer readings are not a valid means of monitoring expansion is transient (<30 minutes) and additional fluid
colloid therapy. 14 therapy must be used. 131 For example, combinations of
Potential side effects of synthetic colloid use are pri­ hypertonic saline and synthetic colloid solutions have
marily related to disruption of normal coagulation. These shown beneficial effects and are described below.
include a decrease in factor VIII and von Willebrand fac­ Although 25% mannitol could also be used as a hypertonic
tor concentrations (beyond just a dilutional effect), fluid, it is less effective at increasing IV volume because the
impairment of platelet function, and interference with osmolarity is approximately half that of 7.5% saline. The
the stability of fibrin clots, which makes them more sus­ use of 23.4% saline intravenously has been reported, but
ceptible to fibrinolysis. 23,139,140,142,157 In addition, the the safety margin is small and this practice is generally
time to clot formation (R), clot strength (maximum discouraged. However, its use for the treatment of brain
amplitude), and the speed of clot strength development injury is ongoing and research thus far looks
(angle) using thromboelastography are all adversely promising. 71,76,115,116, See Table 23-6 for specific
affected by hydroxyethyl starches. 45 The clinical characteristics of hypertonic crystalloids.
manifestations of these changes are variable and depend Hypertonic saline administration is not without
on the status of the patient. Obviously, those patients potential risks. An increase in the concentration of serum
with preexisting coagulopathies, von Willebrand disease sodium and chloride will occur following administration
(VWD), or moderate to severe thrombocytopenia/ (in addition to an increase in osmolarity). A decrease in
thrombocytopathia are at highest risk. Monitoring of potassium, and bicarbonate concentrations should also
the activated partial thromboplastin time (aPTT) may be be anticipated. Typically, these changes are moderate
helpful in assessing the adverse effects and risk level and of minimal clinical importance, except in animals
associated with the use of synthetic colloids, although with preexisting electrolyte derangements or those
there are no precise guidelines and it is difficult to predict receiving repeated doses of hypertonic saline. Hypertonic
which animals will develop clinical bleeding following saline should not be given to dehydrated animals because
administration. In general, the appropriate use of synthetic these patients are interstitially volume depleted, thus
colloid solutions is deemed worth the risk, but judicious limiting the effectiveness of the fluid and predisposing
use of plasma and other blood products may also prove to further dehydration. If hypertonic solutions are
necessary to prevent bleeding complications, especially administered proximal to the heart, arrhythmias might
perioperatively. Caution should also be exercised to pre­ occur. If hypertonic solutions are administered in small
vent volume overload or excessive hemodilution when peripheral veins, hemolysis and phlebitis can result. 115
large volumes of synthetic colloids are given to a patient.
Additional side effects of synthetic colloids in people HYPERTONIC SALINE-SYNTHETIC
include renal impairment and allergic reactions, but COLLOID MIXTURES
similar problems in animals have not been documented. To prolong the effect of the resuscitation fluids, a hyper­
tonic saline/synthetic colloid mixture is often
HYPERTONIC SALINE
Hypertonic (7.0% to 7.5%) sodium chloride administra­ TABLE 23-6 Hypertonic Fluids
tion causes a transient osmotic shift of water from the
þ
extravascular to the IV compartment. Small volumes of Osmolarity [Na ]
approximately 4 to 6 mL/kg can be administered over Fluid Type (mOsm/L) (mEq/L)
10 to 20 minutes. Rates exceeding 1 mL/kg/min may
result in vagally mediated hypotension, bradycardia, 7.5% NaCl 2400 1200
23.4% NaCl 8000 4000
and bronchoconstriction and should be avoided.
25% mannitol 1250
Although hypertonic saline is primarily given to shift
extravascular water into the IV space, there is evidence
Modified with permission from Silverstein DC, Campbell J. Fluid,
to suggest that it may also help to reduce endothelial electrolyte and acid-base therapy. In: Tobias K, Johnston S, editors. Small
swelling, increase cardiac contractility in normal hearts, animal surgical practice. St Louis: Saunders Elsevier (in press).