Page 431 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Perioperative Management of Fluid Therapy 421

affected these functions. Crystalloid fluid administration include normal saline (0.9% NaCl), a lactated polyionic
at 11 mL/kg/hr for 60 minutes to halothane- fluid (e.g., LRS), an acetated polyionic fluid (e.g.,
anesthetized cats did not result in any changes in PCV acetated Ringer’s solution, Normosol-R, Plasma-Lyte
or TP. 16 These cats had undergone thoracotomy for 148, Isolyte S, Polyionic R), or 5% dextrose in water,
placement of catheters and did not start the study with saline, or polyionic solutions.
normal values (PCV ¼ 25%, TP ¼ 4.9 g/dL, colloid
osmotic pressure ¼ 10.2 mm Hg) and thus may be NORMAL SALINE
regarded as similar to compromised animals in a clinical Normal saline is used widely as a replacement solution
situation. In a clinical setting, however, the PCV and intraoperatively. It is the solution of choice for patients
TP often decrease over time due to a combination of fluid with hypercalcemia or hypochloremic alkalosis. This solu-
dilution and blood loss. In a study of dogs undergoing a tion contains higher amounts of chloride than plasma and
tibial plateau leveling operation, the PCV decreased from tends to decrease the strong ion difference, leading to aci-
approximately 48% to approximately 32% and the TP dosis. In classical terms, it dilutes the concentration of
from approximately 6.8 to approximately 5 g/dL with bicarbonate and provides large amounts of chloride for
administration of LRS at 10 mL/kg/hr over a 4-hour reabsorption from the glomerular filtrate, thus leading
period. 19 to hyperchloremic acidosis. The degree of acidosis is
Studies in sheep have examined the redistribution of not likely to be a problem in the healthy patient but
0.9% NaCl during isoflurane anesthesia, and the results may exacerbate acidosis in a compromised patient. Evi-
showed a similar rate of redistribution away from the vas- dence from some studies in humans indicates that urine
cular space, but there was much greater retention in the output may be decreased when patients receive the same
159,195
interstitial space when compared with the awake ani- volume of normal saline as compared with LRS.
mal. 40 This observation was accounted for by a dramatic
reduction in urine output during isoflurane anesthesia, LACTATED RINGER'S SOLUTION
and a recent study in dogs corroborated this finding. 19 (HARTMANN'S SOLUTION)
In that study, dogs undergoing a routine orthopedic pro- LRS is a balanced electrolyte solution containing lactate
cedure received LRS at 10 mL/kg/hr but urine output that contributes to the correction of acidosis and is my
remained less than 0.5 mL/kg/hr. Fluid was retained fluid of choice for most anesthetized patients. Potential
in the extracellular space and a significant increase in body disadvantages of this solution are as follows:
weight occurred. These data suggest that fluid 1. It contains calcium and because blood products gen-
accumulates in the interstitium during anesthesia to the erally are stored using a compound that chelates cal-
detriment of the patient. 144 Further work by this latter cium, it is not ideal to administer LRS through the
group in elderly human trauma patients suggests that same intravenous line as blood products. A 1:10 mix-
excretion of fluid also is decreased in the postoperative ture of blood and LRS resulted in clot formation
period. 178 Careful measurement of respiratory function within 2 minutes at 37 C (see Table 17-1). 155

in awake 59- to 67-year-old people showed some 2. The osmolality of LRS is 272 mOsm/L and the
impairment of respiratory function when they were given sodium content is 130 mEq/L, which means it is a
40 mL/kg LRS over 3 hours. 88 In studies of humans, this hypotonic solution. This hypotonicity could lead to
issue has been examined further by using less (restrictive) greater loss of fluid into the intracellular compart-
or more (liberal) fluid in the perioperative period, but the ment, which in turn may be detrimental in patients
definitions of the terms “restrictive” and “liberal” has with cerebral edema. In models of traumatic brain
varied substantially from study to study and consequently injury, infusion of LRS was associated with an increase
the results are hard to interpret. Regardless, the fluid in ICP. 145,152 In a model of closed-head trauma in
33
retention has been associated with harm to the patient. rats, use of LRS did not affect neurologic outcome
The excessive fluid administration has been implicated in or formation of brain edema. 59 However, the low
longer hospital stays, decreased wound healing, delayed sodium content has been implicated in postoperative
postoperative gastrointestinal activity and even increased hyponatremia in human patients, particularly chil-
postoperative pain. 27 dren, sometimes with disastrous outcomes. 126
This information suggests that use of isotonic fluids at 3. It contains lactate, which mostly is metabolized in the
10 mL/kg/hr is probably excessive under most liver (approximately 56% of normal lactate metabolism
situations encountered in routine practice, but as of yet occurs in the liver). In some LRS, the lactate is in the
no evidence-based criteria for a new approach has been form of L-lactate (e.g., the lactate in Baxter’s product
presented. One recommendation is to provide for ongo- is derived from fermentation), whereas in others, a
ing losses using crystalloids (1 to 2 mL/kg/hr) and man- racemic mixture with equal amounts of the D- and
age relative hypovolemia using colloid solutions. 33 If a L-lactate is used (e.g., the lactate in Hospira’s product
crystalloid is used, a decision still must be made about is derived from chemical production of lactate). The
which crystalloid to use. Commonly available crystalloids L-form is more readily metabolized than is the