Page 430 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

420 FLUID THERAPY

MONITORING CHANGES IN was appropriate for losses occurring during major
FUNCTION abdominal surgery. I have used this approach in many
dogs and cats with few apparent adverse effects. In the
The end result of failure in fluid management is that original studies, blood volume was measured using radio-
164,187
organs begin to fail. In human medicine, a relatively non- active tracers. These techniques are accurate in a
invasive test has been introduced to monitor the func- steady state but may not be accurate when volumes are
tional ability of the liver to clear foreign substances changing during fluid infusion. Later studies evaluated
from the plasma. This system (LiMON, Pulsion Medical the dilution of hemoglobin or albumin, or the change
Systems, Munich, Germany) uses pulse densitometry to in blood water content to assess acute changes in blood
monitor the arterial concentration of an administered volume but these may not be accurate either because they
dye (indocyanine green [ICG]). The rate of clearance do not account for the full circulating volume. 79,172,177
of ICG is a measure of hepatic function. In human Although these initial studies were performed in healthy
patients with sepsis, mortality was 80% when clearance human volunteers, they provide some useful information.
of ICG was less than 8%, whereas it was 11% when clear- In one study, infusions were carried out at different rates
ance was greater than 24%. 91 using two different volumes. 79 The interstitial fluid space
is roughly twice the volume of the intravascular space, and
INTRAOPERATIVE FLUID isotonic replacement solutions redistribute, leaving
approximately 33% of the infused volume in the vascular
MANAGEMENT space. In this study, the volume retained in the vascular
space 15 minutes after the end of the infusion was approx-
Intraoperative fluid management depends on:
imately 20%, and it was approximately 15% after 30
1. How well the patient has been prepared beforehand minutes, indicating rapid redistribution of crystalloid
2. How much fluid loss occurs normally (insensible loss) solutions. The volume of distribution for the balanced
3. How much fluid loss occurs because of the equipment electrolyte solution was similar to the expected plasma
used (e.g., dry gas causes greater water loss than
volume but only 50% to 70% of the expected volume
humidified gas)
for the interstitial space. Regions of the interstitial space
4. Changes in vascular tone and cardiac output
with poor blood supply or rigid structure (e.g., bone)
5. The amount and nature of the tissue exposed during
may be less likely to take up fluid, and this may account
surgery
for the difference in calculated volumes.
6. The amount of blood lost
The authors of these volume-kinetic studies proposed
In most patients, crystalloid solutions are used first,
that their data could be used to calculate infusion rates
and colloids and blood products are added as required.
that would expand the plasma compartment (bolus)
and maintain it at this volume (infusion). To increase
CRYSTALLOIDS blood volume by 5%, the patient would receive 36 mL/
kg/hr for 20 minutes and an ongoing infusion of
The anesthetized animal has ongoing fluid losses of 15 mL/ kg/hr. 177 In another study, nomograms were
approximately 132 BW 0.75 mL/day for the dog and presented for men and women showing the infusion rate
80 BW 0.75 mL/day for the cat, where BW is body and time required to achieve a specific blood volume
weight in kilograms. It is likely that losses will be less than expansion and the infusion rate required to maintain this
predicted by these formulas because the metabolic rate of expansion. 79 Whether these data apply to anesthetized
most anesthetized animals is less than in the awake resting animals is uncertain, but the results suggest that a fluid
state. A maintenance solution would be appropriate rate of 10 mL/kg/hr is relatively conservative if an
merely to replace this loss. However, it is expected that expansion of circulating volume is the aim.
fluid losses will increase during anesthesia because of In a study of healthy dogs undergoing elective
increased loss from the respiratory tract and that there will ovariohysterectomy or castration, the rate of polyionic
be changes in hemodynamics that will require fluid ther- fluid administration was examined to determine how it
apy (see Effects of Anesthesia section). Consequently, it affected hematocrit, total protein concentration, glucose
has been traditional to use isotonic replacement solutions concentration, and systolic blood pressure. 67 The authors
during anesthesia and to expect that the kidneys will tested an acetated polyionic solution given at 0, 5, 10, and
excrete any excess sodium in the postoperative period. 15 mL/kg/hr for 1 to 2 hours. They saw no differences
Replacement solutions do not contain high among groups, suggesting that there was no advantage to
concentrations of potassium and can be given rapidly if fluid therapy in these instances. Even at the highest rate of
necessary without risk of potassium toxicity. fluid administration neither packed cell volume (PCV)
The rate of administration often is set arbitrarily at nor total protein concentration (TP) decreased signifi-
10 mL/kg/hr. This rate of administration is based on cantly. Cardiac output and renal function were not
research in humans in the 1960s suggesting that this rate evaluated, and so it is not possible to say whether fluids