Page 33 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

22 APPLIED PHYSIOLOGY

BASAL (see Table 1-8) were corrected for water intake that bal-
anced evaporative losses, total water intakes were 11 to
Fluid requirements for sick, inappetent small animals have
20 mL/kg/day. This value approximates the accepted
not been well documented. Decreased food intake or
general range for daily urine production in dogs. Thus,
anorexia decreases renal solute load and hence water
if dogs are deprived of food and urine volumes decrease
requirements. However, clinicians frequently base
substantially (renal solute load decreases), the water need
estimates of water requirements for patients on tables
may be small.
derived from the formula for maintenance energy
requirements: 140 body weight (kg) 0.73 19 Haskins 19 Water requirements of sick animals may be increased
.
over basal requirements owing to increased contempo-
commented that the use of tables for water intake based
on this formula might overestimate the water rary fluid losses caused by evaporation (through panting),
requirements of sick patients. In fact, the water require- diarrhea, vomiting, or dilute urine. Clinicians must esti-
ment of an inappetent, sedentary sick animal in a mate how much water needs increase by assessing the vol-
thermoneutral environment may approach basal water ume of these additional fluid losses. However, fluid needs
still may not approach 40 to 60 mL/kg/day.
need. The basal water requirement for a healthy animal
Assessing the basal water needs of dogs and cats from
may be defined analogously to the basal energy need, as
the basal energy requirement provides a high estimate for
water required when the animal is resting is in a
water compared with the minimal requirement
postabsorptive state (i.e., the renal solute load has been
documented in experiments with dogs and cats deprived
excreted) and is not exposed to thermal stress.
of food. This disparity makes estimating basal water needs
Basal water needs of dogs and cats have not been well
of inappetent, quiet dogs problematic. Data on basal
studied. Water intake of healthy dogs and cats in a
water needs of small animals would help clinicians to
thermoneutral environment and deprived of food has
been measured in a few experiments (Table 1-13). Two devise appropriate strategies for fluid therapy in
investigators found that quiet, food-deprived dogs (body inappetent, sick animals by providing a baseline assess-
weights 8 to 15 kg) or cats (approximately 3.5 kg) con- ment from which maintenance or replacement fluid needs
fined to metabolism cages drank about 5 mL/kg of water may be estimated by use of a multiplication factor (i.e.,
daily. A third investigator found that intake was consider- maintenance ¼ 2 basal water need). Current methods
for assessing fluid needs may overestimate the patient’s
ably higher (17.6 2.2 mL/kg/day) in dogs of about
actual requirements because sick patients are inappetent
the same body weight. The dogs in the latter experiment
and inactive. Administration of an excessive volume of
may have been more active and may have had larger evap-
fluid could be detrimental, especially to patients with
orative losses and greater compensatory drinking than
heart failure or oliguric renal failure. Most patients
dogs or cats in the previous experiments. If basal water
respond satisfactorily to currently used standard fluid-
need is estimated by determining the basal energy
1
requirement, using the preceding formula, the water replacement regimens, because the kidneys readily
requirement of a 10-kg dog would be 40 mL/kg/day, excrete excess fluid and solute. When calculating water
assuming 1 mL of water per kilocalorie of energy needs, however, it would be prudent to consider that
inactive, sick animals with decreased or no food intake
required. Data for dogs deprived of food suggest that
may require less water than usual empirical estimates
basal water requirements may be much lower. This fact
may indicate.
is not surprising if we consider that when water intakes
of dogs in the study by O’Connor and Potts 40
TABLE 1-13 Water Consumption in Food-Deprived Dogs and Cats
Body Days Average Water
Number Weight (kg) Starvation Consumption (mL/kg) Reference
10 dogs* NR { 7 17.6 2.2 Cizek 10
5 dogs 8-11 15 4.0 Morris and Collins 34
2 dogs 9.47 4 4.1 (3.2-5.0) Prentiss et al 43
11.71
2 dogs 11.71 9 5.4 (3.0-7.7) Prentiss et al 43
9.47
2 cats 3.59 7 5.2 (3.7-6.7) Prentiss et al 43

*Beagle or hound types of dogs.
{
NR, Not reported.