Page 80 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

70 ELECTROLYTE DISORDERS

clarithromycin) will markedly increase its plasma concen- consider causes such as high ambient temperature (i.e.,
tration, and use with other drugs metabolized by increased insensible water losses), regular prolonged
CYP3A4 (e.g., statins, midazolam, amlodipine) will exercise, water consumption to replace a previous hydra-
increase the plasma concentrations of these drugs and tion deficit, and third-space distribution of consumed
potentially lead to toxicity. water. Excessive administration of parenteral fluids causes
Although considered primarily aquaretics, tolvaptan polyuria without polydipsia. The diagnostic approach to
and lixivaptan may increase sodium excretion at higher polyuria and polydipsia is summarized in Table 3-4 and
dosages, possibly by blocking the sodium-retaining effect Figure 3-15.
of AVP on the thick ascending limb of Henle’s loop. LABORATORY EVALUATION
Unresolved is whether or not V 2 -receptor antagonists
could decrease concentrations of factor VIII and OF POLYURIA AND
von Willebrand factor, which are known to be increased POLYDIPSIA
by AVP.

CLINICAL APPROACH TO ENDOGENOUS CREATININE
POLYURIA AND POLYDIPSIA CLEARANCE
In chronic progressive renal disease, urinary
Normal daily water intake and urine output in dogs and concentrating ability is impaired after two thirds of the
cats are influenced by the nutrient, mineral, and water nephron population has become nonfunctional, whereas
content of the diet. Normal water intake should not azotemia does not develop until three quarters of the
exceed 90 mL/kg/day in dogs and 45 mL/kg/day in nephrons have become nonfunctional. Thus, the main
cats. Normal urine output ranges from 20 to 45 mL/ indication for determination of endogenous creatinine
kg/day in dogs and cats. Dogs with disorders such as psy- clearance is the clinical suspicion of renal disease in a
chogenic polydipsia, CDI, and NDI may have water con- patient with polyuria and polydipsia but normal
sumption as much as five times the normal. BUN and serum creatinine concentrations. The only
Dogs and cats with polyuria and polydipsia are requirements for determination of endogenous creati-
encountered frequently in small animal practice. The nine clearance are an accurately timed collection of urine
causes of polyuria and polydipsia, their pathophysiologic (usually 24 hours), determination of the patient’s body
mechanisms, and the necessary confirmatory laboratory weight, and measurement of serum and urine creatinine
tests are presented in the Table 3-4. The most common concentrations. Failure to collect all urine produced
causes are chronic renal failure in dogs and cats, diabetes results in an erroneously reduced calculated clearance
mellitus in dogs and cats, hyperadrenocorticism in dogs, value. Use of creatinine clearance as an estimate of GFR
and hyperthyroidism in cats. These common causes must is discussed further in Chapter 2.
always be ruled out before beginning an exhaustive diag-
nostic evaluation of the animal. WATER DEPRIVATION TEST
Determination of the specific gravity of a random urine The water deprivation test is indicated in evaluation of
sample from the animal is a logical starting point for eval- animals with confirmed polydipsia and polyuria, the cause
uation of polyuria and polydipsia. If a random USG is of which remains undetermined after the initial diagnos-
greater than 1.030 to 1.035, the clinician should obtain tic evaluation. It is usually performed in animals with
additional history to rule out other disorders that may hyposthenuria (USG <1.007) that are suspected to have
have been confused with polyuria (e.g., urinary inconti- CDI, NDI, or psychogenic polydipsia. An animal that is
nence and dysuria). If a random USG is less than 1.025 dehydrated but has dilute urine has already failed the test
to 1.030, an initial diagnostic evaluation is warranted. and should not be subjected to water deprivation. In such
Many causes of polyuria and polydipsia can be ruled an animal, failure to concentrate urine is probably caused
out by an initial database consisting of a complete history by structural or functional renal dysfunction or adminis-
and physical examination, complete blood count, bio- tration of drugs that interfere with urinary concentrating
chemical profile (including electrolytes), urinalysis, urine ability. The water deprivation test is also contraindicated
culture, and abdominal radiographs. If the animal is oth- in animals that are azotemic. The test should be
erwise healthy, it is helpful to instruct the owner to quan- performed with extreme caution in animals with severe
titate and record the animal’s daily water consumption at polyuria because such patients may rapidly become
home over a 3- to 5-day period. Determination of water dehydrated during water deprivation if they have defec-
intake at home prevents potential reduction in water tive urinary concentrating ability.
intake precipitated by the stress of hospitalization. At the beginning of the water deprivation test, the
With some exceptions (e.g., psychogenic polydipsia), bladder must be emptied and baseline data collected
polydipsia usually occurs as a consequence of polyuria. (body weight, hematocrit, total plasma proteins, skin
If polydipsia occurs without polyuria, the clinician must turgor, serum osmolality, urine osmolality, and USG).