Page 953 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

928 CHAPTER 7

VetBooks.ir Fractional excretion (FE) should be calculated using deprivation. If dehydration becomes apparent, a
loss of 5% of body weight occurs or the urine SG
the following equation:
FE = (([Cr] plasma /[Cr] urine ) × ([X] plasma /[X] urine )) × 100 reaches 1.025, the test should be stopped. If the SG
reaches 1.025, then the ability to concentrate urine
has been proven. Horses with central or nephro-
where [Cr] plasma and [Cr] urine are the creatinine (Cr) genic diabetes insipidus cannot concentrate urine.
concentrations in the plasma and urine, respectively, In some cases, horses with psychogenic polydipsia
and [X] plasma and [X] urine are the concentrations of a may also not be able to concentrate urine because
specific electrolyte or mineral in plasma and urine, of washout of the medullar interstitial osmotic gra-
respectively (Table 7.4). dient. In such horses a partial deprivation of water
Urine and plasma should be collected at the intake at 40 ml/kg/day should restore the osmotic
same time. The suggested reference intervals can gradient in a few days.
be influenced by diet. Concern has been expressed
that fractional urinary excretion of electrolytes does Vasopressin (antidiuretic
not provide reliable information regarding urinary hormone) challenge test
excretion of electrolytes due to considerable physi- Diabetes insipidus is an endocrine cause of polyuria/
ological variation within, and between, days, in uri- polydipsia (PU/PD) syndrome. The lack of vaso-
nary electrolyte excretion even if diet and exertion pressin secretion or the lack of response of renal
are held constant. This has limited its use in the collecting ducts to vasopressin distinguishes cen-
detection of renal disease. tral from nephrogenic diabetes insipidus. They can
be differentiated by exogenous administration of
Water-deprivation test vasopressin. Intravenous administration of 20 µg
The water-deprivation test is a simple test that desmopressin acetate should produce an increase in
determines whether an apparent inability to con- urine SG in normal horses and horses with central
centrate urine is caused by psychogenic polydipsia diabetes insipidus, but not horses with nephrogenic
or diabetes insipidus. The bladder should be emp- diabetes insipidus. Desmopressin acetate nasal spray
tied by catheterisation and a baseline urinalysis had been successfully used i/v in horses.
performed. Baseline serum urea and creatinine
levels and body weight should be recorded before Ultrasonography and radiography
removal of food and water. Water-deprivation Kidneys can be relatively easily examined ultraso-
testing should never be performed in a dehy- nographically. The right kidney can only be imaged
drated or azotaemic horse. Urine SG is measured transabdominally through the dorsolateral extents
12 and 24 hours after the initiation of the test. of the last three intercostal spaces (ICSs) (Fig. 7.8).
Horses should be closely monitored during water The left kidney is imaged transabdominally in the
left paralumbar fossa or transrectally (Fig. 7.9).
A good image is achieved with a 2.5- or 3-MHz
Table 7.4 Reference intervals for fractional probe. A 5-MHz probe is sometimes adequate to
excretion of electrolytes examine the right kidney. In AKI, the kidneys are
normal or increased in size, and the corticomedul-
ELECTROLYTE RANGE (%) lary junction may be indistinct. In CKD the kidneys
Sodium <1.00 are usually decreased in size with increased echo-
Chloride <1.50 genicity. Cystic or mineralised areas can be associ-
Potassium 15–65 ated with chronic renal diseases or, more often, with
Phosphorous <0.50 congenital abnormalities. Calculi within the renal
Calcium <7 pelvis are occasionally seen. Doppler ultrasound may
characterise renal haemodynamics. It unfortunately
Magnesium <15
lacks sufficient specificity to be uniformly applied.