Page 742 - Small Animal Clinical Nutrition 5th Edition

770 Small Animal Clinical Nutrition

by glomerular filtration, and serum urea nitrogen concentra-
Table 37-5. Standard protocol for measuring blood pressure in
VetBooks.ir dogs and cats.* tions are inversely proportional to GFR. However, because urea
is passively reabsorbed in the tubules, especially at reduced
Calibrate the blood pressure measurement device twice yearly.
Standardize the procedure used. tubular flow rates, urea clearance is not an accurate measure of
GFR. Clinical conditions that can increase serum urea nitrogen
• Obtain measurements in an environment that is quiet,
located away from distractions (e.g., other patients) and concentration include gastrointestinal hemorrhage, consump-
with the owner present. tion of high-protein foods and catabolic drugs (e.g., glucocor-
• Restrain patients in a comfortable position, ideally in ventral ticoids). Severe hepatic disease (e.g., portosystemic vascular
or lateral recumbency to limit the distance from the base of
the heart to the measurement cuff. Patients should be calm shunts), feeding a low-protein food and conditions causing
and motionless during the procedure. increased urine volume (e.g., intravenous fluid therapy) can
• Use a cuff that is approximately 30 to 40% of the circum- decrease serum urea nitrogen concentrations independent of
ference of the measurement site in cats and 40% in dogs.
• Have the same trained individual, ideally a technician, per- renal function.
form blood pressure measurements each time. Creatinine results from the nonenzymatic breakdown of
• Determine and record five to seven consecutive and con- muscle phosphocreatine. During steady states, creatinine pro-
sistent (<20% variability) values.
• Discard the first measurement and determine the mean of duction is constant and related to muscle mass. Serum creati-
all remaining values to obtain the patient’s blood pressure nine concentration is less influenced by feeding than serum
measurement. urea nitrogen concentration. However, it may be affected by
• Record the cuff size and site of placement (e.g., limb, tail),
values for all measurements obtained, final (mean) value, breed and body size (Gleadhill, 1995). In a study of retired
details of any additional information (e.g., nervous patient) racing greyhounds, mean values for serum creatinine concen-
and interpretation of results by a veterinarian. tration (1.8 ± 0.1 mg/dl) and GFR (3.0 ± 0.1 ml/min./kg)
*Adapted from Brown S, Atkins C, Bagley R, et al. Guidelines
for the identification, evaluation, and management of systemic were significantly greater than values from control dogs.
hypertension in dogs and cats. Journal of Veterinary Internal However, blood urea nitrogen values were not different
Medicine 2007; 21: 542-558.
(Drost et al, 2006). Increased serum creatinine levels in grey-
hounds may be due to increased muscle mass in this breed. In
contrast, it is possible for serum creatinine concentration to
remain lower than expected or to not be increased in propor-
Evaluation of Renal Function tion to the decrease in GFR in older patients with decreased
The primary functions of the kidneys are to excrete metabolic muscle mass and kidney disease.
wastes (e.g., urea, creatinine), regulate fluid, electrolyte and When considering the magnitude of azotemia it’s important
acid-base balance and produce or activate several hormones to recognize that the relationship between serum urea nitrogen
including erythropoietin, calcitriol and renin. Anatomically, and creatinine concentrations and GFR is not linear (Figure
these functions occur in glomeruli (i.e., glomerular filtration 37-1). Thus, very large changes in GFR early in the natural
and membrane permselectivity), renal tubules (i.e., urine con- course of CKD cause only small changes in serum urea nitro-
centration and tubular resorption) and other areas of the kidney gen and creatinine concentrations. These small changes may
(i.e., erythropoietin, calcitriol, renin). CKD may be associated not exceed the upper limit of the laboratory reference range and
with generalized renal dysfunction or it may involve only one thus may go unrecognized throughout most of stage 1 CKD.
function (e.g., tubular resorptive defect in Fanconi syndrome). Small decreases in GFR cause disproportionately large increas-
es in serum urea nitrogen and creatinine concentrations in more
Glomerular Filtration advanced CKD (stages 3 and 4).
The most commonly evaluated renal function is glomerular fil- Evaluation of serum urea nitrogen and creatinine is used to
tration, which is determined by estimating or measuring GFR. indirectly assess GFR in most patients; however, measuring
Under steady state conditions, serum concentrations of urea GFR is helpful for identifying kidney dysfunction that occurs
nitrogen and creatinine are the time-honored methods for indi- before the onset of azotemia (e.g., breeds known to have famil-
rectly estimating GFR.These tests are useful for detecting large ial kidney disease, patients with polyuria/polydipsia due to kid-
decreases in GFR (75% or greater), but lack sensitivity for ney disease, when potentially nephrotoxic treatment will be
detecting smaller decreases in glomerular filtration (Figure 37- used). Urinary clearance of infused inulin is the gold standard
1). In addition, serum urea nitrogen and creatinine values are for measuring GFR. However, this technique is limited to
affected by nonrenal factors, which contribute to the broad research settings because it requires collection of multiple,
ranges for normal values. timed blood and urine samples and a constant rate infusion of
Urea is produced in the liver from ammonia derived from the inulin. Other methods have been used to estimate GFR but
ornithine cycle, which catabolizes amino acids.The catabolized each has disadvantages. Endogenous creatinine clearance
amino acids come from exogenous (dietary) and endogenous underestimates GFR because non-creatinine chromogens are
proteins. Urea is distributed throughout intracellular and extra- present in plasma. Exogenous administration of creatinine
cellular water and is freely diffusible; therefore, it is common to reduces this potential problem by decreasing the proportion of
use the terms blood urea nitrogen, serum urea nitrogen and non-creatinine chromogens in plasma. A newer creatinine-spe-
plasma urea nitrogen interchangeably.The kidneys excrete urea cific enzymatic analytical method eliminates the problem