Page 343 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Introduction to Fluid Therapy 335

TABLE 14-4 Physical Findings in fat and elastin. A false impression of dehydration also
Dehydration may occur with persistent panting, which may dry the oral
mucous membranes. The urinary bladder should be small
Percent in a dehydrated animal with normal renal function. A
Dehydration Clinical Signs large, urine-filled bladder in a severely dehydrated patient
indicates failure of the normal renal concentrating
<5 Not detectable mechanism.
5-6 Subtle loss of skin elasticity Body weight recorded on a serial basis traditionally has
6-8 Definite delay in return of skin to normal been thought to be the best indicator of hydration status,
position
especiallywhenfluidloss hasbeenacuteand previousbody
Slight prolongation of capillary refill time
weight has been recorded. Loss of 1 kg of body weight
Eyes possibly sunken in orbits
indicates a fluid deficit of 1 L. Unfortunately, previous
Possibly dry mucous membranes
body weight is often unknown in animals presented for
10-12 Tented skin stands in place
treatment. However, records from previous routine hos-
Definite prolongation of capillary
refill time pital visits may provide this information. Despite conven-
Eyes sunken in orbits tional reasoning, clinician estimates of hydration in dogs
Dry mucous membranes and cats admitted to a veterinary teaching hospital inten-
Possibly signs of shock (tachycardia, cool sive care unit did not reliably predict changes in weight
extremities, rapid and weak pulses) after 24 to 48 hours of fluid therapy. 18 Loss of weight in
12-15 Definite signs of shock chronicdiseases includes loss of muscle mass and fluid loss.
Death imminent
An anorexic animal may lose 0.1 to 0.3 kg of body weight
per day per 1000 kcal of energy requirement. 13 Losses in
From Muir WW, DiBartola SP. Fluid therapy. In: Kirk RW, editor.
Current veterinary therapy VIII. Philadelphia: WB Saunders, 1983: 33. excess of this amount indicate fluid loss. Another factor
that must be considered in evaluating body weight is the
possibility of third-space loss. Fluid lost into a third space
peripheral venous distention (e.g., inspection of jugular does not decrease body weight. 23
veins). A decrease in the volume of the interstitial com-
partment leads to decreased skin turgor and dryness of LABORATORY FINDINGS
the mucous membranes. A decrease in plasma volume The hematocrit or packed cell volume (PCV), total
leads to tachycardia, alterations in peripheral pulses, plasma protein concentration (TPP), and urine specific
and collapse of peripheral veins. When these cardiovascu- gravity (USG) are simple laboratory tests that can aid in
lar signs are present, the patient is in shock and should be the evaluation of hydration. It is important to obtain
resuscitated promptly before correction of the hydration these values before initiating fluid therapy. The PCV
deficit. Typically, such signs of hypovolemic shock appear and TPP should be evaluated together to minimize errors
with loss of at least 10% to 12% of the patient’s body in interpretation. The PCV and TPP increase with all
weight. The fluid deficit in a given patient is difficult to types of fluid losses excluding hemorrhage, whereas
determine with accuracy because of the subjectivity of serum sodium concentration increases, decreases, or
skin turgor evaluation and the possibility of undetected remains unchanged depending on the loss (e.g., hypo-
ongoing (contemporary) losses. Thus a crude clinical tonic, hypertonic, isotonic). The effects of the different
estimate of hydration status and the patient’s response types of dehydration on the serum sodium concentration
to fluid administration become important tools in are discussed in Chapter 3. Table 14-5 shows possible
evaluating the extent of dehydration that was present interpretations of various combinations of PCV and
and in formulating ongoing fluid therapy. TPP values. The PCV alone may be an unreliable indica-
Skin turgor is dependent on the amount of subcutane- tor of hemoconcentration in water-deprived dogs, and
ous fat and elastin and on interstitial volume. Detection of although TPP increases, test results may not be above
dehydration by skin turgor is dependent on the animal’s the upper limit of the normal range. 19 In one study of
skin turgor before dehydration developed, the position of dogs and cats admitted to an intensive care unit, baseline
the animal (e.g., standing, recumbent) when the skin is measurements of PCVand TPP were not abnormally high
checked, the site used for evaluation, and the amount in animals judged clinically to be dehydrated, and fluid
of subcutaneous fat. 19 Skin pliability should be tested therapy with crystalloids in dogs had no significant effect
over the lumbar region with the dog in a standing posi- on PCV, although TPP decreased slightly. 18 The USG
tion. When evaluated by skin turgor, obese animals may before fluid therapy is helpful in the preliminary evalua-
appear well hydrated owing to excessive subcutaneous tion of renal function. USG should be high (>1.045)
fat despite being dehydrated. Conversely, emaciated in a dehydrated dog or cat if renal function is normal. This
animals and older animals may appear more dehydrated may not be true if other disorders affecting renal
than they actually are because of lack of subcutaneous concentrating ability, such as medullary washout of