Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 17 Fluid, Electrolyte, and Acid-Base Balance
A clear understanding of the interrelationships between
specific deficits and their clinicopathological consequences
is essential if an appropriate initial diagnosis is to be made.
Once treatment has been initiated, all clinical and clinicopathological
data will be influenced not only by the primary
medical problem and compensating responses but
also by the effects of chemotherapeutic agents and fluid
therapy as well. Therapeutic intervention may not always
be appropriate, organ function may be impaired, and thus
the anticipated clinicopathological responses become less
predictable. These situations represent the bulk of clinical
case material and laboratory data evaluated by clinicians
and clinical pathologists. Under these circumstances, it is
essential to understand the basic mechanisms that underlie
changes in clinicopathological data and how these changes
relate to specific imbalances.
Rational fluid therapy depends on accurate evaluation
of the fluid and electrolyte deficits, the associated acidbase
alterations, and the primary disease processes that
underlie these imbalances. Evaluation must include an
accurate history, a complete physical examination, and, of
course, laboratory evaluation of appropriate parameters.
A . History
An accurate history is absolutely essential for the evaluation
and management of the patient with fluid and electrolyte
imbalances. Basic signalment factors of age, sex,
breed, pregnancy, and stage of lactation are important
because these factors influence the incidence and severity
of many disorders. The presence of a preexisting or coexisting
disease process and an accurate drug history can be
exceedingly important not only in the evaluation of fluid
and electrolyte disorders but also in fluid selection and
patient management. Of particular importance is the history
of prior renal disease, diuretic usage, or exposure to
potentially nephrotoxic drugs. Status of feed and water
intake is exceedingly useful. Most animals that continue
to eat and drink normally are able to maintain fluid balance
even in the face of excessive fluid losses. However,
reduced or restricted fluid intake in the face of normal to
enhanced fluid losses can quickly result in dehydration.
Inadequate fluid intake may result from neurological disorders
or traumatic injuries to the head or neck, whereas
painful or obstructive lesions in the mouth, pharynx, or
gastrointestinal tract may restrict feed and water intake.
Inadequate water intake is often the result of management
errors, broken or frozen water lines, and other factors.
A history of polydipsia suggests that excessive fluid losses
have occurred. Vomiting and diarrhea are obvious causes
of fluid and electrolyte loss, but these findings also reflect
gastrointestinal disorders, which may contribute to inadequate
fluid and electrolyte intake or absorption.
Excessive fluid losses may be associated with vomiting,
diarrhea, polyuria, excessive salivation, copious drainage
from cutaneous wounds or burns, and as the result of
heavy sweat losses in exercising horses. The water losses
that occur in these situations are generally associated with
significant sodium depletion and subsequent decreases in
the effective circulating fluid volume. Vomiting in small
animals ( Clark, 1980 ), gastrointestinal stasis in ruminants
( Gingerich and Murdick, 1975b ), and excessive sweat
losses in endurance horses ( Carlson, 1983b ) are associated
with large losses or compartmentalization of chloride-rich
fluids, which contribute to the metabolic alkalosis that frequently
accompanies these disorders.
B . Clinical Signs
Dehydration is defined as a loss of body water. Clinical
signs of dehydration are said to be first apparent with fluid
losses equivalent to 4% to 6% of body weight. Moderate
dehydration is said to be present with fluid losses of 8% to
10%, and severe dehydration is present when fluid losses
exceed 12% of body weight. It is important to realize that
although these guidelines have been clinically useful, there
is relatively little documentation of this precise quantitative
relationship in most animal species. Accurate measurement
of fluid intake from all sources and output by all routes is
not possible in most clinical situations. In acute situations,
changes in body weight provide the most accurate guide
to change in net water balance. Repeated measurement
of body weight is a key component of the monitoring of
patients on fluid therapy. The clinical signs of dehydration
include weight loss, altered skin turgor, sunken eyes, and
dry mucous membranes. If control of renal function is normal,
urine volume is generally markedly reduced. The clinical
consequences of dehydration depend much more on the
pattern of electrolyte loss than the absolute water deficit.
Clinical signs associated with acute sodium deficits are
largely related to hypovolemia and decreases in the effective
circulating volume. These signs include increased
pulse rate, decreased pulse pressure, delayed jugular distensibility,
increased capillary refill time, and decreased
blood pressure. Urine output is generally decreased and
urine sodium and chloride concentrations are normally
reduced. Decreases in ECF volume are always reflected by
decreases in plasma volume, but the reverse is not always
true. In the absence of blood or protein loss, the PCV and
TPP concentrations increase, reflecting the decrease in
plasma and ECF volume as will be discussed more fully in
Section VIII.A.
VIII . CLINICOPATHOLOGICAL INDICATORS
OF FLUID AND ELECTROLYTE IMBALANCE
A . Packed Cell Volume and Total Plasma
Protein
The packed cell volume (PCV) and total plasma protein
(TPP) concentration are simple, convenient, and useful tools