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

VIII. Clinicopathological Indicators of Fluid and Electrolyte Imbalance
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dog ( Jezyk, 1982 ) and certain quarter horses ( Cox, 1985 ;
Pickar et al. , 1991 ; Spier et al. , 1990, 1993 ; Steiss and
Naylor, 1986 ). In the horse, the condition closely resembles
the heritable disease hyperkalemic periodic paralysis,
which has been reported in human subjects and is due
to an alteration in the voltage-regulated sodium chemical
( Rudolph et al ., 1992a, 1992b ). Sudden marked increases
in serum potassium concentration result from the transcellular
movement of potassium. Serum potassium concentration
can reach 8 to 9 mEq/l (8 to 9 mmol/l) and is associated
with profound electrocardiographic abnormalities and fluid
shifts, which result in marked increases in PCV and protein
concentration. The disease is inherited as an autosomal
dominant, and all affected horses can be traced back to
a common ancestor. A DNA test that can detect the single
base pair substitution responsible for this disease has been
developed (Rudolph et al ., 1992ba). Using this procedure,
it is possible to identify individuals that are heterozygous
or homozygous for this trait.
D . Serum Chloride
It long has been assumed that the anion, chloride, which
combines with sodium to form common salt, simply follows
sodium in the physiological processes that regulate body
fluid and electrolyte balance. It is becoming increasingly
apparent that this may not always be true and that some
of the problems ascribed to sodium retention do not occur
unless chloride is present in excess as well ( Kurtz et al .,
1987 ). Causes of alterations in chloride concentration are
given in Table 17-9 . The hyperchloremia and hypochloremia,
which are normally seen in association with roughly
proportional changes in sodium concentration, are due to
changes in body water balance.
Changes in chloride concentration that are not associated
with a similar change in sodium concentration are
usually associated with acid-base imbalances ( Divers et al .,
1986 ). Chloride concentration tends to vary inversely with
bicarbonate concentration. A disproportionate increase in
chloride most commonly is associated with a normal to
low anion gap hyperchloremic metabolic acidosis such as
renal tubular acidosis and may be seen as a compensating
response for a primary respiratory alkalosis ( Saxton and
Seldin, 1986 ). Disproportionate decreases in chloride characteristically
are seen in a metabolic alkalosis but also may
be seen as part of the compensating response for a chronic
primary respiratory acidosis ( Saxton and Seldin, 1986 ).
E . Osmolality
It has been demonstrated that the concentration of sodium
in serum water is closely correlated with the serum osmolality
over an extremely wide range of physiological and
pathological states provided appropriate corrections are
TABLE 17-9 Causes of Alterations in Chloride
Concentration
Hyperchloremia
With proportional increase in sodium
Dehydration (relative water deficit)
Without proportional increase in sodium
Hyperchloremic metabolic acidosis
Compensation for respiratory alkalosis
Hypochloremia
With proportional decrease in sodium
Overhydration (relative water excess)
Without proportional decrease in sodium
Hypochloremic metabolic alkalosis
Compensation for respiratory acidosis
made for the contributions made by variations in glucose
and urea concentrations ( Edelman et al ., 1958 ). The
measurement of serum osmolality has two specific and
very useful purposes ( Gennari, 1984 ): first, to determine
whether serum water content deviates widely from normal
and, second, to screen for the presence of foreign lowmolecular-weight
substances in the blood. Interpretation of
serum osmolality for these purposes requires simultaneous
comparison of the measured osmolality and the calculated
osmolality as determined from the measured concentrations
of the major solutes in serum (sodium, glucose, and
urea). The difference between the measured and the calculated
osmolality is sometimes referred to as the “ osmolal
gap ” ( Feldman and Rosenberg, 1981 ; Shull, 1978, 1981 ).
Sodium is the principal cation in serum, and sodium is
balanced by a number of different anions (chloride, bicarbonate,
protein, sulfate, and phosphate). Sodium concentration
thus provides a reasonable estimate of the total
electrolyte concentration (anions and cations) in the sample
and, in this calculation, is usually multiplied by a factor of 2.
As mentioned earlier, the water content of serum samples
is approximately 94%. Correction for the water content of
serum is not necessary because, fortuitously, it is counterbalanced
by the fact that sodium chloride does not dissociate
completely and has an osmotic coefficient of 0.93 in serum
(Dahms et al ., 1968 ; Rose, 1984 ; Wolf, 1966 ). The osmolality
can thus be calculated using the measured sodium concentration,
and the concentration of the two nonelectrolyte
components of serum that are normally present in amounts
sufficient to influence osmolality:
mOsm/kg H2O 2
sodium glucose urea (17-15)
This calculation is valid if concentrations of sodium, glucose,
and urea are expressed in mmol/l. Conversion of glucose
concentration from mg/dl to mmol/l requires division
by 18, and urea concentration in mg/dl can be converted to
mmol/l by dividing by 2.8.