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

548
Chapter | 17 Fluid, Electrolyte, and Acid-Base Balance
TABLE 17-4 Reference Values for Blood Gas and Electrolyte Determinations a
Electrolyte or Gas Dog Cat Horse Ox Sheep Goat Pig
Sodium (mEq/l) 140–155 147–156 132–146 132–152 139–152 142–155 135–150
Potassium (mEq/l) 3.7–5.8 4.0–5.3 2.6–5.0 3.9–5.8 3.9–5.4 3.5–6.7 4.4–6.7
Chloride (mEq/l) 105–120 115–123 99–109 97–111 95–103 99–110 94–106
Calcium (mg/dl) 9.0–11.3 6.2–10.2 11.2–13.6 9.7–12.4 11.5–12.9 8.9–11.7 7.1–11.6
Magnesium (mg/dl) 1.8–2.4 2.2 2.2–2.8 1.8–2.3 2.2–2.8 2.8–3.6 2.7–3.7
Phosphorus (mg/dl) 2.6–6.2 4.5–8.1 3.1–5.6 5.6–6.5 5.0–7.3 6.5 5.3–9.6
Anion gap (mEq/l) 15–25 6.6–14.7 13.9–20.2
pH 7.31–7.42 7.24–7.40 7.32–7.44 7.35–7.50 7.32–7.54 7.39
p CO 2 (mmHg) 29–42 29–42 38–46 35–44 37–46 44.3
Bicarbonate (mEq/l) 17–24 17–24 24–30 20–30 20–25 25.6
Osmolality (mOsm/kg H 2 O) 280–305 280–305 270–300 270–300
a
In part from Tasker (1980) .
When sodium-containing fluids are administered to dehydrated
volume-depleted subjects, plasma volume reexpansion
is reflected by the return of PCV and TPP toward the
normal range ( Hayter et al ., 1962 ). Declining PCV and
TPP in response to fluid therapy are the two most useful
laboratory indicators of return of effective circulating fluid
volume and should be correlated with clinical evidence
such as return of normal pulse pressure, capillary refill
time, and jugular distensibility.
B . Serum Sodium
Serum sodium concentration varies within relatively narrow
limits in the normal individual, but there is substantial
interspecies variation in the normal range of sodium, chloride,
and osmolality as indicated in Table 17-4 . A serum
sodium concentration of 134 mEq/l (134 mmol/l), although
quite normal for a horse or cow, represents a significant
hyponatremia in a dog or cat. Before proceeding with a
discussion of the significance of alterations of sodium
concentration, some comment on the methods used for
electrolyte determination is appropriate. In the past, flame
photometry was the standard method for the determination
of both sodium and potassium concentrations. Presently,
electrolyte concentrations in biological fluids are generally
determined utilizing ion-specific electrode technology.
Although consistent differences might have been expected,
ion-specific electrode instruments, which dilute samples,
tend to yield values that are similar to values reported from
the flame photometer. This situation may be a function of
the mathematical algorithm employed in these devices to
convert changes in electric potential to electrolyte concentration.
Interfering substances in the urine of some animal
species render urine potassium determinations inaccurate
when assessed by ion-specific potentiometry ( Brooks
et al. , 1988 ).
1 . Hyponatremia
The common causes of hyponatremia are listed in Table 17-5 .
A falsely low sodium concentration may be noted when
there is marked hyperlipemia or hyperproteinemia. Large
quantities of lipid or protein occupy a significant volume
in a serum or plasma sample, and because electrolytes are
dissolved only in the aqueous phase, the measured concentrations
will be falsely low. The presence of obvious
lipemia or markedly elevated serum protein concentration
should alert the clinician to the probable cause of an
accompanying hyponatremia. Should this information not
be available, the presence of a false hyponatremia can be
determined by comparison of the measured serum osmolality
and the calculated osmolality based on sodium, glucose,
and urea concentration as explained in Section VIII.E.
This potential cause for confusion in interpretation of
hyponatremia can be avoided if ion-specific electrodes are
used for electrolyte determination.
Marked hyperglycemia associated with diabetes mellitus
or the administration of glucose at an excessive rate
generally produces a hyponatremia. As glucose concentration
increases in the ECF, osmotic forces are generated,
which result in the movement of cellular water into
the ECF, diluting serum sodium concentration. The actual