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

VI. Evaluation of Imbalances
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sodium concentration has increased from a normal of
140 mEq/l (140 mmol/l) to 155 mEq/l (155 mmol/l). Fluid
losses are shared proportionately by the ICF and ECF,
and few other clinical or clinicopathological abnormalities
will be noted until the fluid losses become more severe.
Hypernatremia is associated with contraction of the ICF
volume and shrinkage of the cells.
When water losses are associated with proportionate
losses of exchangeable cations (i.e., 130 to 150 mEq [130
to 150 mmol] of sodium plus potassium per l of water lost),
an isotonic fluid volume contraction develops. Despite what
at times may be large water and electrolyte losses, plasma
sodium concentration and osmolality remain unchanged.
The ECF and ICF share the fluid losses in a distribution
that reflects the relative losses of sodium and potassium as
proportional of the initial content in their respective fluid
compartments. In many instances, isotonic dehydration may
occur in which sodium loss exceeds potassium loss. This
type of dehydration is observed with heavy sweat loss in
horses, with acute diarrhea in most species, and with inappropriate
diuretic administration. The effects of an isotonic
fluid loss of 30 l of water, 3800 mEq (3800 mmol) of sodium,
and 400 mEq (400 mmol) of potassium in a 450-kg horse are
illustrated in Figure 17-3 . With this type of isotonic fluid
loss, plasma sodium concentration remains within normal
limits despite the development of large sodium deficits.
These animals manifest clinical signs of inadequate circulating
fluid volume reflecting the sodium deficit and the associated
decrease in plasma and ECF volume as has been shown
in human subjects ( McCance, 1937, 1938 ). When a substantial
portion of the water deficit is replaced by water consumption
or free water administration in these animals, the serum
sodium concentration and osmolality decline, and a hypotonic,
hypovolemic dehydration can develop ( Sufit et al .,
1985 ). The hyponatremia noted in this circumstance is
best considered as an indication of a relative water excess
(Scribner, 1969 ).
2 . Water Excess-Overhydration
The effects of the administration and retention of 30 l of
water in a 450-kg horse are illustrated in Figure 17-4 . In
this example, total body water has been increased by 30 l,
and there is an absolute as well as relative water excess. The
primary effect of this water load is to dilute the electrolytes
in the body fluids producing a substantial decline in plasma
sodium concentration and osmolality. These changes occur
despite the fact that there has been no change in sodium or
potassium balance. In this example, the ECF and ICF share
the water excess proportionately. The hyponatremia is associated
with expansion of ICF and thus swelling of the cells.
Overhydration rarely occurs in normal individuals. The
large water load described in Figure 17-4 , if administered
to a normal animal, would produce only transient changes,
and the excess water would be eliminated by renal excretion.
Even animals with psychogenic polydipsia ordinarily
are able to maintain normal water balance through appropriate
renal water excretion unless sodium depletion and renal
medullary washout occur. However, overhydration can occur
iatrogenically as the result of excessive fluid administration
to patients with compromised renal function. If these fluids
provide free water, as with 5% dextrose, plasma sodium
concentration will decrease reflecting the change in relative
water balance. If these fluids consist of isotonic sodium-containing
replacement fluids such as saline or lactated Ringer’s
solution, there will be little or no change in plasma sodium
concentration ( Carlson and Rumbaugh, 1983 ; Cornelius et
al ., 1978 ), but there will be an increase in plasma and ECF
volume with the potential for cardiovascular overload, pulmonary
edema, or generalized edema formation. In this
instance, the primary problem is sodium retention, and the
changes in water balance are secondary.
B . Sodium
The ECF volume contains approximately one-half to onethird
of the body’s sodium. Most of the remaining sodium
is bound in skeletal bone, relatively little of which is rapidly
FIGURE 17-3 Body fluid compartments in a 450-kg horse with an
isotonic fluid loss of 30 l of water, 3800 mEq (3800 mmol) sodium, and
400 mEq (400 mmol) potassium. Serum sodium remains unchanged at
140 mEq/l (140 mmol/l) despite the development of the substantial sodium
deficit. Fluid losses are borne primarily by the extracellular fluid (ECF)
volume.
FIGURE 17-4 Body fluid compartments in a 450-kg horse after administration
and retention of 30 l of water. Although no sodium or potassium
losses have occurred, serum sodium concentration has declined from
140 mEq/l (140 mmol/l) to 127 mEq/l (127 mmol/l) reflecting a relative
water excess. Water retention results in proportionate expansion of extracellular
fluid (ECF) volume and intracellular fluid (ICF) volume.