Page 64 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

54 ELECTROLYTE DISORDERS

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BOX 3-1 Causes of ECF, and thus one twelfth of the volume loss (¼ /3)is
derived from the intravascular space. The oncotic
Hypernatremia pressure generated by plasma proteins favors retention
of water within vessels, and the plasma compartment
Pure Water Deficit may not share proportionately in the volume loss. 41 As
Primary hypodipsia (e.g., in miniature schnauzers) a result of these factors, volume depletion is usually not
Diabetes insipidus a clinical feature of pure water loss. It is almost impossible
Central for a conscious animal with an intact thirst mechanism
Nephrogenic and access to water to develop hypertonicity caused by
High environmental temperature pure water loss. Thus, hypertonicity associated with pure
Fever
water loss usually implies that water intake has been
Inadequate access to water
defective.
Hypotonic Fluid Loss Consider a normal 10-kg dog with a serum osmolality
Extrarenal of 300 mOsm/kg. We assume that TBW is 60% of body
Gastrointestinal weight, with 40% being intracellular and 20% extracellu-
Vomiting lar, and that the major extracellular (i.e., NaCl) and intra-
Diarrhea cellular (i.e., KCl) solutes are impermeant. The number
Small intestinal obstruction of osmoles of solute in ECF would be 2 L 300
Third-space loss
mOsm/kg ¼ 600 mOsm, and the number in ICF would
Peritonitis
Pancreatitis be 4 L 300 mOsm/kg ¼ 1200 mOsm. Without access
Cutaneous to drinking water, a loss of 1 L of pure water from ECF
Burns would cause water to move from ICF to ECF so as to
Renal equalize osmolality between the compartments
Osmotic diuresis according to the following equation:
Diabetes mellitus
Mannitol infusion New ECF osmolality ¼ new ICF osmolality
Chemical diuretics 600 mOsm= 1 þ xÞ L ¼ 1200 mOsm=ð4 xÞL
Chronic renal failure ð
Nonoliguric acute renal failure
Postobstructive diuresis where x is the volume of water moving between
compartments:
Impermeant Solute Gain
Salt poisoning
600ð4 xÞ¼ 1200ð1 xÞ
Hypertonic fluid administration
Hypertonic saline x ¼ 0:67 L
Sodium bicarbonate
Parenteral nutrition The new volumes and osmolalities are:
Sodium phosphate enema
Hyperaldosteronism ECF : 600 mOsm=1:67 L ¼ 360 mOsm=kg
Hyperadrenocorticism
ICF : 1200 mOsm=3:33 L ¼ 360 mOsm=kg
of hypotonic fluids, or gain of sodium can cause hyperto- Note that the intracellular compartment has lost an
nicity of the ECF and hypernatremia. The causes of amount equal to two thirds of the water deficit
hypernatremia are listed in Box 3-1, and the clinical (0.67 L) and that the final ECF volume (1.67 L) is lower
approach to the patient with hypernatremia is outlined than the original volume (2 L) by an amount equal to one
in Figure 3-7. third of the total water deficit (0.33 L). Thus, the two
compartments have shared proportionately in the water
PURE WATER LOSS loss. These changes are depicted in Figure 3-8.
When a deficit of pure water develops, the ECF becomes Development of a pure water deficit is uncommon in
hypertonic in relation to the intracellular fluid (ICF), and small animal medicine. The main causes of hypertonicity
osmotic forces cause movement of water from the intra- related to pure water deficit are hypodipsia, caused by
cellular to the extracellular compartment. The result is neurologic disease, and diabetes insipidus, which
that the volume loss is shared proportionately between represents abnormal renal loss of water. Other causes of
the extracellular and intracellular compartments. Approx- pure water deficit include respiratory losses during expo-
imately two thirds of the volume loss comes from the sure to high environmental temperature (e.g., panting),
intracellular compartment and one third from the extra- fever, and inadequate access to water (e.g., frozen water
cellular compartment. Plasma volume is one fourth of the bowl, inattentive owner).