Page 56 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

46 ELECTROLYTE DISORDERS

where BUN is blood urea nitrogen. In this equation, the movement and is dependent on the presence of
concentrations of urea and glucose in milligrams per deci- impermeant solutes in the solution. 41 Thus, tonicity
liter are converted to millimoles per liter by the conver- may be thought of as effective osmolality. A solution is
sion factors 2.8 and 18. The measured osmolality hypertonic to a reference solution from which it is
should not exceed the calculated osmolality by more than separated by a semipermeable membrane if its concentra-
10 mOsm/kg. 42,149 If it does, an abnormal osmolal gap is tion of impermeant solutes is greater than that of the ref-
said to be present. This occurs when an unmeasured sol- erence solution. A solution is hypotonic to the reference
ute (i.e., one not accounted for in the equation) is present solution if its concentration of impermeant solutes is less
in large quantity in plasma (e.g., mannitol or metabolites than that of the reference solution. A solution is isotonic
of ethylene glycol) or when hyperlipemia or hyperpro- to the reference solution if its concentration of
teinemia results in pseudohyponatremia (see section on impermeant solutes equals that of the reference solution.
Hyponatremia with Normal Plasma Osmolality). 42,50,56 Tonicity or effective osmolality may be estimated as
P osm BUN/2.8. Consider a dog with the following
SPECIFIC GRAVITY laboratory values: serum sodium, 125 mEq/L; BUN,
The term specific gravity refers to the ratio of the weight 280 mg/dL; and glucose, 90 mg/dL. This patient is
of a volume of liquid to the weight of an equal volume of hyponatremic and azotemic and has plasma hyperos-
distilled water. Specific gravity depends not only on the molality (calculated plasma osmolality ¼ 355 mOsm/
number of particles present in the solution but also on kg) but hypotonicity (effective plasma osmolality ¼ 255
their molecular weight. The clinician can easily measure mOsm/kg). Clinical measurement of osmolality by freez-
specific gravity with a hand-held refractometer. ing-point depression osmometry does not distinguish
Multiplying the last two digits of the urine specific gravity between permeant and impermeant solutes and thus does
(USG) by 36 gives a rough estimate of urine osmolality in not provide direct information about the tonicity of a
71
dogs. This rule may be misleading if the urine sample solution.
contains a large amount of high-molecular-weight solute,
because substances with high molecular weights have a DIURESIS
greater effect on specific gravity than on osmolality. The term diuresis refers to urine flow that is greater than
The effects on urine osmolality of some solutes are shown normal (i.e., >1 to 2 mL/kg/hr in dogs and cats). The
in Table 3-2. term solute,or osmotic, diuresis refers to increased urine
flow caused by excessive amounts of nonreabsorbed sol-
TONICITY OR EFFECTIVE ute within the renal tubules (e.g., polyuria associated with
OSMOLALITY diabetes mellitus, administration of mannitol). During

Changes in the osmolality of ECF may or may not initiate osmotic diuresis, urine osmolality approaches plasma
movement of water between the intracellular and extra- osmolality. The term water diuresis refers to increased
cellular compartments. A change in the concentration urine flow caused by decreased reabsorption of solute-
of permeant solutes (e.g., urea, ethanol) does not cause free water in the collecting ducts (e.g., polyuria associated
movement of water because these solutes are distributed with psychogenic polydipsia or diabetes insipidus). Dur-
equally throughout total body water (TBW). A change in ing water diuresis, urine osmolality is less than plasma
the concentration of impermeant solutes (e.g., glucose, osmolality.
sodium) does cause movement of water because such The term isosthenuria refers to urine with an osmolal-
solutes do not readily cross cell membranes. Tonicity ity equal to that of plasma, and hyposthenuria refers to
refers to the ability of a solution to initiate water urine with an osmolality less than that of plasma. The
term hypersthenuria,or baruria, refers to urine with
an osmolality greater than that of plasma, but this term
TABLE 3-2 Effect of Selected Solutes is rarely used and only to describe urine that is very
on Urine Osmolality* concentrated.
Contribution to TYPES OF DEHYDRATION
Molecular Osmolality Dehydration occurs when fluid loss from the body
Substance Mass (da) (mOsm/kg) exceeds fluid intake. Dehydration may be classified
according to the type of fluid lost from the body and
Albumin 69,000 0.144
the tonicity of the remaining body fluids. Pure water loss
Diatrizoate ion 613 16.313
and loss of hypotonic fluid result in hypertonic dehydra-
Glucose 180 55.555
tion because the tonicity of the remaining body fluids is
*1.0 g/dL of each of the listed solutes added to distilled water would increased. Loss of fluid with the same osmolality as that
increase specific gravity by 0.010, but would have the effects on of ECF results in isotonic dehydration, because there is
osmolality shown in the table. no osmotic stimulus for water movement and the