Page 72 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

62 ELECTROLYTE DISORDERS

Decreased serum sodium concentration
( <140 mEq/L in dogs; <149 mEq/L in cats)
Plasma osmolality determination

Normal P osm Low P osm High P osm
( 290-310 mOsm/kg ) ( < 290 mOsm/kg ) ( > 310 mOsm/kg )

Hyperlipemia? hyperglycemia?
Hyperproteinemia? mannitol infusion?

Pseudohyponatremia?

Evaluation of volume status

Hypervolemia Normovolemia Hypovolemia

Severe liver disease Psychogenic polydipsia Nonrenal route Renal route
Congestive heart failure SIADH
Nephrotic syndrome Antidiuretic drugs
Advanced renal failure Myxedema coma
Hypotonic fluids Gastrointestinal loss Hypoadreno-
• Vomiting corticism
• Diarrhea Diuretic
Third space loss administration
• Pancreatitis
• Peritonitis
• Pleural effusion
• Uroabdomen
Figure 3-12 Clinical approach to the patient with hyponatremia. P osm , Plasma osmolality; SIADH, syndrome
of inappropriate antidiuretic hormone. (From DiBartola SP. Hyponatremia. Vet Clin North Am Small Anim
Pract 1998;28:515–532.)

HYPONATREMIA WITH NORMAL the clinician must be familiar with the laboratory method
PLASMA OSMOLALITY used so as to interpret serum sodium concentrations
properly. The occurrence of a decreased serum sodium
Sodium is present as charged particles in the aqueous
concentration as a result of laboratory methodology in
phase of body fluids. Approximately 93% of plasma vol-
the presence of normal plasma osmolality is called
ume is occupied by water, and the remaining 7% consists
pseudohyponatremia or factitious hyponatremia.
largely of proteins and lipids. Historically, serum sodium
Pseudohyponatremia occurs in conditions associated
concentration has been measured by flame photometry.
Flame photometry measures the number of sodium ions with hyperlipidemia or severe hyperproteinemia.
in a specific volume of plasma or serum. Thus, the sodium Plasma osmolality in patients with pseudohypo-
concentration is measured as if the sodium ions were pres- natremia is normal, because lipids and proteins are very
ent throughout the entire sample volume, whereas actu- large molecules that contribute very little to plasma
ally they are active only in the aqueous phase. Normally, osmolality. If pseudohyponatremia is present, the calcu-
lated plasma osmolality is low because of a spuriously
this error is small. In plasma or serum samples containing
low serum sodium concentration, whereas the measured
a large amount of lipid or protein, however, the error may
osmolality is normal. Thus, when an abnormal osmolal
be larger, and the decrease in measured serum sodium
gap is present and the measured osmolality is normal,
concentration could be misleading to the clinician
pseudohyponatremia should be suspected. The diagnosis
(Fig. 3-13). When serum sodium concentration is
of pseudohyponatremia can be made by visual inspection
measured by direct potentiometry using ion-selective
of plasma for lipemia and by measurement of the total
electrodes, large amounts of lipid or protein in the sample
plasma protein concentration. Hyperlipemia severe
should not affect the measured serum sodium concentra-
tion. However, if the serum sample is diluted before mea- enough to cause pseudohyponatremia is visible to the
surement, large amounts of lipid or protein may still affect naked eye as lactescent plasma. Each milligram per decili-
the measured serum sodium concentration. 89 Therefore, ter of lipid in serum reduces the sodium concentration by