Page 73 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Disorders of Sodium and Water: Hypernatremia and Hyponatremia 63

BOX 3-3 Causes of Normal Hyperlipemic
plasma
plasma
Hyponatremia
93% water 80% water
With Normal Plasma Osmolality
Hyperlipemia
Hyperproteinemia
With High Plasma Osmolality
7% lipid 20% lipid
Hyperglycemia
Mannitol infusion
Serum
sodium
With Low Plasma Osmolality concentration 145 mEq/L 125 mEq/L
And hypervolemia by flame
photometry
Severe liver disease
Congestive heart failure
Actual
Nephrotic syndrome aqueous 145 mEq/L = 156 mEq/L 125 mEq/L = 156 mEq/L
Advanced renal failure concentration 0.93 L 0.8 L
of sodium
And normovolemia
Psychogenic polydipsia Figure 3-13 Effect of increased plasma lipids on serum
Syndrome of inappropriate antidiuretic hormone sodium concentration (pseudohyponatremia or factitious
secretion hyponatremia). (From DiBartola SP. Hyponatremia. Vet Clin North
Antidiuretic drugs (see Fig. 3-6) Am Small Anim Pract 1998;28:515–532.)
Myxedema coma of hypothyroidism
Hypotonic fluid infusion for hyponatremia. Treatment should be directed at the
And hypovolemia underlying disorder causing hyperproteinemia or
Gastrointestinal loss hyperlipidemia.
Vomiting
Diarrhea HYPONATREMIA WITH INCREASED
Third-space loss PLASMA OSMOLALITY
Pancreatitis
If an impermeant solute is added to ECF, water moves
Peritonitis
Uroabdomen from ICF to ECF, and the osmolality of both
Pleural effusion (e.g., chylothorax) compartments increases (see Fig. 3-11). 41 If the added
Peritoneal effusion solute is something other than sodium, the serum sodium
Cutaneous loss concentration is reduced by the translocation of water,
Burns but the plasma osmolality is higher than normal.
Hypoadrenocorticism Hyponatremia with hyperosmolality is usually caused
Diuretic administration by hyperglycemia in diabetes mellitus, wherein each
100-mg/dL increase in glucose may decrease the serum
sodium concentration by 1.6 mEq/L. 82 This correction
factor worked well up to a blood glucose concentration of
0.002 mEq/L (e.g., a serum triglyceride concentration of 440 mg/dL in a study in normal humans made tran-
1000 mg/dL would be expected to reduce the serum siently hyperglycemic by infusion of somatostatin, but
sodium concentration by 2 mEq/L). 118 In the case of the correction factor was much greater at higher blood
hyperproteinemia, each gram per deciliter of protein glucose concentrations. 73 The authors concluded that
above a concentration of 8 g/dL reduces the serum an overall correction factor of a 2.4-mEq/L decrement
sodium concentration by approximately 0.25 mEq/L in sodium for each 100-mg/dL increment in glucose
(e.g., the serum sodium concentration of a patient with would be preferable. In the diabetic patient, both hyper-
a serum protein concentration of 12 g/dL would be lipidemia and hyperglycemia may contribute to decreased
expected to be reduced by 1 mEq/L). 151 At such protein serum sodium concentration. Administration of the
concentrations, the plasma may be viscous, and this is osmotic diuretic mannitol also can cause hyponatremia
likely to occur mainly in patients with plasma cell with plasma hyperosmolality. The calculated osmolality
dyscrasias. Thus, whereas pseudohyponatremia may be is normal, the measured osmolality is high, and the osmo-
intellectually interesting, it is unlikely to be of clinical rel- lal gap is increased in the presence of mannitol, which is
evance in most instances. Furthermore, pseudohypo- an unmeasured osmole. Hyperglycemia does not affect
natremia itself has no consequences for the health of the osmolal gap because the plasma glucose concentra-
the patient. Its importance lies in the ability of the clini- tion is part of the equation used to calculate plasma osmo-
cian to recognize it and refrain from treating the patient lality (i.e., it is a measured osmole).