Page 75 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Disorders of Sodium and Water: Hypernatremia and Hyponatremia 65

ECF ICF vasopressin release, which occur in response to volume
depletion. Furthermore, potassium depletion caused by
1 L H 2 O
diuretics can contribute to hyponatremia because shifting
of intracellular potassium into the extracellular compart-
1200 ment in exchange for sodium may occur. Hyponatremia
1.36 L 3.64 L 158
450 mOsm has been associated with chronic blood loss in dogs.
mOsm It was thought that defective urinary concentrating ability
in these dogs was caused by impaired vasopressin release
in response to plasma hypoosmolality and loss of NaCl
330 330 from the renal medullary interstitium. Some of these dogs
mOsm/Kg mOsm/Kg
had hypoadrenocorticism and gastrointestinal fluid losses
ICF that might have contributed to their hyponatremia. Nor-
ECF mal concentrating ability returned after resolution of
hyponatremia.
Hypervolemic Hyponatremia
1200 (Hyponatremia with Volume Excess)
1.64 L 450 mOsm 4.36 L
mOsm Hyponatremia may occur despite the presence of
increased total body sodium and expansion of the ECF
compartment in patients with ascites or edema. Some
of the pathophysiologic events in these patients impair
275 275
mOsm/Kg mOsm/Kg the excretion of ingested water and exert a dilutional
Figure 3-14 Effect of drinking 1 L of water after a loss of 1 L effect on the serum sodium concentration. Hypervolemic
of hypotonic fluid (150 mOsm/kg) on volume and tonicity of hyponatremia is observed in three clinical conditions:
extracellular fluid (ECF) and intracellular fluid (ICF). (Drawing by congestive heart failure, severe liver disease, and
Tim Vojt.) nephrotic syndrome. In these disorders, there is a percep-
tion of circulating volume depletion by the body, and the
regulatory mechanisms invoked result in volume expan-
sion. This perceived volume deficit has been referred to
hyponatremia can occur. 28 Occasionally, dogs with gastro- as decreased effective circulating volume or decreased
intestinal fluid losses develop electrolyte disturbances effective arterial blood volume.
that mimic hypoadrenocorticism. 30,107 Hyponatremia Three major pathophysiologic mechanisms are opera-
associated with third-space loss of fluid has been reported tive in the pathogenesis of sodium retention and impaired
with pleural effusion related to chylothorax, lung lobe water excretion in these clinical conditions. The renin-
torsion, and neoplasia. 93,155,162,166 In these reports, angiotensin system is activated by reduced renal perfusion
hyponatremia was attributed at least in part to removal of and causes increased sodium retention by the kidneys.
sodium-rich fluid by thoracocentesis. However, many of Decreased renal perfusion, decreased GFR, and increased
these animals had evidence of volume depletion, and it is proximal tubular reabsorption of sodium and water result
likely that nonosmotic vasopressin secretion also played a in decreased delivery of tubular fluid to distal diluting
role in the development of hyponatremia. Affected dogs sites and impairment of free water excretion. A decrease
also had mild hyperkalemia attributed to decreased renal in effective arterial blood volume results in nonosmotic
excretion of potassium caused by volume depletion and stimulation of vasopressin release and further impairment
decreased distal renal tubular flow. Similar findings have of water excretion. Impaired free water excretion causes
been observed in dogs and cats with peritoneal effusion dilution of retained sodium and results in hyponatremia
and indogs inlatepregnancy. 9,85,145 The pathogenesis of despite the presence of increased total body sodium con-
hyponatremiaandmildhyperkalemiaindogswithgastroin- tent and expansion of the ECF compartment. In addi-
testinal losses is probably similar to that described for dogs tion, a primary intrarenal mechanism for sodium
with pleural and peritoneal effusions, but the explanation retention is thought to be operative in patients with the
for the rare occurrence of similar electrolyte abnormalities nephrotic syndrome.
in dogs in late pregnancy is unknown. When the cause In cirrhosis and the nephrotic syndrome, intravascular
of hyponatremia and hyperkalemia is unclear, an ACTH volume may be reduced as a result of decreased oncotic
stimulation test should be performed to rule out pressure caused by hypoalbuminemia. This volume deple-
hypoadrenocorticism. tion causes nonosmotic stimulation of vasopressin release
Diuretics contribute to impaired water excretion and and impaired water excretion. In liver failure, low effective
dilution of sodium in the ECF by decreased distal delivery arterial volume is caused by marked peripheral (primarily
of tubular fluid and nonosmotic stimulation of splanchnic) vasodilation. 53,96,113 Reduction of cardiac