Page 102 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 5

Disorders of Potassium: Hypokalemia

and Hyperkalemia

Stephen P. DiBartola and Helio Autran De Morais

Potassium is the major intracellular cation in mammalian and ECF, however, the serum potassium concentration
cells, whereas sodium is the major extracellular cation. can change without any change in the total body potassium
Normally, the extracellular fluid (ECF) sodium concen- content. One of the most important functions of potas-
tration is approximately 140 mEq/L, and the ECF potas- sium in the body is its role in generation of the normal rest-
sium concentration is approximately 4 mEq/L. This ing cell membrane potential.
relationship is reversed in intracellular fluid (ICF), in
which the sodium concentration is approximately THE RESTING CELL
10 mEq/L and the potassium concentration is approxi- MEMBRANE POTENTIAL
mately 140 mEq/L. In experimental studies of dogs,
control values for ICF sodium and potassium The normal relationship between ECF and ICF potassium
concentrations in skeletal muscle were 8.4 to 13.7 and concentrations is maintained by sodium, potassium-
139 to 142 mEq/L, respectively. 20,107 adenosinetriphosphatase (Na ,K -ATPase) in cell
þ
þ
Total body potassium content in humans is approxi- membranes. This enzyme pumps sodium ions out of,
mately50to55 mEq/kgbodyweight,andalmostallofthis andpotassium ions into,thecell ina3:2Na/K ratio sothat
potassiumisreadilyexchangeable. 6,70 Inonestudyofpotas-
siumdepletionindogs,thecontrolvaluefortotalexchange-
able potassium as determined by 50 Kdilution was 50
47.1 mEq/kg body weight (range, 39.8 to 61.1 mEq/
1
kg). In cats, total body potassium is approximately 55
mEg/kg body weight. 184a As much as 95% or more of total
body potassium is located within cells, with muscle
containing60%to75% ofthispotassium.Muscle potassium 40
contentinnormaldogsandcatsisapproximately400 mEq/ r = .893
kg. 20,107,147,191 Asasolute,intracellularpotassiumiscrucial Serum [K + ] (mEq/L)
for maintenance of normal cell volume. Intracellular potas-
sium also is important for normal cell growth because it is
required for the normal function of enzymes responsible 30
for nucleic acid, glycogen, and protein synthesis.
The remaining 5% of the body’s potassium is located in
the ECF. Maintaining the ECF potassium concentration
within narrow limits is critical to avoid the life-threatening
effects of hyperkalemia on cardiac conduction. In humans, 20
the serum potassium concentration is inversely correlated 0 100 200 300 400 500 600 700 800
with the total body deficit of potassium (Fig. 5-1). Like- K deficit (mEq/70 kg body weight)
+
wise, in dogs with potassium depletion induced by dietary Figure 5-1 Relationship of serum potassium concentration to
restriction, the muscle potassium content was strongly bodily potassium deficit. The data are derived from seven metabolic
correlated (r ¼ 0.87) with the serum potassium concentra- balance studies carried out on 24 human subjects depleted of
tion. 147 During translocation of potassium between ICF potassium. (From Sterns RH et al.: Medicine 60:339, 1981.)

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