Hypokalemia
Hypokalemia increases the resting membrane potential of cells, resulting
in muscle weakness, impaired urine concentrating ability, polydipsia and
arrythmias. It is usually due to gastrointestinal or renal losses of potassium.
Remember that blood K+ values are not always a reflection of total body
K+ stores; K+ values can be normal in blood, despite severe deficits in
total body K+.
Causes of hypokalemia
- Pseudohypokalemia: Severe lipemia will result in a mild hypokalemia
due to solvent exclusion/volume
displacement when potassium is measured using diluted plasma (indirect
potentiometry; the technique routinely used for electrolyte measurement).
This artifact can be overcome by using undiluted plasma (direct potentiometry)
for measuring potassium (i.e. a blood-gas analyzer).
- Decreased intake: This occurs with anorexia in large animals,
including horses (especially foals), camelids and cows. Decreased
intake in small animals rarely results in hypokalemia unless there
are additional losses of potassium. Hypokalemia can be seen in cats
fed low K+ diets.
- Transcellular shifts: Shifting of K+ from ECF to ICF occurs
with metabolic alkalosis resulting in alkalemia, insulin release or
administration, and catecholamine release (from adrenaline stimulating
beta2-adrenergic receptors and activating the sodium-potassium [Na/K]
ATPase pump in muscle). Similarly, endotoxemia may also result in
hypokalemia because endotoxins also stimulate the Na/K ATPase pump
in muscle cells.
- Increased loss
The potassium deficit will be enhanced if intake of potassium is decreased.
1) Gastrointestinal losses: Causes include
vomiting of gastric contents (the loss of chloride enhances K+ excretion
in the kidneys, promoting the hypokalemia), vagal indigestion, and
diarrhea. Diarrhea in horses and cattle often produces a hypokalemia.
Severe diarrhea and vomiting in dogs and cats can also result in hypokalemia.
Saliva is potassium-rich and disorders such as choke in horses and
cattle can result in hypokalemia.
2) Third space losses/sequestration: Accumulation
of fluid in body cavities (e.g. peritonitis) or distended gastrointestinal
system (e.g. volvulus, ileus) can result in hypokalemia. This may
be dilutional from perceived volume depletion due to losses of fluid
from the intravascular space (with secretion of ADH and stimulation
of water intake).
3) Cutaneous losses: Sweating (horses).
4) Renal losses: Renal losses of potassium
can occur via several mechanisms, the main one being aldosterone,
which stimulates sodium absorption in exchange for potassium excretion
in the principle cells of the collecting ducts. Aldosterone is stimulated
by the renin-angiotensin system in response to hypovolemia and decreased
delivery of chloride (hypochloremia) to the macula densa. Hyperaldosteronism
is a rare condition causing severe hypokalemia in dogs and cats and
is usually secondary to adrenal neoplasia (in dogs) or hyperplasia
(cats). Potassium excretion is also enhanced by increased distal tubule
flow rates (any cause of polyuria, e.g. glucosuria, post-obstructive
diuresis), increased lumen electronegativity (high concentrations
of unadsorbable anions in the renal tubule lumen, e.g. penicillin,
ketones), increased distal delivery of sodium and metabolic alkalosis.
Potassium wasting also occurs if there is renal tubule disease that
prevents the normal absorption of filtered potassium, e.g. renal tubular
acidosis, chronic renal disease in cats.
|
