Hypokalemia:
Hypokalemia refers to the condition in which the concentration of potassium in the blood is low. The prefix hypo- means low (contrast with hyper-, meaning high). Kal refers to kalium, the Neo-Latin for potassium, and -emia means "in the blood."
Normal serum potassium levels are between 3.5 to 5.0 mEq[1]; at least 95% of the body's potassium is found inside cells, with the remainder in the blood. This concentration gradient is maintained principally by the Na+/K+-ATPase pump.
[edit] Pathophysiology
Potassium is essential for many body functions, including muscle and nerve activity. The electrochemical gradient of potassium between the intracellular and extracellular space is essential for nerve function; in particular, potassium is needed to repolarize the cell membrane to a resting state after an action potential has passed. Decreased potassium levels in the extracellular space will cause hyperpolarization of the resting membrane potential. This hyperpolarization is caused by the effect of the altered potassium gradient on resting membrane potential as defined by the Goldman equation. As a result, a greater than normal stimulus is required for depolarization of the membrane in order to initiate an action potential.
In certain conditions, this will make cells less excitable. However, in the heart, it causes myocytes to become hyperexcitable. Lower membrane potentials in the atrium may cause arrhythmias because of more complete recovery from sodium-channel inactivation, making the triggering of an action potential more likely. In addition, the reduced extracellular potassium (paradoxically) inhibits the activity of the IKr potassium current[2] and delays ventricular repolarization. This delayed repolarization may promote reentrant arrythmias.
[edit] Causes
Hypokalemia can result from one or more of the following medical conditions:
- Perhaps the most obvious cause is insufficient consumption of potassium (that is, a low-potassium diet). However, without excessive potassium loss from the body, this is a rare cause of hypokalemia.
- A more common cause is excessive loss of potassium, often associated with heavy fluid losses that "flush" potassium out of the body. Typically, this is a consequence of vomiting, diarrhea, excessive perspiration, or losses associated with surgical procedures
- A special case of potassium loss occurs with diabetic ketoacidosis. In addition to urinary losses from polyuria and volume contraction, there is also obligate loss of potassium from kidney tubules as a cationic partner to the negatively charged ketone, β-hydroxybutyrate.
- Hypomagnesemia can cause hypokalemia. Magnesium is required for adequate processing of potassium. This may become evident when hypokalemia persists despite potassium supplementation. Other electrolyte abnormalities may also be present.
- Alkalosis can cause transient hypokalemia by two mechanisms. First, the alkalosis causes a shift of potassium from the plasma and interstitial fluids; perhaps mediated by stimulation of Na+-H+ exchange and a subsequent activation of Na+/K+-ATPase activity.[3] Second, an acute rise of plasma HCO3- concentration (caused by vomiting, for example) will exceed the capacity of the renal proximal tubule to reabsorb this anion, and potassium will be excreted as an obligate cation partner to the bicarbonate. It should be noted that metabolic alkalosis is often present in states of volume depletion, and thus alkalosis is typically not the main cause of hypokalemia seen in in volume-depleted states.
- Disease states that lead to abnormally high aldosterone levels can cause hypertension and excessive urinary losses of potassium. These include renal artery stenosis and tumors (generally non-malignant) of the adrenal glands. Hypertension and hypokalemia can also be seen with a deficiency of the 11β-hydroxylase enzyme which allows cortisols to stimulate aldosterone receptors. This deficiency can either be congenital or caused by consumption of glycyrrhizin, which is contained in extract of licorice, sometimes found in Herbal supplements, candies and chewing tobacco.
- Rare hereditary defects of renal salt transporters, such as Bartter syndrome or Gitelman syndrome, can cause hypokalemia, in a manner similar to that of diuretics. As opposed to disease states of primary excesses of aldosterone, blood pressure is either normal or low in Bartter's or Gitelman's.
- Rare hereditary defects of muscular ion channels and transporters that cause hypokalemic periodic paralysis can precipitate occasional attacks of severe hypokalemia and muscle weakness. These defects cause a heightened sensitivity to the normal changes in potassium produced by catechols and/or insulin and/or thyroid hormone, which lead to movement of potassium from the extracellular fluid into the muscle cells.
[edit] Signs and symptoms
Mild hypokalemia is often without symptoms, although it may cause a small elevation of blood pressure,[4] and can occasionally provoke cardiac arrhythmias. Moderate hypokalemia may cause muscular weakness, myalgia, and muscle cramps (owing to disturbed function of the skeletal muscles), and constipation (from disturbed function of smooth muscles). With more severe hypokalemia, flaccid paralysis, hyporeflexia, and tetany may result. There are reports of rhabdomyolysis occurring with profound hypokalemia. Respiratory depression from severe impairment of skeletal muscle function is not uncommon.
Some electrocardiographic (ECG) findings associated with hypokalemia are flattened T waves, U waves, ST segment depression, and prolongation of the QT interval.
[edit] Treatment
The most important step in severe hypokalemia is removing the cause, such as treating diarrhea or stopping offending medication.
Mild hypokalemia (>3.0 mEq/L) may be treated with oral potassium chloride supplements (Sando-K, Slow-K). As this is often part of a poor nutritional intake, potassium-containing foods may be recommended, such as tomatoes, oranges or bananas[1]. Both dietary and pharmaceutical supplements are used for people taking diuretic medications (see Causes, above).
Severe hypokalemia (<3.0 mEq/L) may require intravenous supplementation. Typically, saline is used, with 20-40 mEq KCl per liter over 3-4 hours. Giving intravenous potassium at faster rates may predispose to ventricular tachycardias and requires intensive monitoring.
Difficult or resistant cases of hypokalemia may be amenable to a potassium-sparing diuretic such as amiloride, triamterene, or spironolactone. In contrast to the more commonly used diuretics like hydrochlorothiazide and furosemide, these potassium-sparing diuretics actually reduce the kidney's excretion of potassium.
When replacing potassium intravenously, infusion via central line is encouraged to avoid the frequent occurrence of a burning sensation at the site of a peripheral IV, or the rare occurrence of damage to the vein. When peripheral infusions are necessary, the burning can be reduced by diluting the potassium in larger amounts of IV fluid, or mixing 3 ml of 1% lidocaine to each 10 meq of kcl per 50 ml of IV fluid. The practice of adding lidocaine, however, raises the likelihood of serious medical errors.[2]
[edit] Hypokalemia in pets
Cats can develop hypokalemia in old age, but Burmese kittens may be genetically prone to the condition if both parents have a defective gene. Symptoms are: staggering, an inability to keep up head which droops alarmingly and animals have good appetite but fail to gain weight. Treatment can include adding ground potassium chloride tablets to the animal's food.[5]
[edit] See also
[edit] External links
- USDA National Nutrient Database for Standard Reference, Release 20]
[edit] References
- ^ Kratz A et al. (2004) Case records of the Massachusetts General Hospital. Weekly clinicopathological exercises. Laboratory reference values. N Engl J Med., 351(15):1548-63; PMID 15470219.
- ^ Sanguinetti MC and Jurkiewicz NK. (1992) Role of external Ca2+ and K+ in gating of cardiac delayed rectifier K+ currents. Pflugers Arch., 420(2):180-6; PMID 1620577.
- ^ Halperin ML and Kamel KS (1998) Potassium. Lancet, 352:155-40; PMID 9672294.
- ^ Krishna GG et al. (1989) Increased blood pressure during potassium depletion in normotensive men. N Engl J Med., 320(18):1177-82; PMID 2624617.
- ^ Feline Hypokalemic Polymyopathy. in The Merck Veterinary Manual, 9th edition By Merck & Co. 2006. ISBN 0-911910-50-6
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