Hypokalemia, also spelled hypokalaemia, is a low level of potassium (K+) in the blood serum. Normal potassium levels are between 3.5 and 5.0 mmol/L (3.5 and 5.0 mEq/L) with levels below 3.5 mmol/L defined as hypokalemia. Mildly low levels do not typically cause symptoms. Symptoms may include feeling tired, leg cramps, weakness, and constipation. It increases the risk of an abnormal heart rhythm, which are often too slow, and can cause cardiac arrest.
Causes of hypokalemia include diarrhea, medications like furosemide and steroids, dialysis, diabetes insipidus, hyperaldosteronism, hypomagnesemia, and not enough intake in the diet. It is classified as severe when levels are less than 2.5 mmol/L. Low levels can also be detected on an electrocardiogram (ECG). Hyperkalemia refers to a high level of potassium in the blood serum.
The speed at which potassium should be replaced depends on whether or not there are symptoms or ECG changes. Mildly low levels can be managed with changes in the diet. Potassium supplements can be either taken by mouth or intravenously. If given by intravenous, generally less than 20 mmol are given over an hour. High concentration solutions (>40 mmol/L) should be given in a central line if possible. Magnesium replacement may also be required.
Hypokalemia is one of the most common water-electrolyte imbalances. It affects about 20% of people admitted to hospital. The word "hypokalemia" is from hypo- means "under"; kalium meaning potassium, and -emia means "condition of the blood".
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Signs and symptoms
Mild hypokalemia is often without symptoms, although it may cause elevation of blood pressure, and can provoke the development of an abnormal heart rhythm. Severe hypokalemia, with serum potassium concentrations of 2.5-3 meq/l (Nl: 3.5-5.0 meq/l), may cause muscle weakness, myalgia, tremor, and muscle cramps (owing to disturbed function of skeletal muscle), and constipation (from disturbed function of smooth muscle). With more severe hypokalemia, flaccid paralysis and hyporeflexia may result. Reports exist of rhabdomyolysis occurring with profound hypokalemia with serum potassium levels less than 2 meq/l. Respiratory depression from severe impairment of skeletal muscle function is found in many patients.
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Causes
Hypokalemia can result from one or more of these medical conditions:
Inadequate potassium intake
Perhaps the most obvious cause is insufficient consumption of potassium (that is, a low-potassium diet) or starvation. However, without excessive potassium loss from the body, this is a rare cause of hypokalemia. Usually only seen in anorexia nervosa patients and people on a ketogenic diet.
Gastrointestinal or skin loss
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 diarrhea, excessive perspiration, or losses associated with muscle-crush injury, or surgical procedures. Vomiting can also cause hypokalemia, although not much potassium is lost from the vomitus. Rather, heavy urinary losses of K+ in the setting of postemetic bicarbonaturia force urinary potassium excretion (see Alkalosis below). Other GI causes include pancreatic fistulae and the presence of adenoma.
Urinary loss
- Certain medications can cause excess potassium loss in the urine. Blood pressure medications such as loop diuretics (e.g. furosemide) and thiazide diuretics (e.g. hydrochlorothiazide) commonly cause hypokalemia. Other medications such as the antifungal, amphotericin B, or the cancer drug, cisplatin, can also cause long-term hypokalemia.
- A special case of potassium loss occurs with diabetic ketoacidosis. Hypokalemia is observed with low total body potassium and a low intracellular concentration of potassium. In addition to urinary losses from polyuria and volume contraction, also an obligate loss of potassium from kidney tubules occurs as a cationic partner to the negatively charged ketone, ?-hydroxybutyrate.
- A low level of magnesium in the blood can also 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.
- An increase in the pH of the blood (alkalosis) can cause temporary hypokalemia by causing a shift of potassium out of the plasma and interstitial fluids into the urine via a number of interrelated mechanisms.
1) Type B intercalated cells in the collecting duct reabsorb H+ and secrete HCO3. Protons are reabsorbed via both H+-K+ATPases and H+ ATP-ases on the apical/luminal surface of the cell. By definition, the H+-K+ATPase reabsorbs one potassium ion into the cell for every proton it secretes into the lumen of the collecting duct of a nephron. In addition, when H+ is expelled from the cell (by H+ATP-ase), cations--in this case potassium--are taken up by the cell in order to maintain electroneutrality (but not through direct exchange as with the H+-K+ATPase). In order to correct the pH during alkalosis, these cells will use these mechanisms to reabsorb great amounts of H+, which will concomitantly increase their intracellular concentrations of potassium. This concentration gradient drives potassium to be secreted across the apical surface of the cell into the tubular lumen through potassium channels (this facilitated diffusion occurs in both Type B intercalated cells and Principal cells in the collecting duct).
2) Metabolic alkalosis is often present in states of volume depletion, such as vomiting, so potassium is also lost via aldosterone-mediated mechanisms.
3) During metabolic alkalosis, the 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. - 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 nonmalignant) of the adrenal glands, e.g., Conn's syndrome (primary hyperaldosteronism). Cushing's syndrome can also lead to hypokalemia due to excess cortisol binding the Na+/K+ pump and acting like aldosterone. Hypertension and hypokalemia can also be seen with a deficiency of the 11-beta-hydroxysteroid dehydrogenase type 2 enzyme which allows cortisols to stimulate aldosterone receptors. This deficiency--known as apparent mineralocorticoid excess syndrome--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.
Distribution away from extracellular fluid
- In addition to alkalosis, other factors can cause transient shifting of potassium into cells, presumably by stimulation of the Na+/K+ pump. These hormones and medications include insulin, epinephrine, and other beta agonists (e.g. salbutamol or salmeterol), and xanthines (e.g. theophylline).
- 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 catecholamines and/or insulin and/or thyroid hormone, which lead to movement of potassium from the extracellular fluid into the muscle cells.
Other
- A handful of published reports describe individuals with severe hypokalemia related to chronic extreme consumption (4-10 l/day) of colas. The hypokalemia is thought to be from the combination of the diuretic effect of caffeine and copious fluid intake, although it may also be related to diarrhea caused by heavy fructose ingestion.
Pseudohypokalemia
- Pseudohypokalemia is a decrease in the amount of potassium that occurs due to excessive uptake of potassium by metabolically active cells in a blood sample after it has been drawn. It is a laboratory artifact that may occur when blood samples remain in warm conditions for several hours before processing.
Pathophysiology
About 98% of the body's potassium is found inside cells, with the remainder in the extracellular fluid including the blood. This concentration gradient is maintained principally by the Na+/K+ pump.
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. Lower potassium levels in the extracellular space 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 to initiate an action potential.
In the heart, hypokalemia cause arrhythmias because of more complete recovery from sodium-channel inactivation, making the triggering of an action potential less likely. In addition, the reduced extracellular potassium (paradoxically) inhibits the activity of the IKr potassium current and delays ventricular repolarization. This delayed repolarization may promote reentrant arrhythmias.
Diagnosis
Blood
Normal potassium levels are between 3.5 and 5.0 mmol/L with levels below 3.5 mmol/L (less than 3.5 mEq/L) defined as hypokalemia.
Electrocardiogram
The earliest electrocardiographic (ECG) findings associated with hypokalemia is a decrease in T waves height. Then, ST depression and T inversion happens as serum potassium reduces further. Due to prolonged repolarization of ventricular Purkinje fibers, prominent U wave occurs (usually seen at V2 and V3 leads), frequently superimposed upon the T wave and therefore produces the appearance of a prolonged QT interval when serum potassium reduces to below 3 mEq/L.
Amount
The amount of potassium deficit can be calculated using the following formula:
Kdeficit (in mmol) = (Knormal lower limit - Kmeasured) × body weight (kg) × 0.4
Meanwhile, the daily body requirement of potassium is calculated by multiplying 1 mmol to body weight in kilogrammes. Adding potassium deficit and daily potassium requirement would give the total amount of potassium need to be corrected in mmol. Dividing mmol by 13.4 will give the potassium in grams.
Treatment
Treatment including addressing the cause, such as improving the diet, treating diarrhea, or stopping an offending medication. People without a significant source of potassium loss and who show no symptoms of hypokalemia may not require treatment.
Mild hypokalemia (>3.0 meq/l) may be treated with oral potassium chloride supplements (Klor-Con, Sando-K, Slow-K). As this is often part of a poor nutritional intake, potassium-containing foods may be recommended, such as leafy green vegetables, avocados, tomatoes, coconut water, citrus fruits, oranges, or bananas. Both dietary and pharmaceutical supplements are used for people taking diuretic medications.
Severe hypokalemia (<3.0 meq/l) may require intravenous supplementation. Typically, a saline solution is used, with 20-40 meq/l KCl per liter over 3-4 hours. Giving IV potassium at faster rates (20-25 meq/hr) may predispose to ventricular tachycardias and requires intensive monitoring. A generally safe rate is 10 meq/hr. Even in severe hypokalemia, oral supplementation is preferred given its safety profile. Sustained-release formulations should be avoided in acute settings.
Difficult or resistant cases of hypokalemia may be amenable to a potassium-sparing diuretic, such as amiloride, triamterene, spironolactone, or eplerenone. Concomitant hypomagnesemia will inhibit potassium replacement, as magnesium is a cofactor for potassium uptake.
When replacing potassium intravenously, infusion by a central line is encouraged to avoid the frequent occurrence of a burning sensation at the site of a peripheral infusion, 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 fluid, or mixing 3 ml of 1% lidocaine to each 10 meq of KCl per 50 ml of fluid. The practice of adding lidocaine, however, raises the likelihood of serious medical errors.
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