Hypokalemia

Jump to navigation

Jump to search

Hypokalemia
Synonyms	Hypopotassaemia, hypopotassemia
An ECG in a person with a potassium level of 1.1 meq/l showing the classical changes of ST segment depression, inverted T waves, large U waves, and a slightly prolonged PR interval.
Specialty	Critical care medicine
Symptoms	Feeling tired, leg cramps, weakness, constipation, abnormal heart rhythm[1]
Complications	Cardiac arrest[1]
Causes	Diarrhea, medications like furosemide and steroids, dialysis, diabetes insipidus, hyperaldosteronism, hypomagnesemia, not enough intake in the diet[1]
Diagnostic method	Blood potassium < 3.5 mmol/L[1][2]
Treatment	Dietary change, supplements[3]
Frequency	20% of people admitted to hospital[4]

Hypokalemia, also spelled hypokalaemia, is a low level of potassium (K⁺) in the blood serum.^[1] Mildly low levels do not typically cause symptoms.^[3]Symptoms may include feeling tired, leg cramps, weakness, and constipation.^[1] It increases the risk of an abnormal heart rhythm, which is often too slow, and can cause cardiac arrest.^[1][3]

Causes of hypokalemia include vomiting, diarrhea, medications like furosemide and steroids, dialysis, diabetes insipidus, hyperaldosteronism, hypomagnesemia, and not enough intake in the diet.^[1] 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.^[1][2] It is classified as severe when levels are less than 2.5 mmol/L.^[1] Low levels can also be detected on an electrocardiogram (ECG).^[1] Hyperkalemia refers to a high level of potassium in the blood serum.^[1]

The speed at which potassium should be replaced depends on whether or not there are symptoms or abnormalities on an electrocardiogram.^[1]Potassium levels that are only slightly below the normal range can be managed with changes in the diet.^[3] Lower levels of potassium require replacement with supplements either taken by mouth or given intravenously.^[3] If given intravenously, potassium is generally replaced at rates of less than 20 mmol/hour.^[1] Solutions containing high concentrations of potassium (>40 mmol/L) should generally be given using a central venous catheter.^[3]Magnesium replacement may also be required.^[1]

Hypokalemia is one of the most common water–electrolyte imbalances.^[4] It affects about 20% of people admitted to hospital.^[4] The word "hypokalemia" is from hypo- means "under"; kalium meaning potassium, and -emia means "condition of the blood".^[5]

Contents

• 1Signs and symptoms
• 2Causes
  • 2.1Inadequate potassium intake
  • 2.2Gastrointestinal or skin loss
  • 2.3Urinary loss
  • 2.4Distribution away from extracellular fluid
  • 2.5Other
  • 2.6Pseudohypokalemia
• 3Pathophysiology
• 4Diagnosis
  • 4.1Blood
  • 4.2Electrocardiogram
  • 4.3Amount
• 5Treatment
  • 5.1Oral potassium supplementation
  • 5.2Intravenous potassium replacement
  • 5.3Potassium-sparing diuretics
• 6See also
• 7References
• 8Further reading
• 9External links

Signs and symptoms[edit]

Mild hypokalemia is often without symptoms, although it may cause elevation of blood pressure,^[6] 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.^{[citation needed]} Respiratory depression from severe impairment of skeletal muscle function is found in many patients.^{[citation needed]}

Causes[edit]

Hypokalemia can result from one or more of these medical conditions:

Inadequate potassium intake[edit]

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[edit]

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[edit]

• 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, β-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).^[7] 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.^[8]
• 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[edit]

• In addition to alkalosis, other factors can cause transient shifting of potassium into cells, presumably by stimulation of the Na⁺/K⁺ pump.^[9] These hormones and medications include insulin, epinephrine, and other beta agonists (e.g. salbutamol or salmeterol), and xanthines (e.g. theophylline).^[10]
• 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[edit]

• A handful of published reports describe individuals with severe hypokalemia related to chronic extreme consumption (4–10 l/day) of colas.^[11] The hypokalemia is thought to be from the combination of the diuretic effect of caffeine^[12] and copious fluid intake, although it may also be related to diarrhea caused by heavy fructose ingestion.^[13][14]

Pseudohypokalemia[edit]

• 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.^[15]

Pathophysiology[edit]

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 I_Kr potassium current and delays ventricular repolarization. This delayed repolarization may promote reentrant arrhythmias.^[16]

Diagnosis[edit]

Blood[edit]

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.^[1][17]

Electrocardiogram[edit]

Hypokalemia leads to characteristic ECG changes (QRS prolongation, ST-segment and T-wave depression, U-wave formation).^[4]

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.^[18]

Amount[edit]

The amount of potassium deficit can be calculated using the following formula:
K_deficit (in mmol) = (K_{normal lower limit} − K_measured) × 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.^[19]

Treatment[edit]

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. Acutely, repletion with 10 mEq of potassium is typically expected to raise serum potassium by 0.1 mEq/L immediately after administration. However, for those with chronic hypokalemia, repletion takes time due to tissue redistribution. For example, correction by 1 mEq/L can take more than 1000 mEq of potassium over many days. ^[6]

Oral potassium supplementation[edit]

Mild hypokalemia (>3.0 meq/l) may be treated by eating potassium-containing foods or by taking potassium chloride supplements in a tablet or syrup form (oral supplements). Foods rich in potassium include leafy green vegetables, avocados, tomatoes, coconut water, citrus fruits, oranges, or bananas.^[20] However, the quantities of potassium present in these foods are often not high enough to correct hypokalaemia and additional potassium is generally required. ^[21] Oral potassium chloride supplements such as Klor-Con, Sando-K, and Slow-K have the advantage of containing precise quantities of potassium, but the disadvantages of a taste which may be unpleasant, and the potential for side-effects including nausea and abdominal discomfort. ^[21]

Intravenous potassium replacement[edit]

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.^[22] Giving IV potassium at faster rates (20–25 meq/hr) may inadvertently expose the heart to a sudden increase in potassium, potentially causing dangerous abnormal heart rhythms such as heart block or asystole.^[16] Faster infusion rates are therefore generally only performed in locations in which the heart rhythm can be continuously monitored such as a critical care unit.^[22] When replacing potassium intravenously, particularly when higher concentrations of potassium are used, infusion by a central line is encouraged to avoid the occurrence of a burning sensation at the site of infusion, or the rare occurrence of damage to the vein. ^[23] When peripheral infusions are necessary, the burning can be reduced by diluting the potassium in larger amounts of fluid, or adding a small dose of lidocaine to the intravenous fluid, ^[22] although adding lidocaine may increase the likelihood of medical errors.^[24] Even in severe hypokalemia, oral supplementation is preferred given its safety profile. Sustained-release formulations should be avoided in acute settings.

Potassium-sparing diuretics[edit]

Hypokalemia which is recurrent or resistant to treatment 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.^[25]