Hypokalemia
Hypokalemia is a low level of potassium in the blood serum. Mild low potassium does not typically cause symptoms. Symptoms may include feeling tired, leg cramps, weakness, and constipation. Low potassium also increases the risk of an abnormal heart rhythm, which is often too slow and can cause cardiac arrest.
Causes of hypokalemia include vomiting, diarrhea, medications like furosemide and steroids, dialysis, diabetes insipidus, hyperaldosteronism, hypomagnesemia, and not enough intake in the diet. Normal potassium levels are between 3.5 and 5.0 mmol/L with levels below 3.5 mmol/L defined as hypokalemia. It is classified as severe when levels are less than 2.5 mmol/L. Low levels may also be suspected based on an electrocardiogram. Hyperkalemia is 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 abnormalities on an electrocardiogram. Potassium levels that are only slightly below the normal range can be managed with changes in the diet. Lower levels of potassium require replacement with supplements either taken by mouth or given intravenously. If given intravenously, potassium is generally replaced at rates of less than 20 mmol/hour. Solutions containing high concentrations of potassium should generally be given using a central venous catheter. 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" comes from hypo- meaning "under", kalium meaning “potassium”, and -emia meaning "condition of the blood".
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, may cause muscle weakness, myalgia, tremor, and muscle cramps, and constipation. 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 some people.Causes
Hypokalemia can result from one or more of these medical conditions:Inadequate potassium intake
Not eating a diet with enough potassium-containing foods or fasting can cause the gradual onset of hypokalemia. This is a rare cause and may occur in those with anorexia nervosa or those 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 post-emetic bicarbonaturia force urinary potassium excretion. Other gastrointestinal 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 and thiazide diuretics 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 Type B intercalated cells in the collecting duct reabsorb H+ and secrete HCO3, while in type A intercalated cells protons are secreted 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, cations—in this case potassium—are taken up by the cell in order to maintain electroneutrality. 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 Metabolic alkalosis is often present in states of volume depletion, such as vomiting, so potassium is also lost via aldosterone-mediated mechanisms.
- 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 of the adrenal glands, e.g., Conn's syndrome. 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, and xanthines.
- 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 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
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 causes arrhythmias because of less-than-complete recovery from sodium-channel inactivation, making the triggering of an action potential less likely. In addition, the reduced extracellular potassium 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 defined as hypokalemia.Electrocardiogram
Hypokalemia leads to characteristic ECG changes.The earliest electrocardiographic findings, associated with hypokalemia, are decreased T wave height. Then, ST depressions and T inversions appear as serum potassium levels reduce further. Due to prolonged repolarization of ventricular Purkinje fibers, prominent U waves occur, frequently superimposed upon T waves, therefore producing the appearance of prolonged QT intervals, when serum potassium levels fall below 3 mEq/L.
Amount
The amount of potassium deficit can be calculated using the following formula:Meanwhile, the daily body requirement of potassium is calculated by multiplying 1 mmol to body weight in kilograms. 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. 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.Oral potassium supplementation
Mild hypokalemia may be treated by eating potassium-containing foods or by taking potassium chloride supplements in a tablet or syrup form. Foods rich in potassium include dried fruits, nuts, bran cereals and wheat germ, lima beans, molasses, leafy green vegetables, broccoli, winter squash, beets, carrots, cauliflower, potatoes, avocados, tomatoes, coconut water, citrus fruits, cantaloupe, kiwis, mangoes, bananas, and red meats.Eating potassium-rich foods may not be the optimal method for correcting low potassium and potassium supplements may be recommended. Potassium contained in foods is almost entirely coupled with phosphate and is thus ineffective in correcting hypokalemia associated with hypochloremia that may occur due to vomiting, diuretic therapy, or nasogastric drainage. Additionally, replacing potassium solely through diet may be costly and result in weight gain due to potentially large amounts of food needed. An effort should also be made to limit dietary sodium intake due to an inverse relationship with serum potassium. Increasing magnesium intake may also be beneficial for similar physiological reasons.
Potassium chloride supplements by mouth 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. Potassium bicarbonate is preferred when correcting hypokalemia associated with metabolic acidosis.