Section 12. Disorders of Nutrition and Metabolism

Chapter 155. Minerals and Electrolytes

Topics: Introduction | Calcium | Copper | Fluoride | Iodine | Iron | Magnesium | Phosphate | Potassium | Selenium | Sodium | Zinc

Potassium

Most of the body's potassium is located inside the cells. Potassium is necessary for the normal functioning of cells, nerves, and muscles.

The level of potassium in the blood must be maintained within a narrow range. A potassium level that is too high or too low can have serious consequences, such as an abnormal heart rhythm or even cardiac arrest. The potassium stored within the cells can be used by the body to help maintain a constant level of potassium in the blood.

Potassium balance is achieved by matching the amount of potassium taken in with the amount lost. Potassium is taken in through food and electrolyte-containing drinks and lost primarily in urine, although some potassium is also lost through the digestive tract and in sweat. Healthy kidneys are able to adjust the excretion of potassium to match changes in dietary intake. Some drugs and certain conditions affect the movement of potassium into and out of cells, which greatly influences the potassium level in the blood.

Hypokalemia

In hypokalemia, the level of potassium in the blood is too low. Excessive potassium loss usually results from vomiting, diarrhea, chronic laxative use, or colon polyps. Very occasionally, excessive loss results from excessive sweating in conditions of extreme heat and humidity. Many foods contain potassium, so hypokalemia is rarely caused by too little intake in people who eat a balanced diet.

There are several reasons why potassium may be lost in the urine. By far the most common is the use of diuretics that cause the kidneys to excrete excess sodium, water, and potassium. In Cushing's syndrome, the adrenal glands produce excess amounts of aldosterone, a hormone that causes the kidneys to excrete large amounts of potassium (see Section 13, Chapter 164). Excessive potassium is also excreted by people who eat large amounts of licorice or chew certain types of tobacco. People with Liddle's syndrome (see Section 11, Chapter 146), Bartter's syndrome (see Section 11, Chapter 146), and Fanconi's syndrome (see Section 11, Chapter 146) have rare defects that interfere with the kidneys' ability to conserve potassium.

Certain drugs (such as insulin and the antiasthmatic drugs albuterol, terbutaline, and theophylline) increase the movement of potassium into the cells and can result in hypokalemia. However, use of these drugs is rarely the sole cause of hypokalemia.

A mild decrease in the potassium level in the blood usually causes no symptoms. A more severe decrease can cause muscle weakness, twitches, and even paralysis. Abnormal heart rhythms may develop, especially in people with heart disease. Even mild hypokalemia is dangerous in people taking the heart drug digoxin. The diagnosis is made by determining that the potassium level in the blood is low.

Potassium usually can be replaced by eating potassium-rich foods or by taking potassium supplements by mouth. Because potassium can irritate the digestive tract, supplements should be taken in small doses with food several times a day rather than in a single large dose. Special types of potassium supplements, such as wax-impregnated or microencapsulated potassium chloride, are much less likely to irritate the digestive tract.

Most people who take diuretics do not need to take potassium supplements. Nevertheless, doctors periodically check the potassium level in the blood so that the drug regimen can be altered if necessary. Alternatively, potassium-conserving diuretics (such as triamterene, amiloride, or spironolactone) can be added to the diuretic therapy, but only in people whose kidneys are functioning normally.

Hyperkalemia

In hyperkalemia, the level of potassium in the blood is too high. Hyperkalemia usually results when the kidneys do not excrete enough potassium. Probably the most common cause of mild hyperkalemia is the use of drugs that decrease blood flow to the kidneys or prevent the kidneys from excreting normal amounts of potassium. Such drugs include triamterene, spironolactone, and angiotensin-converting enzyme (ACE) inhibitors. Hyperkalemia can also be caused by Addison's disease, in which the adrenal glands do not produce sufficient amounts of the hormone aldosterone, which stimulates the kidneys to excrete potassium (see Section 13, Chapter 164). Kidney failure can result in severe hyperkalemia.

Hyperkalemia can also result when a large amount of potassium is suddenly released from the cells. A sudden release of potassium from the cells can result from crush injuries (involving the destruction of large amounts of muscle tissue), severe burns, or overdoses of crack cocaine. The rapid movement of potassium from the cells into the bloodstream can overwhelm the kidneys and result in life-threatening hyperkalemia.

Mild hyperkalemia causes few, if any, symptoms. Usually, hyperkalemia is first detected when routine blood tests are performed or when a doctor notices changes on an electrocardiogram. A high level of potassium in the blood is dangerous. It can cause the heart rhythm to become abnormal. If the level is very high, the heart can stop beating.

For mild hyperkalemia, reducing the potassium intake or discontinuing drugs that prevent the kidneys from excreting potassium may be the only treatment that is needed. If the kidneys are functioning, a diuretic may be given to increase potassium excretion.

For severe hyperkalemia, immediate treatment is essential. A resin that absorbs potassium from the digestive tract and passes out of the body in the stool can be given by mouth or enema. When this treatment is given, doctors also induce diarrhea so that the resin, with potassium absorbed into it, is quickly expelled.

When more rapid treatment is needed, the person may be given an intravenous solution containing calcium, glucose, or insulin. Calcium helps protect the heart from the effects of a high potassium level but does not actually affect the potassium level. This protective effect lasts only a few minutes. Glucose and insulin drive potassium from the blood into the cells, thus lowering the potassium level in the blood. If these measures do not work or if a person has kidney failure, dialysis may be necessary to remove the excess potassium.