Last month EMS World began a four-part look at electrolyte imbalances with a focus on sodium. This month we look at potassium balance disorders.
Potassium Balance Disorders
Potassium is the primary intracellular electrolyte, and because of its positive charge it is considered the primary intracellular cation. The total amount of potassium in the body is related to the person's size, but it averages 3,200 mEq in males and 2,200 in females. Of the body's potassium, 70% is found in skeletal muscle, and 28% is found in the liver and red blood cells, which means that 98% of the potassium in the body is in the intracellular fluid. The remaining 2% is found in the extracellular fluid. It is this 2% that is critical to normal cardiovascular and neuromuscular function, and it is what is measured in a serum potassium reading.
The normal serum potassium is 3.5 to 5.0 mEq/L. Having too little potassium (less than 3.5 mEq/L) is called hypokalemia, while having too much (more than 5.5 mEq/L) is called hyperkalemia. Even minor variations in serum potassium levels can have significant impact on cardiovascular and neuromuscular function. This is because the ratio of intracellular to extracellular potassium is an important determinant of cellular membrane potential. Potassium is constantly moving between the ICF and ECF by way of the sodium-potassium pump.
Adequate renal function is necessary to maintain normal potassium levels. There are two methods by which the kidneys regulate potassium balance. First, potassium and hydrogen ions compete for exchange with sodium ions in the renal tubules. The urine salt content is adjusted by the distal convoluted tubules of the kidney. Second, aldosterone causes the kidneys to retain sodium, which in turn causes retention of water. To retain sodium, the kidneys excrete potassium. Nine-tenths of the potassium excreted daily is excreted in the urine, while the other 10% is lost through the feces and sweat glands. Because the body cannot store potassium, it must be ingested daily—the body needs 40 mEq a day. The normal diet contains 60 to 100 mEq a day, and the potassium balance is maintained by a daily intake and output of 50 to 100 mEq.
Hypokalemia is one of the most common electrolyte disorders. It clinically defined as a serum potassium level of less than 3.5 mEq/L. Moderate hypokalemia is a serum level of 2.5 to 3.0 mEq/L; severe hypokalemia is a serum level of less than 2.5 mEq/L (although life-threatening hypokalemia is rare). Hypokalemia is generally the result of a decrease in total stored potassium, but it can also occur when the body has normal potassium stores in the presence of an alkalotic state. For causes of hypokalemia, see Figure 1.
Potassium plays a key role in the maintenance of pH levels. In alkalosis, where the percentage of hydrogen in the ECF is low, the cells will release hydrogen into the ECF to increase acidity, and will absorb potassium from the serum. This results in a lowering of serum potassium levels. Although total body potassium is normal in this situation, the serum potassium will be lower. If the level drops below 3.5 mEq/L, the patient may exhibit signs and symptoms of hypokalemia despite the fact that alkalosis was their original condition. In the setting of hypokalemia, potassium is released from the ICF to maintain serum potassium levels. In response to this, hydrogen is absorbed into the ICF, creating an alkalotic state. Therefore, regardless of the original condition, hypokalemia and alkalosis commonly coexist, as either condition may cause the other.
The signs and symptoms of hypokalemia are nonspecific and depend on the individual patient. They generally originate in the nervous and muscular systems and are often not present until the potassium levels are less than 3.0 mEq/L. A good history and physical exam are required in the absence of actual potassium levels. As the hypokalemia progresses, the cardiovascular system may become involved. Early symptoms often noted by patients are muscular fatigue and weakness, particularly in the lower extremities (for signs and symptoms, see Figure 2). Death from hypokalemia is usually caused by anoxia secondary to paralysis of the respiratory muscles, which in turn leads to cardiac arrest.