Chapter 250. Potassium and Magnesium Disorders

1. What are the causes of potassium and magnesium disorders?

2. What are the potential consequences of potassium and magnesium disorders?

3. How are potassium and magnesium disorders treated?

4. How are potassium disorders treated in clinical situations with rapid potassium shifts, such as diabetic ketoacidosis, hyperglycemic hyperosmolar state, and periodic paralysis?

Potassium and magnesium disorders are common in the hospital setting. About 12% of hospitalized patients have hypokalemia, 3% have hyperkalemia, and 11% have hypomagnesemia. Potassium disorders are a particular challenge for hospitalists, as the first clinical manifestation of a severe potassium abnormality may be a cardiac arrhythmia.

Potassium (K+) is the most abundant intracellular cation. In a 70 kg adult, the total body K+ is approximately 3500 mmol (50 mmol/kg). About 98% is located in the intracellular compartment, with the majority localized in muscle (70%). Because only 2% of the total body K+ is in the extracellular compartment, laboratory results correlate poorly with total body K+. Normal levels of extracellular potassium are between 3.5 and 5.0 mmol/L, while intracellular K+ levels range between 140 and 150 mmol/L.

Similarly, plasma magnesium (Mg2+) accounts for only 1% of the total body magnesium stores, and plasma Mg2+ correlates poorly with total body magnesium. Plasma magnesium is normally 1.7 to 2.3 mg/dL (0.70–0.95 mmol/L). Magnesium abnormalities can also have major clinical consequences, such as torsades de pointes. Hypokalemia is often accompanied by hypomagnesemia, which renders potassium repletion less effective.

Dietary K+ is excreted mostly in the urine (90%), with a minor component of fecal excretion (10%). In severe chronic kidney disease, stool losses may exceed 25% to 50% of dietary intake. Under normal conditions, the kidney can adjust potassium excretion over a wide range depending on K+ intake. The recommended daily K+ for patients with normal renal function is 4700 mg (120 mmol). If K+ control mechanisms are intact, an increased daily intake of 400 mmol can be tolerated with < 1 mmol/L increase in plasma K+ values. Daily K+ intake for patients on hemodialysis or with severe chronic kidney disease is typically 2000 mg (51 mmol) per day. Conversely, many patients on peritoneal dialysis can tolerate a normal daily intake of K+ due to losses of potassium in the urine and peritoneal fluid.

Etiology

Hyperkalemia is defined as a plasma K+ > 5.0 mmol/L. Hyperkalemia can result from redistribution, reduced K+ excretion, or increased K+ intake. Medications may lead to hyperkalemia by affecting either potassium redistribution or excretion (Table 250-1).