Chapter 68: Hypokalemia

I. PROBLEM

The nurse reports a serum potassium of 2.8 mEq/L in a normal newborn. Normal serum potassium values vary with technique used by the laboratory but are usually between 3.5 and 5.5 mEq/L. Hypokalemia is defined as a serum potassium <3.5 mEq/L. Most sources use the following designations:

1. Mild hypokalemia is 3.0 to <3.5 mEq/L,

2. Moderate hypokalemia is 2.5 to 3.0 mEq/L,

3. Severe hypokalemia is <2.5 mEq/L.

   Hypokalemia causes membrane hyperpolarization and impairs muscle contraction; therefore, the clinical manifestations involve changes to muscle and cardiovascular function. Severe hypokalemia can cause cardiac arrhythmias (can be fatal), respiratory depression, skeletal muscle impairment and weakness, ileus, lethargy, and in extreme cases, rhabdomyolysis. Since 98% of potassium is intracellular and 2% is extracellular, serum potassium levels do not correlate with intracellular potassium levels; therefore, hypokalemia does not reflect total body potassium stores. The body must maintain a normal extracellular potassium in order to avoid any cardiac or neurologic signs or symptoms.

II. IMMEDIATE QUESTIONS

1. What is the central serum potassium? If a low value is obtained by heel stick, central values should be obtained because they may actually be lower than values obtained by heel stick (potassium release from hemolysis of red blood cells). Was the sample sent immediately to the lab? If a sample sat for hours in a warm area, “pseudohypokalemia” can occur.

2. Is the infant on diuretics? Are potassium-wasting medications or digitalis being given? Hypokalemia in a neonate usually occurs from chronic diuretic use. Hypokalemia may cause significant arrhythmias if digitalis is being administered.

3. How much potassium is the infant receiving? Normal maintenance doses are 1 to 2 mEq/kg/d.

4. Are there any gastrointestinal losses from diarrhea, a nasogastric/orogastric tube, or ileostomy? Loss of large amounts of gastrointestinal (GI) fluids can cause hypokalemia. Severe vomiting can also cause hypokalemia such as in infantile hypertrophic pyloric stenosis.

5. What is the infant’s magnesium level? Hypomagnesemia can cause hypokalemia. Consider this diagnosis if the hypokalemia does not correct despite potassium supplementation.

6. Does the infant have hypertension? Consider mineralocorticoid excess: primary aldosteronism, congenital adrenal hyperplasia, or Cushing syndrome.

III. DIFFERENTIAL DIAGNOSIS

Hypokalemia can be caused by a prolonged inadequate intake of potassium, increased GI losses, increased renal losses, transcellular shifts of potassium from extracellular to intracellular spaces, and medications. Medications (diuretics, specifically loop or thiazide diuretics) are the most common cause in the neonatal intensive care unit, followed by increased GI losses from the nasogastric (NG) tube.

1. Pseudohypokalemia can occur if the blood sample has a very high WBC count kept at room temperature for a long time, whereby uptake of extracellular potassium by the WBC occurs. If the blood sample sits at an increased temperature too long (“summertime pseudohypokalemia”), this increases Na⁺/K⁺-ATPase activity, which shifts potassium into cells. A blood specimen collected after insulin administration that sits too long ...