63: Hypokalemia

I. PROBLEM

The nurse reports a serum potassium of 2.8 mEq/L. Normal serum potassium values vary with technique used by the laboratory but are usually between 3.5 and 5 mEq/L. Hypokalemia is defined as a serum potassium <3.5 mEq/L. Mild hypokalemia is 3.0–3.5 mEq/L. Moderate hypokalemia is 2.5–3.0 mEq/L; severe hypokalemia is <2.5 mEq/L. Severe hypokalemia can cause cardiac arrhythmias.

II. IMMEDIATE QUESTIONS

1. What is the central serum potassium? If a low value is obtained by heelstick, central values should be obtained because they may actually be lower than values obtained by heelstick because of the 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 1–2 mEq/kg/d.

4. Are there any gastrointestinal (GI) losses from diarrhea, a naso-gastric/orogastric (NG/OG) tube, or ileostomy? Loss of large amounts of 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.

III. DIFFERENTIAL DIAGNOSIS

Hypokalemia can be caused by a prolonged inadequate intake of potassium, gastrointestinal losses, renal losses, and transcellular shifts or redistrubution. GI and renal losses are more common. Medications (diuretics) are the most common cause in the neonatal intensive care unit (NICU).

1. Pseudohypokalemia can occur if the blood sample sits too long in a warm environment, with a very high white blood cell (WBC) count (uptake of potassium by abnormal WBC), or from a heelstick sample.

2. True hypokalemia (total body deficit)

   1. Inadequate intake (rare) of either maintenance infusion or oral intake of potassium. For a further discussion, see Chapter 9.

   2. Gastrointestinal losses

      1. Loss of fluid via nasogastric tube (common). Unreplaced electrolyte loss from NG tube or excess drainage from ileostomy.

      2. Diarrhea. Congenital chloride diarrhea, any gastrointestinal fistula, short bowel syndrome.

      3. Vomiting. May cause hypokalemia such as infantile hypertrophic pyloric stenosis with vomiting.

      4. Medications. Kayexalate causes fecal potassium loss.

   3. Renal losses

      1. Medications

         1. Diuretic use especially long-term therapy with any thiazide or loop diuretic is the most common medication-related cause.

         2. Steroids and steroid-like medications.

         3. Antibiotics. High-dose penicillin, ampicillin, carbenicillin, vancomycin. Combined treatment of aminoglycosides and vancomycin can cause tubular disturbances in extremely low birthweight (ELBW) infants with renal tubular wasting of potassium.

         4. Magnesium depletion medications such as amphotericin B and aminoglycosides. Amphotericin B can cause direct renal tubular damage with potassium loss.

      2. Any cause of polyuria

      3. Renal tubular losses

         1. Renal tubular acidosis (RTA). Type 1 and 2.

         2. Hypomagnesemia. Exacerbates potassium loss by increasing distal potassium secretion.

         3. Bartter syndrome ...