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I. PROBLEM

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The serum potassium level is 6.5 mEq/L in an extremely low birthweight infant. Normal potassium levels are generally between 3.5 and 5.5 mEq/L. Definitions can vary by weight, but most define hyperkalemia as >6 mEq/L in newborns. Hyperkalemia is common in extremely low birthweight infants. This is the most serious of electrolyte abnormalities because it can cause fatal arrhythmias. If electrocardiogram (ECG) changes relating to hyperkalemia are present, this is an emergency situation. (See Section V.B.)

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II. IMMEDIATE QUESTIONS

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  1. How was the specimen collected? What is the central serum potassium level? Is it a true level or factitious? Blood obtained by heelstick or drawn through a tiny needle may yield falsely elevated potassium levels secondary to hemolysis. Clot formation can also cause falsely elevated potassium. The blood should not be obtained from a heparin-coated umbilical catheter (release of benzalkonium from a heparin-coated umbilical catheter can elevate the potassium reading). Note: Serum potassium level is 0.4 mEq/L higher than the plasma level.

  2. Does the ECG show cardiac changes characteristic of hyperkalemia? This may be the first indication of hyperkalemia. In neonates, serum potassium >6.7 mEq/L is associated with ECG changes. Early cardiac changes include tall, peaked, “tented” T waves, followed by loss or flattened P wave, widening QRS, ST-segment depression, bradycardia, sine wave QRS-T, first-degree atrioventricular block, ventricular tachyarrhythmias, and finally cardiac arrest if the potassium levels continue to increase.

    1. Serum K 5.5–6.5 mEq/L. Tall peaked T waves with a narrow base.

    2. Serum K 6.5–8 mEq/L. Tall peaked T waves, prolonged PR interval, loss or decreased P wave, amplified R wave, widening of QRS.

    3. Serum K >8 mEq/L. Absent P wave, wide bizarre diphasic QRS, progressive QRS widening merging with the T wave, bundle-branch blocks, ventricular fibrillation or asystole.

  3. How much potassium is the infant receiving? Normal amounts of potassium given for maintenance are 1–3 mEq/kg/d.

  4. What are the blood urea nitrogen (BUN) and creatinine levels? What is the urine output and body weight? Elevated BUN and creatinine suggest renal insufficiency. Another indication of renal failure is decreasing or inadequate urine output with weight gain.

  5. Is there associated hyponatremia, hypoglycemia, and hypotension? With low sodium and glucose, high potassium, and hypotension, consider adrenal insufficiency.

  6. Does the infant have any of the common characteristics of premature newborns prone to hyperkalemia? These include small for gestational age, female sex, severe respiratory distress syndrome, very low birthweight, requirement of exogenous surfactant, need for inotropic medications, and delayed feeding. Mildly elevated potassium (>5.6 mEq/L) and phosphate levels (>2 mEq/L) within 6 hours of birth may predict the development of hyperkalemia.

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III. DIFFERENTIAL DIAGNOSIS

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True hyperkalemia can be caused by an increase in potassium intake (usually not a problem if kidneys able to excrete potassium), an increase in potassium release, a decrease or inability in potassium excretion by the kidneys, or by a shift of potassium into the extracellular space or an impaired ...

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