1. Radiographic studies. Rarely helpful in the acute setting but may help identify underlying abnormalities responsible for hypokalemia. Abdominal ultrasound or CT scan may help identify adrenal tumors, and MRI scan of the brain may identify pituitary abnormalities associated with increased cortisol release. These studies should be performed based on an index of suspicion from history and laboratory results.
2. ECG. Of paramount importance, especially as serum potassium level drops below 3 mEq/L. Rapid recognition of cardiac disturbances is critical. It is also important to monitor cardiac status as therapy is instituted. (ECG changes, outlined earlier at II, A, include PACs, PVCs, flattened T waves, ST segment changes, and U waves). The classic finding of a U wave is poorly understood. It is believed to represent delayed repolarization of cardiac muscle. The U wave appears after the T wave. As hypokalemia worsens, the T wave flattens and the U wave becomes more pronounced, producing what appears to be a prolonged QT interval.
V. Plan. Treatment is variable, depending on severity of the potassium deficit as well as presence of symptoms and associated conditions. Serum potassium level is not a good indication of total body potassium deficit. Patients with diabetic ketoacidosis often present with a normal to high serum potassium level, yet have a severe total body deficit. In adult patients (70-kg man) an estimate of total body potassium deficit can be approximated as 150 mEq for each 1 mEq/L decrease in serum potassium from 4 mEq/L. No such physiologic studies have been performed in children to produce a reliable estimate. A. Mild, Asymptomatic Hypokalemia (serum K+ 3-3.5 mEq/L).
Depending on cause, may resolve without therapy or require oral supplementation of potassium chloride (KCl). Must consider ongoing loss when correcting serum level as well as daily requirement of 2-3 mEq/kg/day.
B. Severe or Symptomatic Hypokalemia (serum K+ < 3 mEq/L). Requires more rapid assessment and therapy as well as more stringent cardiac monitoring. Typically IV administration of KCl is required. Usual dose is 0.5 mEq/kg IV, to be given over 1 hour and not to exceed 40 mEq total. Infusion of as much as 1 mEq/kg/h may be used in severe, life-threatening hypokalemia. It is important to perform all infusions with appropriate cardiac monitoring and to reassess serum potassium level frequently.
C. Recalcitrant Hypokalemia. Correct serum magnesium and reconsider severity of ongoing potassium loss.
VI. Problem Case Diagnosis. Hypokalemia in the 6-month-old infant was secondary to GI loss from severe gastroenteritis. Patient responded well to IV administration of KCl.
VII. Teaching Pearl: Question. A patient has a serum potassium level of 3 mEq/L and is also anemic; a blood transfusion is ordered. How should you approach correcting the potassium level?
VIII. Teaching Pearl: Answer. Stored blood is often relatively high in potassium due to red cell hemolysis. If the patient is asymptomatic, it may be wise not to treat a mild hypokalemia when giving blood. Repeating a potassium level after transfusion may show that you have accomplished your goal.
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