USMLE/COMLEX - Step 2 - Hypokalemia
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USMLE/COMLEX - Step 2 - Hypokalemia
What is the cut-off value for hypokalemia?
Serum potassium less than 3.5 mEq/L is hypokalemia.
What are the common etiologies of hypokalemia?
Transcellular shift
- Alkalosis
- Medications e.g., insulin, ?2-adrenergic agonists
- Vomiting
- Nasogastric tube drainage
- Diarrhea
- Laxative abuse
- Diuretics (e.g., loop and thiazide diuretics)
- Primary or secondary hyperaldosteronism
- Decreased circulating volume
- Renal tubular acidosis (type I and II)
- Hypomagnesemia
- Bartter syndrome (mimics loop diuretic)
- Gitelman syndrome (mimics thiazide diuretic)
- Drugs (e.g., amphotericin, gentamicin)
What are the clinical manifestations of hypokalemia?
Patients can present with fatigue, paresthesia, muscle cramps and weakness. Muscle weakness usually begins in the lower limbs and progress to trunks and upper limbs. Respiratory muscle weakness can also occur which can lead to respiratory failure. Gastrointestinal muscle weakness can lead to nausea, vomiting, anorexia, distention and ileus. Rhabdomyolysis can also develop.
What are the ECG findings in hypokalemia?
ECG findings include:
Flattening of T waves
Presence of U waves
Depression of the ST segment
Prolongation of QT interval
How will you manage hypokalemia?
The management of hypokalemia aims to treat the underlying cause of hypokalemia and replace potassium deficit to prevent life-threatening complications.
Potassium can be replaced via oral or intravenous routes.
What is the preferred route of potassium replacement?
Oral is the preferred route of potassium replacement due to inexpensiveness, and safety. However, intravenous route can be undertaken in patients who are unable to take oral medications or as an add-on to oral therapy.
Which electrolyte imbalance can lead to difficulty in potassium replacement?
Hypomagnesemia can lead to difficulty in potassium replacement and is a well-known cause of refractory hypokalemia.
How can hypomagnesemia lead to hypokalemia?
Potassium is secreted by renal outer medullary potassium (ROMK) channels in the cortical collecting tubules of the kidney. This process is inhibited by intracellular magnesium. Decreased intracellular magnesium in the setting of hypomagnesemia relieves the inhibitory effect of magnesium on potassium secretion. Therefore, potassium loss increases. Normalization of magnesium levels restores ROMK channels and prevents potassium depletion. Therefore, serum magnesium levels should be measured in patients with hypokalemia and replaced if low.