Hypokalemia

Key Features

Essentials of Diagnosis

• Serum K⁺ < 3.5 mEq/L (< 3.5 mmol/L)

• Severe hypokalemia may induce arrhythmias and rhabdomyolysis

• Assessment of urine potassium excretion (urine potassium to creatinine ratio) can distinguish renal from non-renal loss of potassium

General Considerations

• Hypokalemia can result from

  • Insufficient dietary potassium intake

  • Intracellular shifting of potassium from the extracellular space

  • Potassium loss (renal or extra-renal)

• A low dietary potassium intake is usually not sufficient to cause hypokalemia because the kidneys can lower urine potassium excretion to very low levels (< 15 mEq/L)

• Shift of potassium into cells is increased by insulin and beta-adrenergic stimulation

• Excess potassium excretion by the kidneys is usually due to increased aldosterone action in the setting of preserved delivery of sodium to the distal nephron

• Magnesium

  • An important regulator of potassium handling

  • Low levels lead to persistent renal excretion of potassium; hypokalemia is often refractory to treatment until the magnesium deficiency is corrected

• Loop diuretics (eg, furosemide) cause substantial renal potassium and magnesium losses

Clinical Findings

• Usually asymptomatic

• When severe, may lead to muscle weakness and cardiac arrhythmias

• Involvement of gastrointestinal smooth muscle may result in constipation or ileus

• Rhabdomyolysis with associated acute kidney injury may occur with potassium levels < 2.5 mEq/L

• May additionally present as polyuria and polydipsia due to diminished concentrating ability of the kidney (nephrogenic DI)

• Chronic hypokalemia can lead to kidney disease (tubulointerstitial nephritis)

Diagnosis

Laboratory Tests

• Transient hypokalemia is generally secondary to intracellular shift, while sustained hypokalemia is secondary to potassium wasting or rarely inadequate intake

• Assessment of renal potassium excretion can help distinguish renal from non-renal causes

• A 24-hour urine collection is the most accurate method for assessing renal handling of potassium, with a level < 25 mEq/day compatible with appropriate renal potassium retention, and higher values corresponding to renal potassium wasting

• A more immediate assessment can be made by measuring a urine potassium to creatinine ratio (U_K/U_Cr) on a spot urine sample

• In the setting of hypokalemia, a U_K/U_Cr ratio < 13 mEq/g (or 1.5 mEq/mmol) is suggestive of a nonrenal etiology, most commonly gastrointestinal losses, intracellular potassium shifts, or inadequate dietary intake; higher values imply renal potassium wasting

Diagnostic Procedures

• Characteristic progression of electrocardiogram (ECG) changes as the hypokalemia becomes more severe: initially T wave flattening, subsequently ST depressions and T wave inversions, ultimately U waves

• Typical ECG patterns may be not be observed in all patients

Treatment

• Any underlying conditions should be treated and causative drugs discontinued

• Magnesium deficiency should be corrected, particularly in refractory hypokalemia

• Oral potassium supplementation

  • Safest for mild to ...