Which is replaced first, Magnesium (Mg) or Potassium (K), in a patient with depletion of both?

Magnesium Should Be Replaced Before Potassium in Patients with Depletion of Both

Magnesium should be replaced first before potassium in patients with depletion of both electrolytes, as hypomagnesemia can cause refractory hypokalemia that will not correct until magnesium levels are normalized. 1, 2

Rationale for Replacing Magnesium First

• Magnesium deficiency impairs potassium repletion due to increased urinary potassium losses, making potassium supplementation ineffective until magnesium is corrected 1, 3
• Hypomagnesemia is associated with secondary hypokalemia that becomes refractory to treatment without concurrent magnesium replacement 2
• Hypomagnesemia is frequently underdiagnosed in clinical practice, occurring in up to 12% of hospitalized patients and 60-65% of critically ill patients 4

Clinical Implications of Magnesium-Potassium Relationship

• Hyperaldosteronism (from sodium depletion) increases renal retention of sodium at the expense of both magnesium and potassium, which are lost in high amounts in the urine 5
• To correct hypokalemia effectively, especially in patients with high output stoma or other conditions causing electrolyte depletion, sodium/water depletion must first be corrected to avoid hyperaldosteronism, and serum magnesium should be brought into the normal range 5
• Both potassium and magnesium deficiencies are associated with increased ventricular ectopy and may increase the risk of sudden unexpected death 1

Special Considerations in Different Clinical Scenarios

• In patients undergoing continuous kidney replacement therapy (CKRT):

  • Hypomagnesemia occurs in up to 65% of critically ill patients on CKRT 4
  • The risk is particularly high when regional citrate anticoagulation is used, as ionized magnesium is chelated by citrate and lost in the effluent 5
  • Dialysis solutions containing magnesium (along with potassium and phosphate) should be used to prevent electrolyte disorders during CKRT 5

• In patients with short bowel syndrome:

  • Significant magnesium losses occur, particularly in those with jejunostomy 4
  • Rehydration to correct secondary hyperaldosteronism is the crucial first step before electrolyte supplementation 5, 4
  • Magnesium supplementation may require 12-24 mmol daily, preferably administered at night when intestinal transit is slowest 4

Practical Approach to Replacement

• Assess and correct volume status first to reduce secondary hyperaldosteronism 5, 4
• Replace magnesium to normal levels before aggressive potassium replacement 1, 2
• Monitor both electrolytes during replacement therapy 4
• Consider intravenous magnesium replacement in severe deficiency or when oral supplementation is ineffective 4, 6

Common Pitfalls to Avoid

• Failing to measure serum magnesium levels in patients with hypokalemia 1, 2
• Attempting aggressive potassium replacement without addressing concurrent magnesium deficiency 1, 2
• Overlooking that serum magnesium levels may not accurately reflect total body magnesium status, as less than 1% of magnesium is found in the blood 4
• Neglecting to consider magnesium replacement in patients on diuretics or with gastrointestinal losses 3