Mar 01 2014

How to Replace Electrolytes

Electrolyte replacement is one of the first things new interns learn when they start on their inpatient rotations. It is very easy to just order the electrolyte replacement protocol and forget about replacing the electrolyte abnormalities. However, it is important to know what you are ordering and understand how to replace these electrolytes on your own. This post will cover the replacement of potassium, magnesium, phosphorus, and calcium. The replacement of sodium is a large topic in and of itself and will not be covered in this post. This post should be considered to be general information on how to replace electrolytes in the inpatient setting and should not be used to make any clinical decisions. You should always consult your specific hospital policies for electrolyte replacement.

Hypokalemia

How is hypokalemia defined?

Hypokalemia is defined as a serum potassium < 3.5 mEq/L.

How is hypokalemia replaced?

Potassium can be replaced through the oral or IV routes. The oral route is best for mild hypokalemia if the patient can take pills, because it doesn’t burn going in like the IV form. If the patient is unable to take PO or has severe hypokalemia (or is symptomatic) replacement should be through the IV route. The potassium should be mixed in normal saline and not dextrose as this may worsen the hypokalemia. Administration through a femoral central line is ideal because the local concentration of K+ around the heart can be rapidly increased by infusion through an internal jugular or subclavian line.

What formulations of potassium replacement are available?

Potassium is usually replaced in the form of potassium chloride (KCl), but can also be given as potassium phosphate, potassium bicarbonate, or potassium citrate depending on the clinical situation.

What is the usual dose of potassium chloride?

The pills can be given in up to 40 mEq doses up to every 4 hours. The highest concentration of potassium chloride infused IV is usually 40 mEq/L through a peripheral line or 100 mEq/L through a central line and should not be infused faster than 20 mEq/hr. However, it can be infused faster if there are ongoing critical complications from the hypokalemia and the patient should be monitored in the ICU.

How much will the serum potassium be raised?

A general rule is that for every 10 mEq of K+ given it will raise the serum K+ by 0.1 mEq/L.

Why should hypomagnesemia be replaced along with the hypokalemia?

Concomitant hypomagnesemia should also be replaced because low magnesium levels inhibit the Na+K+-ATPase pump and cause more potassium to be excreted in the urine. Low magnesium levels also allow increased potassium secretion in the distal nephron.

How many mEq of potassium are in one inch of banana?

Approximately 1 mEq

Hypomagnesemia

How is hypomagnesemia defined?

Hypomagnesemia is defined as a serum magnesium < 1.3 mEq/L.

How is hypomagnesemia replaced?

Magnesium can be replaced through the oral or IV routes. It should be replaced orally if the patient is asymptomatic or IV if the patient is symptomatic.

What formulations of magnesium replacement are available?

There are multiple oral formulations including:

  • Mag-Ox (240 mg elemental magnesium per 400 mg tablet)
  • Uro-Mag (84 mg elemental magnesium per 140 mg tablet)
  • Slow-Mag (64 mg elemental magnesium per tablet)

The IV form of magnesium comes as magnesium sulfate (96 mg elemental magnesium per 1 g IV) and can be given as an IV push if the patient is symptomatic (e.g. arrhythmia).

What is the usual dose of magnesium?

A usual oral dose is one tablet of Mag-Ox per day for mild hypomagnesemia, and up to three tablets daily for severe hypomagnesemia.

How much will the serum magnesium be raised?

A general rule is that for every 2 g of MgSO4 given IV it will raise the serum Mg by 0.5 mEq/L.

Hypophosphatemia

How is hypophosphatemia defined?

Hypophosphatemia is defined as a serum phosphorus < 2.8 mg/dL.

How is hypophosphatemia replaced?

Phosphorus can be replaced through the oral or IV routes. IV replacement should be considered when the serum phosphorus is < 1.0 mg/dL or the patient is unable to take PO.

What formulations of phosphorus replacement are available?

There are several oral formulations including:

  • Neutra-Phos (250 mg elemental phosphorus and 7 mEq Na+ and K+ per capsule)
  • Neutra-Phos K (250 mg elemental phosphorus and 14 mEq K+ per capsule)
  • Fleet Phospho-soda (815 mg phosphorus and 33 mEq Na+ per 5 mL)

The IV form of phosphorus comes as potassium phosphate (1.5 mEq potassium/mmol phosphate) and sodium phosphate (1.3 mEq sodium/mmol phosphate). One mmol phosphate is equal to 31 mg phosphorus.

What is the usual dose of phosphorus?

A usual dose is 0.5 to 1.0 g elemental phosphorus two to three times daily. The IV dose is weight based with the usual dose being 0.08 to 0.16 mmol/kg given IV over six hours.

Hypocalcemia

How is hypocalcemia defined?

Hypocalcemia is defined as a serum calcium < 8.4 mg/dL or an ionized calcium < 4.2 mg/dL.

How is hypocalcemia replaced?

Calcium can be replaced through the oral or IV routes. The IV formulations should only be given for severe or symptomatic hypocalcemia.

If hyperphosphatemia is present, this should be corrected first as the calcium may precipitate with the phosphorus. In addition, hypomagnesemia should also be corrected first in order to adequately treat the hypocalcemia.

Before you begin replacement the serum calcium should be corrected for the albumin to rule out pseudohypocalcemia. The corrected calcium equals [(4 – albumin) x 0.8] + measured calcium.

What formulations of calcium replacement are available?

There are two main formulations of oral calcium including:

  • Calcium carbonate (40% elemental calcium)
  • Calcium acetate (25% elemental calcium)

The IV form of calcium comes as calcium chloride or calcium gluconate. Calcium gluconate is preferred due to less risk of tissue toxicity with extravasation.

What is the usual dose of calcium?

A usual dose is 1 to 2 g of elemental calcium orally three times daily. The calcium should be given with Vitamin D to increase the intestinal absorption.

One ampule (10 mL) of calcium gluconate has approximately 90 mg of elemental calcium. A usual dose is 1 to 2 g of calcium gluconate given IV over 10 to 20 minutes.

How much will the serum calcium be raised?

A general rule is that for every 1 g of calcium gluconate given it will raise the serum calcium by 0.5 mg/dL.

This concludes the discussion on the replacement of these key electrolytes. Keep in mind that this applies only to the inpatient setting where you are able to have adequate monitoring of the patient including frequent lab checks of these electrolytes.

References:
Moses, Scott, MD. “Dietary Potassium.” FPnotebook.com. Family Practice Notebook, 17 Feb. 2014. Web. 28 Feb. 2014. <http://fpnotebook.com/Renal/Pharm/DtryPtsm.htm>.
Mount, David B. “Fluid and Electrolyte Disturbances.” Harrison’s Principles of Internal Medicine. By Dan L. Longo. 18th ed. Vol. 1. New York: McGraw-Hill, Medical, 2012. 353-54. Print.
Sankarpandian, Bala, and Steven Cheng. “Fluid and Electroylyte Management.” The Washington Manual of Medical Therapeutics. By Corey Foster. 33rd ed. Philadelphia, PA: Wolters Kluwer/Lippincott Williams & Wilkins, 2010. 385-88, 394-97, 399-400, 402-03. Print.
Swaroop, Bindu, MD. “Strategies for Electrolyte Replacement.” Medicine.uci.edu. University of California, Irvine School of Medicine, n.d. Web. 28 Feb. 2014. <http://www.medicine.uci.edu/residency/powerpoint/Electrolyte.ppt>.

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