Fluids, Electrolytes, Acid-Base Disorders, and Nutrition Support

iv.

Empiric intravenous magnesium sulfate dosing. This algorithm (Table 13) was designed

primarily for trauma and thermally injured patients but can likely be universally applied to

other critically ill patient populations (Nutrition 1997;13:303-8). The therapy may need to be

adjusted according to renal function and ongoing severity of losses. An empiric approach to

dosing intracellular electrolytes for patients with significant renal impairment is to give one-

half the recommended dose. However, electrolyte therapy for patients with renal impairment

must be individualized according to patient response. Certain patient populations, such as

those with cholinergic disorders, may be harmed with administration of magnesium sulfate;

hence, an adequate patient history is paramount for patient safety.

Magnesium concentrations are often elevated for several hours or longer after an infusion

because it takes about 48 hours for the magnesium to fully redistribute to the body tissues

and levels should continue to be monitored daily to ensure an appropriate assessment of

magnesium is made after redistribution of previous bolus doses.

Serum Magnesium (mg/dL)

Dose (g/kg)a

1.6–1.8

0.05

1–1.5

0.1

< 1

0.15

aFor ease of use and preparation, intravenous magnesium sulfate should be ordered in 2-g increments (e.g., 2 g, 4 g, 6 g). The drug should be mixed in 50-100 mL of

normal saline or 5% dextrose and given at a rate no faster than 1 g (8 mEq) per hour (Nutrition 1997;13:303-8). Commercially available sources of magnesium sulfate

are available in 2 gram/50 mL and 4 mEq/100 mL premixed bags. Maximum dose should be held at a ceiling of 8–10 g per administration. Intravenous magnesium may

be administered by the peripheral or central venous route.

vi.

Oral magnesium: It can be difficult to successfully replenish magnesium if given by the

oral route in critically ill patients because of the adverse GI effects of oral magnesium (e.g.,

diarrhea) and the high elemental magnesium doses required to achieve repletion. Although it

has been inferred that certain oral magnesium products are better tolerated than others (e.g.,

gluconate vs. oxide), this tolerability likely pertains to the elemental magnesium content of

the products. The lower the elemental magnesium content, the more tolerable the oral product.

However, the lower the magnesium content, the more difficult it is to achieve magnesium

repletion for a patient with significant magnesium depletion. Oral magnesium replacement is a

reasonable option for patients who are asymptomatic and tolerating oral intake as long as their

magnesium concentration is not below 1 mg/dL.

Salt Form

Strength

(mg)

Elemental Magnesium Content

(mEq)

Usual Dosing

Oxide

19.8

6.9

1–2 tablets two or three times daily

Gluconate

2.2

1–2 tablets two or three times daily

Chloride

2.6

1–2 tablets two or three times daily