Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Magnesium
Case Synopsis
A 23-year-old pregnant woman at 34 weeks and 3 days’ gestation with severe preeclampsia is brought emergently to the operating room for cesarean delivery under general anesthesia. She is currently receiving a magnesium infusion for treatment of her preeclampsia. Forty-five minutes after uneventful rapid-sequence induction with propofol and rocuronium, followed by an uneventful delivery, the patient has continued profound neuromuscular blockade (train of four 0/4 with no posttetanic facilitation). After waiting 30 minutes in the operating room with no improvement, she is brought intubated to the intensive care unit; she is extubated 6 hours later.
Problem Analysis
Definition
Magnesium Homeostasis
Magnesium is the fourth most abundant cation in the body, with total body stores of about 2000 mEq. Normal serum magnesium (Mg 2+ ) concentrations are between 1.4 and 2.1 mEq/L, equivalent to 1.7 and 2.5 mg/dL, respectively (see Table 90.1 for unit conversions); however, serum Mg 2+ concentrations correlate poorly with total body stores, reflecting less than 1% of the total. Gastrointestinal absorption in the duodenum and jejunum represents the principal source of Mg 2+ (8 to 9 mEq/day). The amount of Mg 2+ lost from the body via gastrointestinal secretions is relatively constant (2 mEq/day). In contrast, the kidney can dramatically affect losses in response to lowered serum Mg 2+ concentrations due to reabsorption of Mg 2+ in the proximal renal tubules and the loop of Henle.
TABLE 90.1
Unit Conversions for Magnesium Compounds and Serum Concentrations
| Compound | Unit Conversions |
|---|---|
| Magnesium sulfate (MgSO 4 ) | 1 g = 8.13 mEq of Mg 2+ |
| Magnesium oxide (MgO) | 1 g = 46 mEq of Mg 2+ |
| Magnesium acetate (MgC 4 H 6 O 4 ) | 1 g = 9.35 mEq of Mg 2+ |
| Magnesium chloride (MgCl 2 ) | 1 g = 9.75 mEq of Mg 2+ |
| Serum concentrations (all compounds) | 1 mg/dL = 0.83 mEq/L = 0.415 mmol/L |
Role in Cellular Function
Magnesium serves as an essential cofactor for many important cellular enzymes (e.g., adenylyl cyclase, Na + ,K + -ATPase). In addition, the magnesium complex, with adenosine triphosphate, serves as a substrate for the enzymatic reaction mediating muscle contraction and relaxation. Magnesium also regulates cellular function by antagonizing the effects of calcium and modulating several potassium currents ( Box 90.1 ).
BOX 90.1
Mechanisms for Magnesium’s Effect on Cellular Function
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here
