|(RDA) of magnesium is 4.5 mg/kg which is a total daily allowance of 350-400 mg for adult men and 280-300 mg for adult women. During pregnancy the RDA is 300 mg and during lactation the RDA is 355 mg. Dave’s tip: Generally do not exceed ~40meq (490mg elemental Mg++)/day with oral supplements to reduce incidence of diarrhea.
|INTRAVENOUS MAGNESIUM REPLACEMENT: For Mg levels < 1.2 mg/L or symptomatic or patient unable to take oral. (~1 gram IV/hour)SYMPTOMATIC/ASYMPTOMATIC
WEIGHT OR Mg < 1.2 mg/dl AND Mg > 1.2 mg/dl
< 50 kg 2-3 gm Mg Sulfate 1-2 gm Mg Sulfate
>50 kg 3-4 gm Mg Sulfate 2-3 gm Mg Sulfate
Additional doses of 1-2 gms/day of Mg sulfate may be required for several days if the patient has not previously been receiving magnesium. Renal insufficiency (CLcr < 20ml/min) may require lower doses of magnesium. Caution should be used when replacing magnesium in any patient with renal insufficiency.
MAXIMUM CONCENTRATION: 1 gm in 10 ml D5W or NS
Serum magnesium: 1.5-2.5 mg/dL (MW: 24.3) Overview Dietary Sources: Average diet provides a daily Mg intake ranging form ~17 to ~ 50 mEq ( 200 – 600 mg). Mg is ubiquitous in food, but it is particularly abundant in dairy products, bread and cereals, vegetables (specially the leafy types), meat, and nuts (specially almonds).
Mg++ is an important ion that is required by the human body in relatively large amounts. It is essential for the optimal function of over 300 key enzymes involved in energy transformation, protein synthesis and nucleic acid metabolism. It is also essential for the stability and normal function of the cell membranes of excitable tissues. Thus, Mg abnormalities can have profound effects on neuromuscular transmission and cardiac conduction. Also, a normal body Mg content is necessary for the maintenance of electrolyte balance particularly for Ca++and K+.
Transplant patients are particularly prone to the development of Mg deficiency due to a direct effect of tacrolimus and cyclosporine on the renal tubules which results in enhanced urinary Mg loss.
Distribution and Balance
Average body Mg content is about 1000 mmoles (14 mmoles / kg of body wt), of which ~50% resides in soft tissues and the remainder in bones. The extracellular space contains <1% of total body Mg. The plasma Mg level normally varies within a relatively wide range (1.5 – 2.5 mEq/L). Clinical laboratories typically measure total plasma Mg, of which ~30% is bound to plasma proteins, ~ 20% is complexed with such ions as phosphate and citrate, and ~50% exists in the ionized (physiologically active) state. Due to the relatively low protein binding, variations in the plasma protein level have little influence on the total plasma Mg concentration. This is unlike the case plasma calcium of which ~45% is protein-bound. To correct for low plasma protein levels:
|Corrected Ca level = Actual Ca level + 0.8 (4 – Alb).
Corrected Mg level = Actual Mg level + 0.08 (4 – Alb)
Although serum Mg may not accurately reflect the overall body Mg balance, clinical symptoms of Mg deficiency correlate well with serum Mg. Levels < 1.0 mEq/L usually indicate significant total body Mg depletion. However, the severity of the symptoms among patients with similar degrees of hypomagnesemia vary widely. The so-called Mg retention test should not be used in patients with renal impairment or in transplant patients receiving cyclosporine or tacrolimus which cause urinary Mg wasting.
Mg balance is primarily a renal function (Mg intake does not appear to be regulated). The average diet provides 20 – 30 mEq daily, but the net absorption is only about 7 mEq. The kidneys normally excrete an equal amount in order to maintain Mg balance. However, in the presence of Mg deficiency urinary Mg excretion can be reduced to a minimum of about 2 mEq/day. Renal Mg reabsorption takes place primarily in the proximal tubule (30% of the filtered load) and the thick ascending limb of Henle’s loop (65%). Overall renal reabsorption appears to be saturable, so that a higher Mg intake results in a proportional increase urinary excretion. For this reason, it is difficult for patients with normal renal function to develop hypermagnesemia.
Causes of hypomagnesemia
- GI problems
- Diarrhea, NG suction, fistulas, etc
- Poor intake (as in alcoholics)
- Poor absorption (malabsorption, ileal bypass, etc]
- Renal Losses
- Acute alcohol consumption.
- Osmotic or saline diuresis
- Primary hyperaldosteronism
- Hypercalciuric disorders
- Tubulo-interstitial nephropathies
- Drugs: diuretics, aminoglycosides,
cyclosporine A, tacrolimus, cis-platinum, etc.
- Primary Renal Mg Wasting
- Internal Redistribution (from ECF to ICF)
- IV administration of glucose or aminoacids (TPN)
- Treatment of diabetic ketoacidosis (DKA)
Symptoms of hypomagnesemia
- Neural and Neuromuscular:
Neural and neuromuscular abnormalities are the most common clinical signs of hypomagnesemia. Lowered excitability threshold may be manifested as irritability, psychosis, esophageal spasms (leading to dysphagia), and convulsions. Neuromuscular manifestations include tremor, fasciculations, and tetany. The latter occurs almost exclusively in presence of hypocalcemia.
Chronic, whole-body Mg depletion may be associated with lethargy, poor appetite, nausea, muscle cramps, paresthesias, and mental abnormalities (irritability, confusion, disorientation, etc.).
- ECG changes: Prolonged PR & QT intervals and flattening of the T waves.
- Ventricular dysrhythmias: Premature contractions, tachycardia, and fibrillation. These occur almost exclusively in patients receiving digoxin therapy because both digoxin and hypomagnesemia promote the loss of K from myocardial cells. Thus, hypomagnesemia, like hypokalemia, predisposes patients to digitalis toxicity.
Although an acute fall in plasma Mg tends to stimulate the release of parathormone (PTH), chronic hypomagnesemia has the opposite effect, resulting in hypoparathyroidism and hypocalcemia. Also, hypomagnesemia is associated with target-tissue resistance to the actions of PTH. Correction of the hypocalcemia induced by Mg deficiency requires the repletion of Mg stores as an essential first step.Because Mg++ is necessary for the activation of the Na-K-ATPase, Mg deficiency is almost always associated with intracellular K+ depletion. Also, Mg depletion induces renal K + loss possibly by inhibiting K + reabsorption in the proximal tubules. The hypokalemia associated with Mg deficiency can be corrected only through the administration of both ions.
Note: Oral magnesium is not generally adequate for repletion in patients with serum magnesium concentrations <1.5 mEq/L (1.2 meq/L)?
|Magnesium hydroxide ( Milk of Magnesia): Dosing (Adults): Laxative ( Onset of action: Laxative: 4-8 hours): Oral:> 12 years: 30-60 mL/day or in divided doses. Antacid: Oral: 5-15 mL up to 4 times/day as needed. Renal Insufficiency: Patients in severe renal failure should not receive magnesium due to toxicity from accumulation.|
|Magnesium oxide: Mag-Ox ® 400, Uro-Mag®. Dosing (Adults): Dietary supplement: Oral: 20-40 mEq (1-2 tablets) 2-3 times/day. Product labeling: Mag-Ox 400®: 1-2 tablets daily with food ( Do not take more than 2 tablets in a 24-hr period, except under the advice and supervision of a physician.) Uro-Mag®: 1-2 tablets 3 times/day with food.|
|Magnesium gluconate: Magonate® : Dietary supplement: Oral: 54-483 mg/day in divided doses. Absorption: Oral: 15% to 30%.|
|Slow-Mag: enteric-coated tablet – 64 mg elemental magnesium and 106 mg elemental calcium. Magnesium chloride formulation that is enteric-coated to help prevent the stomach upset commonly associated with oral magnesium supplements. Tablets provide magnesium chloride for increased absorption versus magnesium oxide. Dosing: dietary supplement: take 2 tablets daily or as directed by a physician.|
Magnesium Tables (formulations)
| Oral Supplementation
[Oral absorption is variable – 20 – 50% of an oral dose is absorbed]
[aggressive oral supplementation can lead to diarrhea]
|Mag Oxide, 400 mg (Mag-Ox®)||241.3 mg (20.1 mEq)|
|Uro-Mag® cap 140 mg||(84.5 mg) 6.93 mEq|
|Mag Gluconate, 500 mg||27 mg (2.25 mEq)|
|Mag Gluconate, liquid (Magonate)||54 mg (4.5 mEq) per 5mL|
|Mg Chloride (Slow-Mag®), 535 mg||64 mg (5.33 mEq)|
|Mg Hydroxide (MOM) (1200 mg / 15 mL)
(500mg Mg2+/15 ml)
|166.7 mg (13.7 mEq) per 5 mL.
(start with 5 mL tid = 41 meq)
|Mg L-Aspartate HCl (Maginex™)||615 mg (5 mEq) / tablet|
|1230 mg (10 mEq) / packet|
|Mg lactate (Mag-Tab SR)||84 mg (7 mEq)
(start with 14 mEq bid)
|Exists as magnesium sulfate heptahydrate – MgSO4.7H2O MW=246.47 Calculations: (1gram MgSO4 (2ml of 50% soln)/246.47) x 1000= 4.057 or ~ 4.06 mmol x valence (2) = 8.12 meq. (contains 10% elemental magnesium)|
|Magnesium Salt||M.Wt.||% Mg||mEq/g|