Magnesium Dosing
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References
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Magnesium |
(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.
|
Product |
Amount needed to
obtain ~40 meq of Mg++/day |
|
MgOxide 400 mg |
2 tablets |
| Mag
Gluconate 500 mg |
~17 tablets |
| Mg
Chloride (Slow-Mag®), 535 mg |
7.5 tablets |
| Mg
Hydroxide (MOM) (1200 mg / 15 mL) (500mg Mg2+/15
ml) |
15 ml |
[Oral absorption is
variable - 20 - 50% of an oral dose is
absorbed] [aggressive oral
supplementation can lead to diarrhea]
Cathartic dose: 80-160 meq Mg/day. |
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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
MAXIMUM INFUSION RATE: 1 gm over 7 minutes (150
mg/min)
(Requires ECG monitoring; cases involving potentially lethal
ventricular arrhythmias may require higher doses under close
medical supervision.)
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Magnesium
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)
- Pancreatitis
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.).
- Cardiovascular
- 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.
- Metabolic
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.
Magnesium Products
Products
Note: Oral magnesium
is not generally adequate for repletion in patients with
serum magnesium concentrations <1.5 mEq/L (1.2 meq/L)? |
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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. |
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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. |
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Magnesium gluconate:
Magonate® : Dietary supplement: Oral: 54-483 mg/day in
divided doses. Absorption: Oral: 15% to 30%. |
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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) |
|
IV |
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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 |
| Sulfate [MgSO4.7H2O] |
246.5 |
10 |
8.1 |
| Chloride [MgCl2.6H2O] |
203.2 |
12 |
9.8 |
| Oxide [MgO] |
40.3 |
60 |
49.6 |
| Hydroxide [Mg(OH)2] |
58.3 |
42 |
34 |
| Citrate |
451 |
16 |
13 |
| Gluconate [.2H2O] |
450 |
5.4 |
4.5 |
| Lactate |
202.5 |
12 |
9.8 |
| L-Aspartate HCl |
245.9 |
9.9 |
8.1 |
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(1) Flink EB. Magnesium deficiency. Etiology and clinical spectrum.
Acta Med Scand Suppl 1981;647:125-37
"The documentation of normal renal function is absolutely necessary
for maximum doses. The order of magnitude of dose is 1.0 meq Mg/kg on
day 1, and 0.3 to 0.5 mEq/kg per day for 3 to 5 days. In emergencies
such as convulsions or ventricular arrhythmias, a bolus injection of
1.0 gm (8.1 meq) of MgSO4 is indicated. Therapy of Mg deficiency in
the presence of renal insufficiency requires smaller doses and
frequent monitoring. Complete repletion occurs slowly."
(2) Abbott LG, Rude RK. Department of Endocrinology, LAC+USC Medical
Center, Los Angeles 90033. Clinical manifestations of magnesium
deficiency. Miner Electrolyte Metab 1993;19(4-5):314-22.
(3) Berkelhammer C, Bear RA. A clinical approach to common
electrolyte problems: 4. Hypomagnesemia. Can Med Assoc J 1985 Feb
15;132(4):360-8.
(4) DiPalma JR. Magnesium replacement therapy. Am Fam Physician 1990
Jul;42(1):173-6
(5) al-Ghamdi SM, Cameron EC, Sutton RA. Magnesium deficiency:
pathophysiologic and clinical overview. Am J Kidney Dis. 1995
Jun;25(6):973. //
Am J Kidney Dis 1994 Nov;24(5):737-52.
(6) Tso EL, Barish RA. Magnesium: clinical considerations. J Emerg
Med 1992 Nov-Dec; 10(6):735-45.
(7) Kingston ME, Al-Siba'i MB, Skooge WC. Clinical manifestations of
hypomagnesemia.
Crit Care Med 1986 Nov;14(11):950-4 .
(8) Whang R, Hampton EM, Whang DD. Magnesium homeostasis and clinical
disorders of magnesium deficiency. Ann Pharmacother 1994
Feb;28(2):220-6.
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