Magnesium Sulphate (MgSO₄)
Magnesium Physiology:
➧ Magnesium is the fourth most abundant cation in the body and is the second most abundant intracellular cation in the human body (potassium being the first).
➧ It has numerous physiological activities including activation of enzymes involved in energy metabolism, protein synthesis, regulation of vasomotor tone, neurotransmission, and signaling.
➧ It serves as a cofactor for more than 300 enzyme reactions that involve adenosine triphosphate (ATP). One of the magnesium-dependent enzyme systems is the membrane pump that generates the electrical gradient across cell membranes. As a result, magnesium plays an important role in the activity of electrically excitable tissues.
➧ Magnesium regulates the movement of calcium into smooth muscle cells, which gives it a pivotal role in the maintenance of cardiac contractile strength and peripheral vascular tone.
➧ Magnesium is required for the transformation of thiamine into thiamine pyrophosphate, so a magnesium deficiency can promote thiamine deficiency in the face of adequate thiamine intake. For this reason, the magnesium status should be monitored periodically in patients receiving daily thiamine supplements.
➧ Magnesium also has antinociceptive effects in animal and human pain models. Magnesium antinociceptive effects appear to be relevant not only to chronic pain but also, to the duration and intensity of postoperative pain. These effects are primarily based on physiological calcium antagonism, that is voltage-dependent regulation of calcium influx into the cell, and noncompetitive antagonism of NMDA receptors which produces a reduction of NMDA-induced currents.
Magnesium Balance:
➧ The average-size adult contains approximately 24 g (1 mole, or 2000 mEq) of magnesium.
➧ Half of the total body content of magnesium is found in bone.
➧ The muscle and liver are the soft tissues that contain the greatest amount of magnesium.
➧ Thirty percent of extracellular magnesium circulates bound to protein. Therefore, albumin concentration must be known to interpret total magnesium levels.
Corrected Serum Mg = Mg x 0.42 + 0.05 (4 - albumin in g/dL)
➧ Less than 1% of magnesium is located in plasma. This lack of representation in the plasma limits the value of the plasma magnesium concentration as an index of total body magnesium stores.
➧ This is particularly true in patients with magnesium deficiency, in whom serum magnesium levels can be normal in the face of total body magnesium depletion.
➧ The serum is favored over plasma for magnesium assays because the anticoagulant used for plasma samples can be contaminated with citrate or other anions that bind magnesium. The normal range for serum magnesium depends on the daily magnesium intake, which varies according to geographic region.
➧ Only 67% of the magnesium in plasma is in the ionized (active) form, and the remaining 33% is either bound to plasma proteins (19% of the total) or chelated with divalent anions such as phosphate and sulphate (14% of the total).
➧ The daily oral intake is 8–20 mmol (40% of which is absorbed). Magnesium absorption occurs throughout the small intestine and is enhanced by 1,25(OH)₂D₃. Regulation of magnesium balance is mainly by the kidneys. Like calcium, magnesium is reabsorbed in the kidney tubules.
➧ When magnesium intake is deficient, the kidneys conserve magnesium and urinary magnesium excretion falls to negligible levels and the serum magnesium remains in the normal range. This illustrates the relative value of urinary magnesium over serum magnesium levels in the detection of magnesium deficiency.
➧ The normal range for healthy adults residing in the United States is shown in Table 1:
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| Table 1: Magnesium Reference Ranges |
Magnesium Sulphate Preparations:
➧ The standard intravenous preparation is magnesium sulphate (MgSO₄). Each gram of magnesium sulphate has 8 mEq (4 mmol) of elemental magnesium.
➧ Saline solutions should be used as the diluent for magnesium sulphate. Ringer's solutions should not be used because the calcium in Ringer's solutions will counteract the actions of the infused magnesium.
Uses of Magnesium Sulphate:
1- Prevention of eclampsia in women with pre-eclampsia and it is also a recommended treatment for established eclampsia.
2- Used to treat atrial fibrillation, to achieve both rate control and reversion to sinus rhythm in many settings, including post-cardiac surgery, and in the emergency department.
3- Intravenous magnesium can suppress digitalis-toxic arrhythmias, even when serum magnesium levels are normal. Intravenous magnesium can also abolish refractory arrhythmias (i.e., unresponsive to traditional antiarrhythmic agents) in the absence of hypomagnesemia. This effect may be due to a membrane-stabilizing effect of magnesium that is unrelated to magnesium repletion.
4- It May be beneficial for patients with acute severe asthma either given intravenously or nebulized.
5- May also prevent delayed cerebral ischemia due to vasospasm in patients with subarachnoid hemorrhage.
6- Used in hypomagnesemia and hypomagnesemia associated with cardiac arrhythmias.
7- Magnesium sulphate when used systemically, has shown antinociceptive effects and has decreased postoperative opioid requirements. A limitation to the parenteral application of magnesium for modulation of antinociception via NMDA channel antagonism is insufficient blood-brain barrier penetration to achieve effective CSF concentrations.
8- Intrathecally administered magnesium has antinociceptive effects in animals. Intrathecal magnesium, an NMDA antagonist, has been demonstrated to prolong analgesia without notable adverse effects in healthy parturients and in various surgical procedures like lower limb surgeries and in patients undergoing total abdominal hysterectomy.
Contraindications:
➧ Hypocalcemia, heart block (risk of arrhythmias), and oliguria.
Interactions:
➧ Potentiates neuromuscular blockade (nondepolarizing/depolarizing).
➧ Potentiates CNS effects of anesthetics, hypnotics, and opioids.
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