Magnesium Sulphate (MgSO₄)

Magnesium sulphate is a **physiological Ca²⁺ antagonist** and **NMDA blocker** providing **anti-arrhythmic**, **anticonvulsant**, **vasodilator**, and **bronchodilator** effects. 

It treats **torsades de pointes**, **digoxin toxicity**, **eclampsia**, and **hypomagnesaemia**, and blunts catecholamine response post-CPB. 

Typical dose: **1–2 g IV over 10–15 min**, maintenance **0.5–1 g/h**; monitor reflexes, RR, and serum Mg, especially in renal impairment.

# 1) Receptor-level action → clinical effects

**Primary mechanisms:**

Magnesium is a **physiological calcium antagonist** and **membrane stabiliser**. It modulates several receptor systems:

| Target                                            | Effect                                                 | Clinical implication                                                   |

| ------------------------------------------------- | ------------------------------------------------------ | ---------------------------------------------------------------------- |

| **Voltage-gated Ca²⁺ channels**                   | ↓ Ca²⁺ influx into presynaptic and smooth-muscle cells | Vasodilation, anti-arrhythmic, tocolytic, anticonvulsant               |

| **NMDA receptor (non-competitive antagonist)**    | ↓ excitatory glutamate activity                        | Anticonvulsant, neuroprotection, analgesic adjunct                     |

| **Na⁺/K⁺-ATPase / Na⁺ channels**                  | Membrane stabilisation                                 | Anti-arrhythmic effect                                                 |

| **ACh release at neuromuscular junction**         | ↓ ACh release → reduced transmission                   | Potentiates neuromuscular blockade                                     |

| **Catecholamine release (adrenal + sympathetic)** | Inhibition                                             | Anti-tachyarrhythmic, ↓ stress response                                |

| **Myocardium**                                    | Mild negative inotropy                                 | Useful in tachyarrhythmias but may cause hypotension in fragile hearts |

**Summary of clinical effects**

* **Cardiovascular:** vasodilation (↓ SVR, ↓ PVR), ↓ catecholamine surge, stabilises myocardium, treats torsades de pointes and digoxin toxicity.

* **CNS:** anticonvulsant (e.g., eclampsia), mild sedation, neuroprotective.

* **Respiratory:** bronchodilation (asthma, status asthmaticus adjunct).

* **Neuromuscular:** potentiates relaxants; overdose → loss of reflexes, respiratory paralysis.

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# 2) Vial strength, preparation & basic pharmacokinetics (+ disadvantages)

**Formulations:**

* Common: **50% solution = 500 mg/mL** (each mL = 4 mmol Mg²⁺).

* Dilute in 0.9% saline or 5% dextrose for IV use.

**Pharmacokinetics:**

* **Onset (IV):** immediate.

* **Duration:** 30 min–2 h (depending on renal clearance).

* **Distribution:** extracellular and intracellular compartments (slow equilibration).

* **Elimination:** **renal** (95%).

* **Half-life:** ~4 h (normal kidneys), prolonged in renal impairment.

**Disadvantages:**

* Requires **IV titration**; narrow therapeutic-to-toxic window.

* **Hypotension, bradycardia, muscle weakness, respiratory depression** in overdose.

* **Accumulation** in renal failure.

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# 3) Practical dosing

## A. Anaesthesia / Cardiac practice

**Indications:**

* **Arrhythmias** (torsades de pointes, digoxin-induced VT/VF, refractory VF).

* **Attenuation of catecholamine stress** response to laryngoscopy or CPB.

* **Pulmonary hypertension** (↓ PVR).

* **Post-bypass atrial/ventricular arrhythmia prophylaxis.**

**Typical dosing:**

* **Bolus:** 1–2 g IV (≈ 8–16 mmol) over 10–15 min (dilute in 50 mL 0.9% NaCl).

* **Arrhythmia (torsades):** 2 g IV over 1–2 min (may repeat once in 5–15 min).

* **Infusion (maintenance):** 1–2 g/h (4–8 mmol/h) titrated to effect.

* **Max daily dose:** ~8–12 g in adults (unless in severe eclampsia protocols).

**Post-bypass example:** 2 g (8 mmol) bolus after declamping → reduces arrhythmias, blunts catecholamine surge, smooths extubation.

## B. ICU

**Indications:**

* Torsades de pointes, digoxin toxicity, eclampsia, severe asthma, hypomagnesaemia, refractory arrhythmias, pulmonary hypertension, tachyarrhythmia control in catecholamine excess.

* **Hypomagnesaemia replacement:**

  * Mild (serum 0.6–0.8 mmol/L): **2 g (8 mmol) IV over 1 h**.

  * Severe (<0.5 mmol/L): **4–8 g IV over 4–6 h**, then infusion 0.5–1 g/h.

  * Continuous infusion: **0.5–1 g/h (2–4 mmol/h)** to maintain serum 0.8–1.0 mmol/L.

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# 4) Special populations — dosing cautions

### Pregnancy

* Widely used in **eclampsia**: 4 g IV load (over 5–10 min), then 1 g/h infusion (Pritchard or Zuspan regimens).

* Crosses placenta → transient neonatal hypotonia possible; monitor reflexes in neonate.

### Lactation

* Excreted into breast milk in small quantities; short-term maternal therapy considered safe.

### Hepatic impairment

* Not hepatically metabolised → **no adjustment needed.**

### Renal impairment

* **Major concern:** risk of accumulation/toxicity → **reduce dose by 50%**; monitor **reflexes, RR, urine output, serum Mg²⁺**.

* Avoid continuous infusions unless under strict monitoring.

### Obesity

* Dose by **actual body weight** up to 100 kg; after that, consider adjusted body weight for maintenance infusions.

### Paediatrics

* **Arrhythmia:** 25–50 mg/kg (max 2 g) over 10–20 min.

* **Maintenance:** 10–20 mg/kg/h (max 2 g/h).

* **Bronchodilator adjunct:** 25–75 mg/kg over 20 min (max 2 g).

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# 5) Drug interactions (clinically key)

* **Neuromuscular blockers:** additive → **potentiates both depolarising (sux) and non-depolarising** relaxants → dose reduction needed.

* **Calcium channel blockers:** exaggerated **hypotension and heart block**.

* **Digoxin:** stabilises myocardium, but high Mg²⁺ may increase AV block.

* **Opioids / sedatives:** additive respiratory depression in overdose.

* **Loop/thiazide diuretics:** increase Mg loss → may require higher maintenance.

* **Lithium:** increased neurotoxicity risk.

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# 6) Significant complications & management

| Complication                           | Mechanism / Features                  | Management                                                                   |

| -------------------------------------- | ------------------------------------- | ---------------------------------------------------------------------------- |

| **Hypotension, flushing**              | Vasodilation from Ca²⁺ blockade       | Slow infusion; fluids; vasopressor if severe.                                |

| **Bradycardia / heart block**          | SA/AV nodal depression                | Stop infusion; calcium gluconate 10% 10 mL IV slow push; atropine if needed. |

| **Respiratory depression / arrest**    | Neuromuscular blockade                | Stop drug, ventilate, give **calcium gluconate 10% 10–20 mL IV**.            |

| **Loss of reflexes / muscle weakness** | Early sign of toxicity (Mg >4 mmol/L) | Stop Mg; IV calcium; monitor RR, reflexes.                                   |

| **Hypermagnesaemia**                   | Accumulation in renal impairment      | Supportive; **calcium**, diuretics, dialysis if severe.                      |

**Toxicity thresholds:**

* Loss of reflexes: >4 mmol/L

* Respiratory paralysis: >5 mmol/L

* Cardiac arrest: >6 mmol/L

**Antidote:** 

Calcium gluconate 10% 10 mL IV over 5 min** (can repeat).

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