I1iii / 21A18 / 19B04 / 15B10 / 14B20: Outline the pharmacology of intravenously administered magnesium sulphate

21A18: Exam Report

Outline the pharmacology of intravenous magnesium sulphate.

74% of candidates passed this question.

Overall answers were well structured. However, a lack of detail and inaccurate pharmacokinetics was common. Better answers included a discussion of the mechanism of action of Mg++ including Ca++ antagonism, presynaptic cholinergic effects and NMDA receptor antagonism. Adverse effects were not discussed in detail by many candidates and  contraindications were commonly omitted.

19B04: Exam Report

Outline the pharmacology of intravenously administered magnesium sulphate.

57% of candidates passed this question.

The best answers appropriately addressed the pharmacology of magnesium sulphate, rather than diverting into physiology. They noted that the question concerned intravenous magnesium sulphate and did not discuss other routes. They included pharmaceutics, important examples of the wide-ranging indications, listed potential modes of action and considered the full range of body systems affected including potential adverse effects. Drug interactions, such as potentiation of neuromuscular blocking agents, and pharmacokinetics (including stating that magnesium is not metabolised) were described.

15B10: Exam Report

Describe the pharmacology of magnesium sulphate.

27% of candidates passed this question.

The standard “pharmacology template” approach would have served well to cover this question. Answers were generally lacking in detail and focussed on extraneous physiology rather than pharmacology. Toxicity and side-effects were important to emphasise, especially in the context of infusions for treatment of asthma and/or pre-eclampsia.

14B20: Exam Report

Describe the pharmacology of magnesium sulphate.

4% of candidates passed this question.

This was a repeat question and very poorly answered for such a commonly used agent. A structured approach to describing any drugs pharmacology was often not used.
Most answers were lacking depth and detail. The questions asked the pharmacology NOT physiology of magnesium sulphate. This was best answered with a standard template addressing:
Presentation, Uses, Main actions, Pharmacodynamics, Pharmacokinetics, Mode of Action, Toxicity, and any Special Points

I1iii / 21A18 / 19B04 / 15B10 / 14B20: Outline the pharmacology of intravenously administered magnesium sulphate

Magnesium Sulphate

Magnesium Sulphate

Chemical

Magnesium Sulphate

Use

  1. Electrolyte replacement
  2. Tx Eclampsia & Pre-Eclampsia
  3. TdP
  4. Severe Asthma
  5. Box jellyfish envenomation
  6. Irukandji syndrome

Presentation

Ampoule, 10mmol/5ml

Dose

10-20mmol infused at a rate of 10mmol/20mins

Route

IV/IO

Onset

Immediate

DoA

30mins

MoA

  1. Co-factor of enzyme systems
  2. NMDA receptor antagonism – anticonvulsant
  3. Reduces Ach release at NMJ – reducing striated muscle contraction
  4. Inhibits Ca++ influx through dihdropyridine sensitive voltage-gated channels – relaxing smooth muscle

PD

PK

A

N/A

D

25-30%

M

None

E

Excreted by kidney, proportional to GFR

Adverse Effects

Hypotension

ECG changes

Diarrhoea

Urinary retention

CNS depression

Loss of deep tendon reflexes

Respiratory depression

Contradictions

Renal impairment, heart block, myasthenia gravis