Wed24November0743AM 3

A 75-year-old woman is scheduled surgery to a fractured neck of femur.

Induction of anaesthesia is achieved using 1 mg midazolam followed by 75 mg propofol.

Which one of the following options best describes the pharmacological interaction between these two drugs?

(Please select 1 option)

Summation

Potentiation

Additive

Synergism Correct

Infra-additive interaction

Explanation

These are examples of drug interactions:

Additive interaction (summation) (1 + 1 = 2)

The actions of combinations of intravenous agents such as ketamine and thiopentone or ketamine and midazolam are described as additive. They have different mechanisms of action. Ketamine is an NMDA receptor antagonist whereas thiopentone and midazolam are GABAA receptor agonists. Another example is nitrous oxide and halothane.

Synergism (supra-additive interaction) (1 + 1 >2)

Refers to the administration of two drugs with similar pharmacological properties and closely related sites of action that produces an effect in combination that is greater than would be expected from the summation of the contribution of each component. These can be interpreted and understood by construction of an isobologram. The hypnotic effect of benzodiazepines and intravenous induction agents such as propofol is the best example. Midazolam is often administered prior to propofol as part of a co-induction technique.

Potentiation

Volatile agents potentiate the effects of neuromuscular blocking agents in a dose dependent manner. Effects of the neuromuscular blocking agents can be increased by electrolyte disturbance (hypermagnesaemia), Penicillin and probenecid (the latter has no similar pharmacological activity).

Infra-additive interaction (antagonism) (1 + 1 <2)

This can be sub-classified as:

Pharmacokinetic interference, for example, one drug affecting the absorption of another by the gastrointestinal tract or influencing metabolism by hepatic microsomal enzyme induction

Chemical antagonism, for example, heparin and protamine or heavy metals and chelating agents

Competitive reversible, for example, opioids and naloxone, and irreversible antagonism of receptors

Answer Statistics

1

4%

2

16%

3

15%

4

65%

5

2%

Times answered: 259