Pharmacological and physiological interactions of muscle relaxants

Question 1

Volatile anaesthetics

Answer 1

Prolong blockade.

Depression of somatic reflexes in CNS (reducing transmitter release at the NMJ)

Question 2

Aminoglycosides (large intraperitoneal doses), polymyxins and tetracycline

Answer 2

Prolong blockade.

Decreased ACh release possibly by competition with Ca (which unpredictably reverses the block)

Question 3

Local anaesthetics

Answer 3

Variable effect on blockade.

Low doses of LA may enhance blockade by causing a degree of Na channel blockade

Question 4

Lithium

Answer 4

Prolonged blockade.

Na channel blockade.

Question 5

Diuretics

Answer 5

Variable effect of blockade.

Variable effect on cAMP. May have effects via serum K+

Question 6

Ca channel antagonists

Answer 6

Prolonged blockade.

Reduced Ca influx leading to reduced ACh release.

Question 7

Hypothermia

Answer 7

Prolonged blockade.

Reduced metabolism of muscle relaxant.

Question 8

Acidosis

Answer 8

Variable effect on blockade.

Prolonged in most but reduced for gallamine.
Tertiary amine group of dTC becomes pronated increasing its affinity for the ACh receptor

Question 9

Hypokalaemia

Answer 9

Variable effect.

Acute hypokalaemia increases (i.e. makes more negative) the resting membrane potential. Depolarizing agents are antagonised and NDMRs are potentiated.

Question 10

Hyperkalaemia

Answer 10

Variable.

Makes the resting membrane potential more positive. Depolarising relaxants are potentiated and NDMRs are antagonized.

Question 11

Hypermagnesaemia

Answer 11

Prolonged blockade.

Decreased ACh release by competition with Ca and stabilization of the post junctional membrane. When used at supranormal levels Mg can cause apnoea via the same mechanism.