muscle relaxants

Question 1

Define ED95 use in muscle relaxant.

Answer 1

-Dose required to produce 95% suppression of single twitch response measured at adductor pollicis.
(0.5)

• Use to compare potency among muscle relaxants. (0.25)

-Dose x2-3 ED95 required to produce optimal intubation conditions. (0.25)

Question 2

After an intubating dose of rocuronium, explain the determinant of onset and offset of neuromuscular block at the diaphragm, larynx, and adductor pollicis.

Answer 2

-ONSET (4marks): Larynx > Diaphragm > Adductor Pollicis (0,5)
-Onset of action depends on their ability to diffuse from circulation to NMJ (move down concentration gradient), competitive antagonism of nAChR at pre & post-synaptic receptors. (0.5)
-Speed of onset depends on:
* Muscle blood flow (0.5): High blood flow-> Delivery of Roc to effect site allowing more rapid equilibrium (Blood flow to diaphragm & larynx > than adductor pollicis) (0.5)
*Muscle size (0.5): Small fine muscles blocked before larger ones (Larynx > Diaphragm) (0.5)
-Muscle type (0.5): Slow ‘oxidative’ twitch fibres have less density of nAChR -> more quickly reach threshold receptor occupancy -> Faster onset
-Larynx & diaphragm ->fast ‘glycolytic’ twitch fibres with increase no of nAChR -> require more roc to be paralysed) (0.5)

-OFFSET (total 3marks): Diaphragm > Larynx > Adductor Pollicis (0.5)
-Offset of action depends on redistribution of roc from NMJ into plasma. (0.5)
-Speed of offset depends on:
* Muscle blood flow (0.5): Greater muscle BF -> faster redistribution of roc out of NMJ -> faster offset (0.5)
-Muscle type (0.5): Fast twitch muscles recover more quickly than slow as they have more nAChR (0.5)

Question 3

What are the clinical implications of these differences(onset and offset of neuromuscular block)?

Answer 3

-Onset and offset of neuromuscular blockade at diaphragm and larynx and adductor pollici vary.
-For intubation, monitoring of onset of muscle relaxation at larynx and diaphragm is difficult.
* Not reflected well by adductor pollicis muscle which has slowest onset.
* Monitoring of corrugator supercilli muscle more reflective of onset of muscle relaxant at diaphragm and larynx

-For extubation, the recovery for upper airway muscles (geniohyoid muscles) and diaphragm are more reflected by adductor pollicis muscle (slowest to offset). For administration of reversal for neuromuscular blockade and extubation should be guided by monitoring adductor pollicis muscle