Neuromuscular monitoring

[RDM6:p1552-p1555; CEACCP 2004 Vol 4(1) "Pharmacology of neuromuscular blocking drugs"; SH4:p216-219]

[To be added later...

What are the common sites for monitoring

Ulnar

Red (positive) lead is placed on the proximal point.

Principles of peripheral nerve stimulation

Response of a single muscle fibre
= All-or-none pattern
Response of a muscle
= Variable depending on the stimulation and the number of fibres activated
Reduction in the muscle response to a supramaximal stimulus is proportional to the degree of blockade
--> Supramaximal stimulus is usually at least 20-25% above what is neccessary for a maximal response

Normally ACh also act on prejunctional nAChRs
--> ACh mobilised within the prejunctional terminal for immediate release
--> Increase its own release during high frequency stimulation (>2Hz) (i.e. positive-feedback)
Non-depolarising NMBDs block the prejunctional nAChRs
--> Impairs mobilisation
Fade response to continuous stimulation (50-100Hz) or train-of-four stimulation (TOF) reflect antagonism at the presynaptic receptors

Supramaximal stimuli applied at frequencies raning from 0.1 - 1Hz
* 0.1Hz is usually used
Response to single-twitch stimulation depends on frequency
* At frequencies above 0.15Hz, the response will gradually decrease and settle at a lower level
* Results obtained using different frequencies are not comparable
The strength of response to single-twitch stimulation is the same for both depolarising and non-depolarising blockade (partial blockade)

Four supramaximal stimuli are given every 0.5 seconds (2Hz)
* If given repeatedly, usually each set (of TOF) are repeated every 10 - 20 seconds
TOF ratio (fade)
= Amplitude of the fourth response divided by amplitude of the first response
During a partial non-depolarising block, the ratio is inversely proportional to the degree of the blockade
* i.e. low ratio = higher degree of blockade
During a partial depolarising block, there is no fade
--> TOF ratio is approximately 1.0
No response if the blockade was complete (depolarising or non-depolarising)

Very rapid stimulation
* Usually 50Hz stimulation given for 5 seconds
During normal NMJ transmission or partial depolarising NMJ blockade
--> A sustained response (no fade)
During a partial non-depolarising NMJ block (or a phase II block)
--> Fade occurs

Very painful
May produce a lasting antagonism of NMJ blockade
--> The response of the tested muscle is no longer representative of other muscle groups

Post-tetanic facilitation of NMJ transmission refers to the increase in twitch tension after tetanic stimulation during a partial nondepolarising blockade
Caused by increase in mobilisation and synthesis of ACh after tetanic stimulation
Disappears after about 60 seconds of tetanic stimulation

Different from post-tetanic facilitation
A muscular phenomenon
* Not accompanied by an increase in the compound muscle action potential
Occurs before NMBDs were given

Tetanic stimulation (50Hz for 5 seconds)
--> Followed by single-twitch stimulation given at 1Hz starting 3 seconds afterwards
During very intense blockade, there will be no response to any stimulation
When the very intense blockade wears off, this method may elicit response before the first response to TOF occurs
Used when the NMJ blockade is still very strong and there is no response to TOF or single twitch stimulation
Due to possible antagonism of NMJ blockade, this should not be done more often than every 6 minutes

DBS consists of:
* Two bursts of tetanic stimulation, separated by 750 msec
* Each burst consist of 3 impulses (duration 0.2 msec) at 20 msec intervals (50Hz)
In normal NMJ transmission, the two responses are of equal strength
In partial blockade, the second response is weaker than first
DBS ratio correlates closely to TOF ratio
* But may be easier to evaluate manually

[??? reference]

TOF ratio of >0.9 --> Less risk of aspiration
Other indicators include:
* Grip strength
* Ability to sustain head lift
* Vital capacity measurement
* Generation of negative inspiratory pressure