Monitoring neuromuscular block

Following stimulation of the relevant nerve.

Nerve stimulators must generate a supramaximal stimulus (60–80 mA) to ensure that all the composite nerve fibres are depolarized.

Duration 0.1 ms.

Negative electrode directly over the nerve. Positive electrode  placed where it cannot affect the muscle in question.

Five main patterns of stimulation

Single twitch stimulation

Simplest form

Requires a baseline twitch height

Reduction in twitch height observed until 75% of NMJ receptors have been occupied by muscle relaxant 

Only a small number of receptors are required to generate a summated mini end-plate potential, which triggers an action potential

Partial NMJ block with depolarizing muscle relaxants (DMRs) and non-depolarizing muscle relaxants (NDMRs) reduce the height of single twitch stimulation.

Tetanic stimulation

Individual stimuli are applied at a frequency >30 Hz

Twitches observed in the muscle become fused into a sustained muscle contraction – tetany

Most stimulators deliver stimuli of 0.1 ms duration at a frequency of 50 Hz

Partial NDMR the tetanic stimulation fades with time

Partial DMR block reduces but does not exhibit fade

Post-tetanic potentiation and count

Following tetanic stimulation, subsequent twitches are seen to be large

Due to increased synthesis and mobilization of ACh and/or increased Ca 2+ in the synaptic terminal.

Stimuli at 1 Hz are started 3 seconds after a tetanic stimulation

Number of twitches is inversely related to the depth of block

Best used when the degree of receptor blockade is >95%,

Efects of tetanic stimulation may last for up to 6 minutes, may give a false impression of inadequate block to single twitch or train-of-four analysis.

Partial DMR block does not exhibit post-tetanic potentiation.

Train-of-four

Four 0.1 ms stimuli delivered at 2 Hz.

Ratio of the fourth twitch height to the first twitch height (T4:T1): TOF ratio

Number of twitches: TOF count

Does not require a baseline twitch height.

TOF ratio 

Partial non-depolarizing muscle relaxant blockade, that is, T 4< T1

Receptor occupancy above 70%- T 4 start to decrease in size. 

When T 4 has decreased by 25% - T 1 starts to decrease (75–80% receptor occupancy). 

T 4disappears when T 1is approximately 25% of its original height

TOF ratio is diicult to assess in practice.

TOF ratio in the presence of partial DMR block is 1.

TOF count

Receptor occupancy exceeds 90% - T 4 disappears

only T 1is present at 95% receptor occupancy

TOF count assesses the degree of deep NDMR block.

Double burst stimulation

Delivery of two bursts of stimulation separated by 0.75 s.

Three 0.2 ms stimuli separated by 20 ms  at a 50 Hz

Easy manual detection of small amounts of residual NMJ blockade

When magnitude  of the two stimuli are equal, clinically significant residual NMJ blockade does not exist

No more sensitive than TOF