Train of four (TOF)

four stimuli {T1-T4} are given at a frequency of 2 Hz (0.5 sec between the stimuli)

Each stimulus in the train causes the muscle to contract and the ‘fade’ in the response allow us to evaluate the neuro muscular blockade

The ratio T4:T1 ( i.e. Dividing the amplitude of the fourth response by the amplitude of the first response ) indicates the degree of neuromuscular block.

Non-depolarizing NMBAs produce a decrease in magnitude of the first twitch compared with a pre-relaxant stimulus, and a progressive reduction in magnitude of T1–T4.

The number of elicited twitches indicates the degree of receptor occupancy.

Disappearance of T4, T3, T2, T1 corresponds to 75%, 80%, 90% and 100% occupancy.

With recovery of neuromuscular function the twitches appear in the reverse order.

Accepted values for TOF count are:

1 twitch for tracheal intubation

1–2 twitches during established anaesthesia

3–4 twitches before reversal of neuromuscular blockade is attempted.

Double burst stimulation

Consists of two bursts ( the duration of each square wave impulse in the burst is 0.2 sec ) at 50 Hz with each triple burst separated by 750 ms.

DBS with 3 impulses in each of the two tetanic bursts is commonly used

These manifest visually as two separate stimuli (T1 and T2).

The ratio is related to the TOF ratio and is easier for the operator to interpret reliably.

Used under light paralysis where train of four ratio is difficult to distinguish

Post-tetanic Count (PTC)

PTC is used when there is no response to TOF stimuli and also when we want to eliminate sudden movements of the patient completely as during ophthalmic and neurosurgery

Uses tetanic stimulation at 50 Hz for 5 s to mobilize presynaptic ACh (to ‘kick start’ the nerve under deep paralysis) .

After a recovery time of 3 sec , it’s followed by 20 pulses at 1-2 Hz twitch stimulation

The number of twitches generated (i.e. the post-tetanic count) reflects the degree of neuromuscular blockade.

Shows fade response earlier than train of four

Used under deep paralysis to estimate time to recovery

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Reference: frca.uk Anesthesia Monitoring Techniques , Miller’s Anesthesia , 7/e