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Train-of-Four

General Surgery · Final Professional

Definition

The train-of-four (TOF) is the commonest method of monitoring neuromuscular block with a peripheral nerve stimulator. Four supramaximal electrical stimuli are delivered to a peripheral nerve (e.g. The ulnar nerve) over 2 seconds, and the evoked muscle twitches are assessed.

Interpretation

The number of twitches reflects the depth of block (fewer = deeper; none = profound). With a non-depolarising block the twitches show fade (each weaker than the last), and the TOF ratio (fourth/first twitch) measures recovery — a ratio > 0.9 indicates adequate recovery for extubation. A depolarising phase I block shows reduced but equal twitches (no fade).

CLINICAL PEARL

Fade on TOF = non-depolarising block; a TOF ratio > 0.9 confirms adequate recovery. Twitch count gauges depth; fade gauges recovery.

Other Stimulation Patterns

Besides the train-of-four, the nerve stimulator can deliver other patterns suited to particular depths of block: a post-tetanic count is used when the block is so deep that there is no train-of-four response at all, applying a tetanic stimulus followed by single twitches to estimate how profound the block is; and double-burst stimulation, two short bursts, makes any residual fade easier to detect by palpation at the end of surgery. Selecting the appropriate pattern for the clinical situation is part of using the monitor well.

CLINICAL PEARL

Read the train-of-four two ways: the number of twitches tells you how deep the block is, and the fade (T4/T1 ratio) tells you how far it has recovered — aim above 0.9.

Note

Because clinical signs miss residual weakness, this objective ratio is what actually keeps the airway safe after extubation.

DANGER / REMEMBER

In everyday use the ulnar nerve at the wrist is stimulated and the thumb watched, and the anaesthetist palpates or measures the fourth response against the first to judge recovery before removing the tracheal tube from the patient.

A TOF ratio above 0.9 is required before safe extubation.

TOF count	Approximate block	Clinical state
0 twitches	Over 95% receptors blocked	Deep block
1–2 twitches	85–90%	Adequate surgical relaxation
3 twitches	80%	Block wearing off
4 with fade	70–75%	Reversal possible
TOF ratio above 0.9	Minimal	Safe for extubation

KEY POINT

Key points TO remember

TOF: 4 supramaximal stimuli to a peripheral nerve; observe twitches.
Twitch count = depth of block; fade = non-depolarising; no fade (equal, reduced) = depolarising phase I.
TOF ratio (T4/T1) tracks recovery; > 0.9 = adequate for extubation.
Objective monitoring detects residual block that clinical signs miss.

EXAM TIP

Sources: Morgan & Mikhail’s Clinical Anesthesiology; Miller’s Anesthesia; Ajay Yadav’s Short Textbook of Anaesthesia.

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