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NEW Written to fill a gap — nerve stimulation patterns, TOF, tetanus, post-tetanic count and double burst, in plain terms.

Neuromuscular monitoring

How we check how "paralysed" a patient is: the nerve stimulator, the five stimulation patterns and their frequencies, what the twitches tell you, and how to judge recovery.

The basics

We give a small electric shock to a nerve and watch the muscle it supplies. The weaker the twitch, the deeper the block.

The usual set-up

Nerve: the ulnar nerve at the wrist. Muscle: adductor pollicis — you watch or feel the thumb move in.
Current: supramaximal — a bit stronger than needed to fire every nerve fibre, so a weaker twitch can only mean the drug. Usually 50–60 mA.
Each pulse is very short: 0.2 ms (200 microseconds), square-shaped.
Electrodes: black (negative) one distal, red (positive) one about 3 cm closer to the elbow.

What "Hz" means — in one line

Hz = pulses per second. So divide 1 by the number to get the gap between pulses:
0.1 Hz = 1 pulse every 10 seconds
1 Hz = 1 pulse every second
2 Hz = 2 pulses a second = 1 every half second — this is TOF
50 Hz = 50 pulses a second — so fast the muscle cannot relax between them, so it stays contracted. That is tetanus.

The five patterns at a glance

Drawn to show the shape of each pattern, not to exact scale.

Pattern	Frequency	What the machine does	Used for
Single twitch	0.1 or 1 Hz	One pulse at a time	Watching onset. Needs a baseline twitch before the drug to compare against
Train-of-four (TOF)	2 Hz × 4	4 pulses, 0.5 s apart. Repeat no more than every 12–15 s	Routine monitoring and recovery — no baseline needed, because the 4 twitches are compared with each other
Tetanus	50 Hz for 5 s	A continuous run of pulses	Seeing if the contraction holds or fades. Painful — only in anaesthetised patients
Post-tetanic count (PTC)	50 Hz 5 s → wait 3 s → 1 Hz twitches	Tetanus, a pause, then up to 20 single twitches; count how many you see	Deep block, when TOF shows nothing. Repeat no more than every 6 min
Double burst (DBS)	Two mini-bursts at 50 Hz, 750 ms apart	Usually 3 pulses + 3 pulses (DBS 3,3)	Feeling fade by hand — two strong jerks are easier to compare than four small twitches. Repeat no more than every 15 s

Train-of-four in detail

Two numbers come from one TOF: the count (how many twitches) and the ratio (how big the 4th is compared with the 1st).

1 · TOF count — how deep is the block?

The twitches disappear from the 4th backwards as the block deepens, and come back 1st first on recovery.

Twitches seen	Receptors blocked (approx.)	What it means
4	under 75%	Light block or recovering — but there may still be fade
3	75–80%	Moderate block
2	80–90%	Good surgical relaxation
1	90–95%	Deep
0	over 95%	Deep or intense — use the post-tetanic count to see how deep

Key idea: the twitch looks normal until about 75% of receptors are blocked. There is a big safety margin, so "normal-looking" does not mean "drug-free".

2 · TOF ratio — is recovery complete?

TOF ratio = height of 4th twitch ÷ height of 1st twitch.
≥ 0.9 — adequate recovery; safe to extubate.
0.7–0.9 — the patient may breathe and lift their head, but the throat muscles are still weak → aspiration and airway obstruction risk.
Your fingers and eyes cannot detect fade once the ratio is above about 0.4. That is why a quantitative monitor (one that prints the number) is recommended.

3 · Using it for reversal

Neostigmine: give it once all 4 twitches are back — it cannot reverse a deep block.
Sugammadex (rocuronium or vecuronium): 2 mg/kg at TOF count 2 · 4 mg/kg at PTC 1–2 · 16 mg/kg for immediate reversal.

Fade — the pattern that tells you the type of block

Response	Non-depolarising (roc, vec, atracurium)	Depolarising — phase I (sux)	Phase II (too much sux)
TOF	Fade	No fade — all 4 equally reduced	Fade
Tetanus	Fades — contraction not sustained	Sustained (but weaker)	Fades
Post-tetanic facilitation	Yes — twitches bigger after tetanus	No	Yes
Effect of neostigmine	Reverses	Makes it worse	Unpredictable

Why it happens — simply

Fade: the nerve ending has its own nicotinic receptors that act as an accelerator, releasing more acetylcholine when stimulated fast. Non-depolarising drugs block this accelerator, so each pulse in a rapid series releases less acetylcholine → each twitch is smaller.
Suxamethonium does not block the accelerator in the same way, so there is no fade — every twitch is smaller by the same amount.
Post-tetanic facilitation: the 5 s of tetanus makes the nerve ending stockpile acetylcholine (and calcium). For a short while afterwards, each pulse releases extra → twitches you could not see before reappear. That is what the post-tetanic count uses.
Phase II block: after large or repeated doses of suxamethonium (roughly > 3–5 mg/kg, or an infusion), the block starts behaving like a non-depolarising one.

Post-tetanic count — reading it

Used only when the TOF count is 0.
PTC 0 = intense block — nothing will reverse it except high-dose sugammadex.
PTC 1–2 = deep block. Sugammadex 4 mg/kg works here.
The higher the count, the closer the first TOF twitch is to returning. With rocuronium, a PTC of about 8–10 usually means TOF twitches are about to come back.
Wait 6 minutes before repeating — the tetanus leaves a stockpile that falsely strengthens the next measurement.

Double burst (DBS) — reading it

Invented because fingers are bad at spotting fade in four small twitches. Two strong jerks are much easier to compare.

Version	What is delivered	Note
DBS 3,3 — the usual one	Burst of 3 pulses at 50 Hz → gap of 750 ms → another burst of 3 pulses at 50 Hz	Each burst feels like one single jerk
DBS 3,2	3 pulses, then 750 ms later only 2	Second jerk is naturally a little weaker — makes fade even easier to spot

Two equal jerks = no fade you can feel. Second jerk weaker = fade = residual block.
By hand, DBS picks up fade up to a TOF ratio of about 0.6 — better than TOF by hand (about 0.4).
But it still misses ratios between 0.6 and 0.9 — so "no fade on DBS" does not prove the patient is safe to extubate. Only a quantitative monitor showing ≥ 0.9 does.
Wait at least 15 s between DBS tests.

Recovery — from deep block to extubation

The block wears off in stages. Each stage is measured with a different pattern, and each has its own best reversal.

Sugammadex doses are for rocuronium or vecuronium. For atracurium and cisatracurium, wait for recovery and use neostigmine.

Residual block — why a ratio below 0.9 is dangerous

The throat muscles are weak → the patient cannot swallow properly → aspiration.
The upper airway flops closed → obstruction, especially in recovery.
The carotid bodies respond less to low oxygen → the patient does not breathe harder when hypoxic.
More postoperative chest complications, and a frightening feeling of weakness for the patient.

Bedside tests — useful, but not good enough on their own

5-second head lift, sustained hand grip, leg lift, biting on a tongue depressor against pull (the most sensitive of these).
Many patients pass these tests with a TOF ratio of only 0.5. They tell you recovery has started, not that it is complete.

Which muscles recover first

Diaphragm and larynx first — so a patient can breathe and cough while still partly paralysed.
Adductor pollicis next.
Pharyngeal and upper airway muscles last — the ones that protect the airway. A thumb TOF ratio of ≥ 0.9 is used because by then these have usually recovered too.

What makes a block last longer

Body state: hypothermia, acidosis, low potassium, low calcium, high magnesium, dehydration.
Drugs: volatile agents, aminoglycosides (gentamicin), clindamycin, magnesium sulphate, lithium, local anaesthetics, calcium channel blockers.
Organs: kidney or liver failure (rocuronium, vecuronium), old age.
Disease: myasthenia gravis (very sensitive to non-depolarisers), Lambert–Eaton.
Suxamethonium specifically: low or abnormal plasma cholinesterase — genetic, pregnancy, liver disease, neostigmine.

Which muscle you watch matters

Site	Nerve	Behaviour
Adductor pollicis (thumb)	Ulnar	The standard site. Relatively sensitive — so when the thumb has recovered, the airway muscles usually have too
Corrugator supercilii (eyebrow)	Facial	Behaves like the diaphragm and larynx: blocks faster and recovers sooner. Handy when the arms are tucked away, but can make recovery look better than it is
Diaphragm, larynx	—	Most resistant muscles — onset quick, recover first. That is why a patient may breathe or cough while the thumb is still weak

At the eye, avoid stimulating the muscle directly — it twitches even when the nerve is blocked, and falsely reassures you.

Types of monitor

Type	How it works	Note
Subjective (qualitative)	You look at or feel the thumb	Cannot detect fade above a ratio of about 0.4 — misses residual block
Acceleromyography	A sensor on the thumb measures its acceleration	The commonest quantitative monitor. The baseline ratio can read above 1, so aim for ≥ 0.9 of baseline
Electromyography (EMG)	Measures the muscle's electrical response	The thumb need not move freely — works with arms tucked
Mechanomyography	Measures the force of thumb contraction	Gold standard, but research only
Kinemyography	A bending sensor between thumb and finger	Quantitative, less common

Current AAGBI (2021) standards: use a quantitative monitor whenever a neuromuscular blocker is given, from induction until recovery is confirmed.

Quick recall

TOF 4 pulses at 2 Hz

0.5 s between TOF pulses

Tetanus 50 Hz for 5 s

PTC 50 Hz 5 s, 3 s, then 1 Hz

DBS 2 × 50 Hz bursts, 750 ms apart

Single 0.1–1 Hz

≥ 0.9 TOF ratio to extubate

0.4 fade you can feel

75% blocked before twitch falls

Sux no fade, no PTF

0.2 ms pulse, 50–60 mA

Ulnar → adductor pollicis