Neuromuscular Monitoring

Neuromuscular monitoring assesses the degree of paralysis induced by neuromuscular blocking agents (NMBAs) and guides safe reversal and extubation. Peripheral Nerve Stimulators (PNS) deliver electrical stimuli to a motor nerve (usually the ulnar or facial nerve), measuring muscle response.

Train-of-Four (TOF) & TOF Ratio (TOFR)

  • TOF = 4 stimuli at 2 Hz over 2 seconds.

  • TOFR = Ratio of 4th twitch to 1st twitch amplitude (T4/T1).

  • Interprets: Depth of blockade and readiness for reversal/extubation.

TOF Response

TOFR (%)

Interpretation

Clinical Utility

0 twitches

0

Profound blockade

Cannot reverse with neostigmine

1 twitch

<10

Deep blockade

Wait or consider sugammadex (4 mg/kg)

2 twitches

10–25

Moderate-deep blockade

Sugammadex possible (2 mg/kg); neostigmine not yet

3 twitches

~30–40

Moderate blockade

Neostigmine may work but incomplete reversal risk

4 twitches, TOFR <90%

40–89

Residual blockade

Sugammadex (2 mg/kg) or neostigmine if TOFR >40%

4 twitches, TOFR ≥90%

≥90

Minimal/no residual blockade

Safe for extubation; no reversal may be needed

Tetanic Stimulation

  • 50 Hz for 5 seconds.

  • Not used routinely; highly uncomfortable in awake patients.

  • Used to elicit post-tetanic facilitation in deep blockade.

Post-Tetanic Count (PTC)

  • Used when TOF = 0.

  • 50 Hz tetanus (5 sec) → pause 3 sec → single twitches every second.

  • PTC correlates with depth:

    • PTC = 1–2 → Profound block; expect prolonged recovery.

    • PTC >5 → May start seeing TOF twitches soon.

  • Sugammadex 4 mg/kg may be given if PTC ≥1.

    • Base sugammadex dosing on baseline PTC count (before any tetanus).

    • If PTC = 1–2, give 4 mg/kg.

    • If TOF is present, and ≥2 twitches, you can use 2 mg/kg.

    • If you mistakenly think the block is lighter due to PTF, you might underdose sugammadex → leading to incomplete reversal.

    • Neostigmine is not effective at this level.

Reversal Agents: Neostigmine vs Sugammadex

Neostigmine + Glycopyrrolate

  • Mechanism: Anticholinesterase → ↑ACh competes with NMBA.

  • Time to effect: ~7–10 minutes.

  • Co-administered with: Glycopyrrolate to block muscarinic side effects.

  • Effective if:

    • TOFR >0.4–0.5

    • At least 3–4 TOF twitches present

  • Not effective:

    • Deep or profound block (TOF <3)

    • When PTC <3

Sugammadex

  • Mechanism: Encapsulates aminosteroid NMBAs (e.g., rocuronium).

  • Onset: Rapid (~2–3 min).

  • Dose based on depth:

    • 2 mg/kg: TOFR ≥0.4 with 2–4 TOF twitches

    • 4 mg/kg: 1–2 PTC twitches (deep block)

    • 16 mg/kg: Immediate reversal (e.g. can't intubate/can't ventilate)

  • Advantages:

    • Effective in deep blockade

    • Predictable reversal

    • Fewer side effects than neostigmine

Clinical Summary

Block Depth

TOF/TOFR/PTC

Reversal Option

Notes

Profound

TOF = 0, PTC ≤2

Sugammadex 4–16 mg/kg

Neostigmine ineffective

Deep

TOF = 0, PTC ≥3

Sugammadex 4 mg/kg

Moderate

TOF = 2–3, TOFR <0.4

Sugammadex 2 mg/kg

Neostigmine risky

Light/residual

TOF = 4, TOFR >0.4

Neostigmine 50 µg/kg + glyco

TOFR ≥0.9 ideal for safe extubation

Full recovery

TOFR ≥0.9

None or neostigmine (if needed)

May not need reversal if stable airway

Pitfalls to Avoid

  • Reversing too early with neostigmine → inadequate recovery.

  • Assuming 4 twitches = safe extubation → always check TOFR ≥0.9.

  • Using neostigmine in deep block → ineffective and dangerous.

  • Not matching sugammadex dose to block depth.

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