2010A02 Describe the methods of determining depth of neuromuscular block and list the advantages and limitations of each.

 

List:

·      Intro: rationale and methods

·      Tests for non-depolarisers

·      Tests for depolarisers

·      Muscle location comparison

·      Drug type comparison

 

Intro:

Rationale

·   Residual neuromuscular blockade common in PACU

·   Risks: distress, injury, resp failure, aspiration

·   Hence nerve stimulator test essential for every relaxant GA

Accelero-myography

·   Negative electrode over nerve e.g. ulnar at wrist crease

·   Positive electrode away from muscle e.g. forearm

·   Accelerometer moves due to muscle contraction e.g. thumb

·   Twitch strength acceleration potential difference

·   Always supramaximal stimulus e.g. 60-80mA to ensure all motor units depolarized

·   Most common in anaesthesia

·   Ideal for train-of-four, post-tetanic count

·   Unable to monitor tetanus or double-burst stimulation

Other methods

·   Mechanomyography

·   Electromyography

·   Observation or palpation

·   Clinical examination

 

Test types: non-depolarising blockade

Single twitch

·   Single supramax stimulus

·   Magnitude measured by AMG or MMG

·   No ↓magnitude until 75% receptor occupancy

o Only a small number of receptors are required for summation of MEPP to AP

·   Pros: simple, better tolerated in awake patient.

·   Cons: baseline calibration needed. Requires AMG. Less sensitive than TOF.

Double burst

·   Two bursts separated by 0.75 second

·   Each burst: 3 x 0.2msec at 50Hz

·   Assess magnitude second : magnitude first

·   Pros: more sensitive for manual/visual detection of residual blockade; no baseline needed.

·   Cons: no more accurate than TOF when mechanically assessed; less accurate than TOF with AMG.

Train of four

(TOF)

·   Four supramaximal stimuli 0.2msec long, 4Hz

·   ≥10 seconds between trains

·   Use AMG.

·   TOF count: number of twitches 1-4

o 3 at 90% occupancy = reversal possible

o 0-1 at 95%

·   TOF ratio: height T4:T1

o 1.0 until 70% occupancy

o 0.5 at 80%

o e.g. TOF ratio ≥0.9 indicates adequate reversal

·   Pros: no baseline needed*

o But normal TOF ratio can be >1 due to facilitation!

o Hence should calibrate post-induction, pre-paralysis

·   Cons: manual/visual assessment inaccurate. Need monitor and accelerometer.

Tetany

·   e.g. 0.2msec 50Hz for 5 seconds

·   Fade blockade

·   Pros: most sensitive of all.

·   Cons: painful++ in awake patient. Quantitative assessment rarely available.

Post-tetanic count (PTC)

·   5 seconds 0.2msec 50Hz, 3 second gap, supramaximal stimuli at 1Hz

·   Number of twitches 1/depth

·   Use AMG.

·   PTC 9: TOF count 1

·   PTC 5: 10 minutes to TOF count 1

·   PTC 2: 20 minutes to TOF count 1

·   TOF count 1 ≈ PTC 9

·   Pros: assess blockade at profound depth, when TOF count = 0, occupancy >95%

·   Cons: post-tetanic facilitation for 6mins after tetany -> falsely strong

Clinical tests

·   e.g. head lift 5 seconds, poke out tongue, grip strength

·   Pro: no equipment, easy

·   Con: no guarantee safe TOF ratio!

 

Test types: depolarizing blockade

Single twitch

·   Needs prior calibration

·   Other methods: e.g. EMG

 

Muscle location comparison:

 

Diaphragm, larynx

Orbicularis oculi, pharynx

Adductor pollicis

Blood flow rate

Higher

Higher

Lower

ACh vesicle release

Higher

Lower

Lower

ACh receptor number

Higher

Lower

Lower

Rate of onset

Faster

Faster

Slower

Max depth

Lesser

Greater

Greater

Rate of recovery

Faster

Faster

Slower

 

Drug type comparison:

 

Partial depolarising phase 1

Depolarising phase 2

Partial non-depolarising

Single twitch height

TOF ratio

>0.7

<0.7

<0.7

Response to tetany

Sustained

Fade

Fade

Post-tetanic facilitation

N

Y

Y

Add depolarizing

Augment

Antagonise

Antagonise

Add non-depolarising

Antagonise

Augment

Augment

Add neostigmine

Augment

Antagonise

Antagonise

 

 

Feedback welcome at ketaminenightmares@gmail.com