· Intro: rationale and methods
· Tests for non-depolarisers
· Tests for depolarisers
· Muscle location comparison
· Drug type comparison
|
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 |
|
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, 2Hz · ≥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! |
|
Single twitch |
· Needs prior calibration · Other methods: e.g. EMG |
|
|
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 |
|
|
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 |
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