Vecuronium For Anaesthetists

What is a lyophilized powder?

Lyophilized ‘freeze dry’

With the ice being removed by sublimation (transition from Solid > Gas state without being a liquid) in a near vacuum.

Preserves drugs, and creates a stable powder that will last, and can be reconstituted at leisure.

Agno Pharma, have a pretty good webpage describing it : https://agnopharma.com/blog/lyophilization-of-pharmaceuticals-an-overview/

Intro To Neuromuscular monitoring

Neuromuscular monitoring is a cornerstone of safe anaesthetic care when neuromuscular blocking agents are used.

There are subjective and objective methods to assessing someone for neuromuscular blockade.

Subjective can be subdivided into clinical signs and the visual interpretation of muscle contraction amplitude when using a testing device.

Objective involves a testing device that also has a means of measuring the resultant movement, or muscle signals associated with the stimulus by a nerve.

This stimulus must be supramaximal, to depolarise the motor nerve. This is generally >60mA (don’t forget that this is quite painful, attach the twitcher to yourself and turn it right down, and incrementally zap yourself, 20mA is spicy, it is not a competition…. but maybe it could be.)

Testing devices often provide three core tests.

TOF – Train of four, 4 maximal amplitude stimuli 0.5 seconds apart – | | | |

DBS – Double Burst stimulus, a pair of signals 750ms apart – | |

PTC – Post Tetanic Count – Constant electrical stimulus is delivered for five seconds, followed by a stimulus every second until you stop – }{}{}{}’ZAP‘{}{}{}{ |. |. |. |. |. |. |. |. |. |. |. |. |. |

PTC can be used to explore deep neuromuscular blockade, as the tetany liberates more acetylcholine into the NMJ, which will slowly fade into the abyss and you will see fade across your twitches

Train of Four

Commonly used, the consistent spacing and the length of time the test is over improves operator perception of fade.

Fade, the diminished amplitude of contraction over repeat contraction due to insufficient acetylcholine activity in the NMJ. ACh would normally stimulate pre-junctional nicotinic receptors at the motor nerve, near the terminal bouton. This stimulus encourages it to mobilise ACh vesicles towards the nerve ending ready to deploy if another contractile stimulus is received. Antagonised by Nicotinic AChRs! You will not see fade with suxamethonium, just diminished contractions with every zap of the same amplitude.

 

The goal post at which using neostigmine/glycopyrolate to reverse the residual effects of blockade has repeatedly moved over the decades, now you need to see four twitches, with fade before it being acceptable to reverse the patient, once upon it was fair game to only see three! (CHECK THIS!)

4 Twitches with fade = ~70%!! receptor occupancy

3 Twitches = 80%

No twitches = >90% of all nicotinic receptors blocked

Target TOF RATIO of >0.9 pre extubation is the goal.That is, the amplitude of the last twitch is >90% of the first twitch. You get there by waiting, sugammadex or neostigmine. But you dont want to be in a situation where the neostigmine wears off and ‘re-curarisation’ occurs as the rocuronium that was outcompeted by the increased concentration of acetylcholine start to turn the tide of battle (once more unto the breach! – Rocuronium V – Shakespeare) As such trying to get the neostigmine in a bit early, may need a double dose, but it is recommended to wait and use a bit less NMB agent next time.

Don’t Fire off a TOF see a little and then immediately fire off another TOF that will give you false reassurance, as you’ve juiced up that NMJ with acetylcholine, you need to give a reasonable gap of 10+ seconds.

Subjective clinical Signs

Signs of Recovery:

Sustained head raise from pillow

Sustained arm raise above head

Tidal volumes >500mls?

?Strong sustained hand grip?

Signs of Non-recovery:

Beyond obvious paralysis doctor…

Jerky, twitched movement that is unsustained, hoarse/non voice if extubated

Respiratory distress in recovery areas

Subjective approaches especially of the clinical sign variety have large interoperator variability, and lack sensitivity. especially when we consier that some muscle units recover faster from blockade than others (diaphragm fast, geniohyoid as a surrogate of airway tone, slower recovery.)

Objective methods

Need to be calibrated pre paralysis (so you anaesthetise, ensure deep enough that wont recall the pain of being zapped with a stimulus) and then paralyse.

Electromyography (EMG) Action potential of muscle detected

Acceleromyography (AMG) Accelerometer measure how much a thumb or toe swings

Kinesiomyography (KMG) – a bend sensor between thumb and hand

Mechanomyography (MMG) (mostly in research land)

Where to stick it?

You need a motor nerve near the body surface which operates a muscle that you can see the action of easily.

Common Choices

Ulnar nerve, will trigger thumb abduction, but also all hand interossei, and the lumbrical muscles of small and ring fingers.

Facial Nerve – you will probably catch temporal or zygomatic nerves, depending on where you put it. They recover from block earlier than diaphragm, and earlier than the ulnar nerve.

Posterior Tibial / Common peroneal (handy if the surgeons are fiddling with a head/chest and you cant get at the former sites.

Which way around do the stimulator electrodes go, and why?

The electrodes are placed with black (negative) distal this delivers energy more effectively!

‘red closest to the heart’

References

• The ‘New’ Relaxants is an excellent & informative paper – Torda TA. The ‘New’ Relaxants. A Review of the Clinical Pharmacology of Atracurium and Vecuronium. Anaesthesia and Intensive Care. 1987;15(1):72-82. doi:10.1177/0310057X8701500110
• Neuromuscular block management: evidence-based principles and practice Rodney, G. et al. BJA Education, Volume 24, Issue 1, 13 – 22