Neuromuscular blockade monitoring.. Flashcards
how can we assess the degree of NMB present in a patient?
Qualitatively
- It may become apparent during anaesthesia that NMB is wearing off
- e.g. fighting ventilator, surgeons complaining of abdominal muscle movements.
- although this is a good assessment during anaesthesia, it is not a safe measure when extubating
Quantitatively
- nerve stimulation to cause muscle contraction indirectly.
- the muscle contraction can be measured via a number of methods - visual, tactile, mechanomyography, acceleromyography and electromyography.
what is a nerve stimulator?
a device that produces direct current of specified amplitude and frequency and duration to depolarise a peripheral nerve via contact with electrodes.
depolarisation results in an action potential and contraction of the muscle.
consists of
- a power source
- constant current generator
- oscillator - adjusts current produced (freq, amplitude, duration) based on users selection
- display/ control units
- electrodes - needle or skin electrodes - anode and cathode.
when are nerve stimulators used by anaesthetists?
checking for NMB - skin electrodes
localisation of nerves in regional anaesthesia - needle electrodes
(can also be used in diagnosis of neuromuscular diseases, by surgeons to localise nerves)
what factors determine whether a nerve is stimulated?
amplitude - usually supramaximal stimulus is used. usually around 15-30mA. can be larger in obesity.
duration - usually short stimulus is used as these will activate larger motor neurons with less effects of smaller C fibres. usually 0.2-0.3ms
frequency - depends on mode of test e.g. TOF 2 Hz, PTC 50Hz
proximity - the more direct the current is in contact with the neuron the better the response as less current lost through impedance of tissues.
electrode position - cathode (negative electrode) will depolarise the nerve due to creating negative surrounding (hence potential becomes less negative). This is placed more proximal along the course to promote AP transmission.
what is the difference between Rheobase and Chronaxie
rheobase = the minimum amount of current needed to cause an action potential at unlimited duration (I.e constant current)
chronaxie = the minumum duration needed at 2x rheobase to create an AP
which nerves are commonly stimulated in NMB monitoring?
ulnar = adductor pollicis brevis = thumb adduction
facial nerve = black electrode near tragus of ear over temporal brach. then can place red either on masseter or forehead = obicularis occuli or masseter contraction
posterior tibial = behind medial malleolus. big toe, plantar flexion
peroneal = at neck of fibula - foot dorsiflexion
what is meant by a supramaximal stimulus?
current amplitude that is 25% more than the current needed to stimulate all neurons within an axon to produce maximal muscle contraction.
this is used in NMB monitoring so that the size of stimulus is not confounding the response given. hence if max stimulus is applied, any weakened response can be down to NMB agents.
why is it important to place black electrode as close to the wrist as possible?
prevent other ulnar branches being stimulated
e.g. finger movement
what skin eelctrodes are used?
silver/ silver chloride - same as ECG
how would the energy being delivered to the nerve be calculated?
energy (joules) = current^2 x resistance x time
( energy = power x time )
Power = I x V
Therefore
Energy = I x Vx T
V= IR
So E= I^2 xRx T
describe the different ways muscle response can be recorded?
visual / tactile - can visually count number or twiches in TOF or PTC. Easiest and cheapest method however less accurate especially with TOF ratio as hard to compare 1st and last.
accelaromyography - main method used. measures the accelaration of a fixed mass attached to patients thumb/ muscle. more accurate than visual but more convenient than other methods.
electromyography - record the compound AP in muscle via separate electrodes. this is very accurate but too sensitive, v small movements picked up (no longer used)
mechanomyography - small weight hung from muscle to maintain isometric contraction. strain gauge measures tension generated in muscle. more accurate, used in research, not practical for anaesthesia.
what are the advantages and disadvantages of nerve stimulators?
CONS
what are the different modes used for NMB testing?
single twitch - 50mA, 1 hz, each twitch 0.2ms. no fade because time to recover in between. simple way of looking at if there is or not NMB e.g. after succinylcholine before atracurium
TOF - 50mA, 2Hz, each twitch 0.2ms. fade seen in non depolarising. can compare 1st and last twitch height to give a TOF ratio
PTC - 50mA, 50hz for 5 second, then a pause (3sec) and single twitches delivered. tetanic stimulation potentiates contraction as releases ACh into cleft. count the number of twitches seen after
Double burst stimulation = 2 sets of 3 50mA 50Hz for 0.2ms and gap of 750ms in between. easier to assess fade between them by visual/ tactile methods as only comparing 2.
tell me more about the TOF ? how is it delivered and what is measured?
Train of four is a method for assessing degree of NMB
it consists of delivery of 4x stimulus each of 50mA and 0.2ms duration at a frequency of 2Hz
the number of twitches seen and the presence of fade and the TOF ratio can give infomation on the degree and type of blockade.
e.g.
4 twitch = 70% or less receptors occupied
3 twitches = 75%
2 twiches = 80%
1 twich = 90%
0 twitches = near 100
the twitch height also can be used. reduced height in depolarising agents but no fade. in non depolarising or phase 2 block . Fade is seen - this is where 1st twitch is taller than 2nd and 3rd and 4th is smallest.
from this the TOF ratio can be calculated by the 1st and 4th twitch height ratio. this gives another assessment of degree of reversal and can help guide management.
what is the single twitch method?
50mA stimulus delivered as single twitches each lasting 0.2ms and at a frequency of 1Hz
no fade is seen as time to recover in between
can be useful for assessing presence/absence of block e.g. after sux use.
what is the double burst stimulation?
A method of assessing NMB via delivering 2 bursts of stimulus
each block is 50mA, 50Hz each of 0.2ms.
separated by 750ms pause
can demonstrate fade so can be used to assess fade visually more accurately as easier to compare 2 stimuli back to back than 4.
what is the PTC?
PTC is used to assess deep blocks.
Often when TOF =0 a clinician may want to use PTC to see how deep the block is
it works via tetanic potentiation - tetanic stimulation causes release and build up of ACh in the cleft.
tetanic stimulus is delivered - 50mA, 50Hz, 5 second. then a 3s pause followed by single twitches.
the deeper the blocker, the fewer twitches post tetanic stimulation.
TOF =1 is roughly PTC of 12
PTC <5 = profound block.
what is fade?
A phenomena seen in non-depolarising blockade or depolarising blockade.
Due to blocking pre-synaptic Ach receptors preventing the movement of Ach vesicles in the reserve pool to the release pool.
hence the nerve tires quickly and shows fade- the reduction in force of contraction with repeated stimulus.
not seen with depolarising agents unless repeated use and phase 2 block characteristics are present.
what is an acceptable TOF for intubation/ extubation?
intubation - 1 twitch
extubation TOF >0.9
neostigmine - when TOF count is 3
how are nerve stimulators used in regional anaesthesia?
the needle used to inject local anaesthetic is a specialised needle which acts as a cathode. only the tip is able to produce current to help in accuracy. the anode is connected to dry clean skin to complete the circuit.
initially current of 1-3mA is used at a frequency of 2Hz where each stimulus has a 0.2ms duration.
As the needle approaches a nerve, muscle contraction will be seen, the user then turns down the current and continues to get closer to the nerve until muscle contractions are seen between 0.3-0.5 mA. this suggests very close to the nerve however not intraneural.
USS guidance is also used to improve accuracy.
what might indicate you are injecting local into a nerve during regional?
high resistance to injection
muscle twitches as 0.2mA or less suggests too close
what are the pros and cons of using nerve stimulation in regional?
pros:
- improves accuracy - thus effectiveness and safety.
- new needles have echogenic coating to be visualised under USS
-
cons:
- still a risk of neural injury
- unconfortable in awake patient
- disconnection may result in no twitches and user thinking they are not near the nerve - however usually disconnection alarms now.
- subjective interpretation of twitches.
- risk of burns, arrhythmias and pacemaker dysfunction. (however low current used so unlikely)
why is a frequency of 2hz used in regional nerve stimulation?
dont want constant current as this will cause fade/ tiring of the muscle
but want the frequency to be high enough so that you dont pass the nerve during needling
why is current used for regional anaesthesia much lower than NMB monitoring?
closer to nerve,
less impedenance
hence current reaching the nerve will be more than if 3mA used at the surface
what are the properties of an ideal nerve stimulator?
produces a constant current (not voltage) as a square waveform for 0.2ms
has the ability to function as TOF, PTC, DBS, single twitch with preset timings for these
user can adjust the magnitude of current and mode
able to produce a supramaximal stimulus
indicated polarity - i.e. anode and cathode clearly labelled
(from RCOA)
why is Neuromuscular monitoring useful in surgery?
monitor depth of the block - some surgerys it is crucial for blockade e.g. neurosurgery , other times it may just need to be optimised
monitor reversal / for safe extubation
may need to ensure it is worn off for the surgical proceedure e..g parotid surgery may need to test fascial nerve so need to ensure there is no NMB
it is particularly useful in liver/renal disease or neuromuscular disease where the offset / metabolism is less predictable
in PTC and DBS a frequency of 50Hz is used, what does this mean in terms of timing.
during the tetanic period a 50Hz stimulus is applied. each stimulus lasts 0.2ms and they are delivered at a frequency of 50 every second which is every 20ms
in a DBS 3x 0.2ms stimuli are delivery each 20ms apart (i.e. equivalent to 50Hz)
what common physics law does the acceleromyograph use?
newtons 2nd law
f=ma
mass is constant therefore accelaration is proportional to force
newer devices can look at accelaration in multiple axis
how is a nerve stimulator calibrated?
should be calibrated after induction before NMBA are given
what neuromuscular blockade monitoring devices do you know ?
The TOF watch - simple batery powered twitcher
the NMT monitor which can be connected to monitoring. These mostly uses a mechanosensor via a piezoelectric sensor
describe clinical assessment of neuromuscular blockade?
sustained head lift for 5 seconds
ability to generate tidal volume of 10ml/kg
sustained hand grip
ability to protrude tongue
however not very reliable and may be positive when still 50% of receptors are blocked
when may PTC be useful?
to see how far away from TOF 1 i.e. how long until you can use neostigmine
for surgery where a profound block is needed e.g. neuro
what is post operative residual curarisation?
residual paralysis post anaesthesia where NMBA have been used
can result in aspiration risk, airway obstruction, poor ventilation leading to hypoxia.
name 2 factors which determine the likelihood of a nerve to propagate an AP when stimulated by a nerve stimulator?
The amplitude of current reaching the nerve
the duration of current applied
