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.
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)
