Peripheral Nerve Stimulator – Flashcards
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What are the two types of nerve stimulators?
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Electrical and magnetic
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Who invented TOF stimulation?
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Ali and Lee
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Advantages of using nerve stimulators (8 advantages)
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1) permit titration of drug dose to effect 2) determine onset of blockade 3) diagnose an unusual sensitivity to relaxants 4) avoidance of under or overdosing 5) determine if block is reversible 6) assess adequacy of recovery from NMB 7) locate nerves for regional anesthesia 8) determine extent of sensory blockade
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What is the greatest impediment to current flow? What can it be affected by (5 factors)
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Skin resistance is the greatest impediment Can be affected by: 1) moisture 2) thickness 3) hair 4) temperature 5) creams + lotions
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What is Ohm's Law
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Voltage = Current x Resistance V = I x R
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Current is measured in what unit?
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Amps
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What does voltage mean?
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Push or pressure behind current being used
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How much resistance does the nerve stimulator apply?
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18 ohm's of resistance
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How much current does the nerve stimulator apply?
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0.5 amps
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What type of battery does the nerve stimulator use? What does this mean for the current and resistance?
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9 volt battery Ohm's Law: V = I x R 9 volts = 0.5 x 18
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How do you obtain a maximal stimulus?
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> 20-25% needed for a maximal response - 50mA (current) over at least 1000 ohms resistance to get maximal stimulus
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What is the definition of frequency?
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Expressed in hertz (hz): 0.1 Hz = 1 stimulus every 10 seconds 1 Hz = 1 cycle per second 10 Hz = 10 stimuli every second
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What can happen if you are constantly testing a patient's TOF?
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Can increase blood supply to muscle, shunts blood away from other muscles, which will INCREASE the blood going to the muscle being tested
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What does the stimulus waveform look like?
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rectangle and monophasic
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What can occur if you have biphasic waveforms?
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- can produce repetitive stimulation - leads to underestimation of block
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What should duration of the TOF stimulus be? Why?
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0.2 milliseconds or less - If its longer, it may trigger a second action potential. - if > 0.5 milliseconds can stimulate muscle DIRECTLY, which can cause multiple firings and you can't tell if pt is blocked or not
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Response of muscle groups to neuromuscular blockade (from groups that need the LEAST amount of drugs to the groups that require the MOST amount of drugs to paralyze the muscle)
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1) Eye 2) Pharyngeal 3) Masseter 4) ADDuctor Pollicis 5) ABdominal rectus 6) Orbicularis oculi 7) Diaphragm 8) Vocal Cords
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How do paralytics affect the airway muscles compared to more peripheral muscle groups?
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For the airway muscles, the onset is quicker and duration of action shorter than peripheral muscles. - 1.5-2x more dose required for diaphragm than to block adductor pollicis
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What are the potential sites for nerve stimulation monitoring?
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1) ulnar nerve 2) facial nerve 3) median nerve 4) posterior tibial 5) common peroneal
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What muscle group does facial nerve stimulation affect?
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orbicularis oculi
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Describe placement of the nerve stimulator on the ulnar nerve
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1) placed on volar (palmar) side of wrist 2) place DISTAL electrode 1cm PROXIMAL to creases 3) place PROXIMAL electrode 2-7cm ABOVE DISTAL 4) the RED lead is the POSITIVE lead. The BLACK lead is NEUTRAL and should be put as close to nerve you are stimulating
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Describe placement of the nerve stimulating on the median nerve
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**this area is larger and less superficial than ulnar nerve 1) medial to electrode placement for ulnar nerve 2) adjacent to brachial artery at elbow 3) stimulating results in thumb adduction
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When monitoring recovery from paralysis, which site is optimal and why?
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Adductor pollicis is better to monitor recovery because the diaphragm recovers faster than the adductor pollicis; so if the adductor pollicis has recovered, then you KNOW that the DIAPHRAGM is recovered
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Is it possible for your patient to have no twitches and buck on the ventilator?
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Yes because the diaphragm recovers faster than the adductor pollicis
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Describe the placement of the nerve stimulator on the facial nerve
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three ways to place it: 1) negative electrode over nerve and positive electrode behind earlobe 2) electrode below lateral canthus of eye, 2nd electrode (black electrode) anterior to earlobe 3) negative electrode placed anterior to earlobe; positive electrode placed over lateral margin of opposite eyebrow
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Describe the placement of the nerve stimulator on the peroneal nerve
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- electrodes placed on lateral aspect of the knee - stimulation causes dorsi-flexion of the foot - used in peds
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Where can you place the nerve stimulator? 5 places
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1) Ulnar Nerve 2) Median Nerve 3) Facial Nerve 4) Peroneal Nerve 5) Posterior Tibial Nerve
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Describe the placement of the posterior tibial nerve stimulator
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- place electrodes behind medial malleous anterior to achielles tendon - positive electrode placed proximal to negative electrode - stimulation causes PLANTAR flexion of great toe
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What are some limitations of each nerve stimulation site?
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- peripheral nerve stimulation does not preclude possibility of diaphragmatic movement - posterior tibial - limited by neuropathy, PVD, foot deformity
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What occurs when the orbicularis oculi muscle is stimulated?
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closes and twitches eyelid and aids drainage and passage of tears
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What is accelerometry?
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- allows quantitative measurement of blockade - transduces acceleration into electrical potential - based on Newton's 2nd law (force = mass x acceleration)
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What is EMG?
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- records electrical activity of muscle in response to nerve stimulation - used intraop to eval integrity of peripheral nerves
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Patterns of nerve stimulation (5)
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1) single twitch 2) TOF 3) tetanic stimulation 4) post-tetanic stimulation 5) double-burst suppression
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Describe the single twitch stimulation pattern
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- fixed current - square wave stimulus - frequencies range from 0.1Hz to 1.0 Hz (more frequent stim dec. response and could lead to overestimation of blockade)
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Advantages of Single Twitch (3)
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1) establish supramaximal stimulus 2) identify optimum time for intubation 3) used in conjunction with PTC to monitor deep level of blockade
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Disadvantages of single twitch
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1) affected by change in temperature 2) cannot distinguish between depolarizing and nondepolarizing blcok 3) need control twitch prior to relaxant 4) full twitch height does not guarantee full recovery from block
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Describe the stimuli elicited by the TOF
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4 supramaximal stimuli given every 0.5 seconds
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What is used to determine the TOF ratio?
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the fade in the response is used to determine the TOF ratio - divide twitch height of 4th twitch by height of 1st twitch
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% blocked after disappearance of 4th twitch, 3rd twitch, 2nd twitch
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disappearance of: 4th twitch: 75% blocked 3rd twitch: 80% blocked 4th twitch: 90% blocked no twitches: 95% blocked
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How many twitches can you have and have adequate surgical relaxation?
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1-2 twitches
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Advantages of TOF stimulation (5)
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1) easily quantified without use of force transducer 2) more sensitive indicator of residual blockade than single twitch 3) no control twitch required 4) distinguishes between nondepolarizing and depolarizing blockade 5) detects phase II block
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Disadvantages of TOF stimulation (4)
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1) not useful to estimate deep level of blockade 2) not very sensitive in assess adequacy of reversal 3) difficult to determine value of TOF 0.9 which is required to ensure adequate recovery 4) skin temperature (dec. temp will dec. twitches)
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How many Hz are used for tetanic stimulation?
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anywhere from 30-100 Hz
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What do depolarizing and nondepolarizing block look like with tetanic stimulation?
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depolarizing - tetanic response is sustained and no PTF occurs nondepolarizing - tetanic response fades and PTF occurs
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Depend of fade with tetanic stimulation depends upon 2 things
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1) extent of neuromuscular blockade 2) frequency of applied stimulus - greater at higher frequencies - should not be repeated more often than every 2 minutes - 50 Hz most commonly used
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With tetanic stimulation is fade pre or post synaptic ?
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Pre-synaptic - larger amounts of Ach released with tetany - time needed to achieve equlibrium between synthesis and mobilization - normal muiscle able to sustain contraction as amount Ach released to greater than amount needed to stimulate post-synaptic receptors
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When is post tetanic facilitation seen?
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during non depolarizing block
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after tetanic stimulation, you can see an increase in twitch height with post tetanic facilitation. Why?
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the role of calcium --> calcium goes into nerve during stimulus but has a hard time coming out of cell, leading it to accumulate
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What is the BEST method to quantify intense degrees of nondepolarizing muscle blockade?
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Post-tetanic count stimulation - allows clinician to estimate recovery time of intense blockade - more twitches seen as block resolves - PTC of 2 - TOF comes back in 20-30 min - PTC of 5 - TOF comes back in 10-15 min
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How do you perform post-tetanic count stimulation?
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TOF then apply 50 Hz of tetany for 5 seconds, then observe response of single twitch given at 1 Hz 3 seconds after end of tetany
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If you have a PTC of 2 then 5, what does this mean?
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TOF will come back in 20-30....TOF will come back in 10-15 min
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Describe double burst stimulation
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two short bursts of 50 Hz stimuli, separated by 0.75 seconds - used to detect residual neuromuscular blockade - duration of each wave impulse is 0.2 milliseconds - each group of 3 stimuli produce ONE twitch
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Double burst stimulation is similar to what other type of stimulation and has what advantage?
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Similar to tetany, but is LESS PAINFUL
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Which method of stimulation has fade and tactile response superior to TOF?
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double burst suppression
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What will double burst suppression look like in the semi-paralyzed patient?
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the second response is weaker than the first
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Three phases of nondepolarizing neuromuscular blockade
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1) intense blockade - aka period of no response 2) surgical blockade 3) recovery
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Describe the intense blockade stage of neuromuscular blockade
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- occurs 3-6 min after injection of NDMB - AKA "period of no response" - length of this stage depends on drug used
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What is the most useful pattern to monitor during maintenance?
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TOF
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How many twitches provide sufficient relaxation for surgery?
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1-2 twitches out of 4
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Ease of reversal is _________ related to the depth of blockade at the time of reversal
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inversely
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The return of the ______ __________ indicates recovery phase
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4th twitch
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Recovery is governed by ______________ of muscle and rate of ____________ _______________ of drug
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sensitivity plasma elimination
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Which muscles recover first? Name 3
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diaphragm, laryngeal muscles, and orbicularis oculi recover first
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Which muscles are better to monitor during recovery? Why?
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Peripheral muscle - better indication of complete recovery --> lower chance of residual weakness of laryngeal muscles and diaphragm
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Which muscles should you NOT use during recovery?
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facial muscles --> resistance to NMB may overestimate degree of recovery
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What would you expect to see when your TOF < 0.4? (4 things) What does it MEAN when your TOF is < 0.4?
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no head lift decreased TV decreased VC decreased NIF means that 4th twitch height is 40% height of 1st twitch (significant fade)
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What would you expect to see when your TOF < 0.6? (5 things)
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3 second head lift opens eyes stick out tongue decreased VC decreased NIF
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What would you expect to see when your TOF < 0.7-75?
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Effective cough 5 sec head lift weak hand grip unable to sit unassisted
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What would you expect to see when your TOF < 0.8
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Normal VC Normal NIF may have diploplia May have facial weakness May have fatigue
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Why do you need a TOF of 0.9?
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guanrantees adequate recovery of neuromusucular function
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What is the more effective pattern of stimulation to estimate residual paralysis than TOF?
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double burst stimulation
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What are some adequate extubation criteria for the patient that is asleep?
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- adequate tidal volume - inspiratory force of at least 25 cm H2O negative pressure
