Neuromuscular Tranmission (NMT) monitoring Flashcards
Why do we monitor neuromuscular transmission?
- To know how much NMB is still on board
- Need to know this prior to extubating your patient
Action Potential
NM twitch creatse an aciton potential, which elicits a response in a nerve
Motor Neuron
NMJ
Where and how do NMBD work?
-Our reversals do not decrease the non depolarizing blocker at the junction. Instead we increase the amount of ACH at the NMJ
Presynaptic Nicotinic Receptors
- Facilitate the recruitment of ACH
- Normally, in times of repeated muscle stimulation (like exercise), presynaptic nicotinic receptors are recruited to relase more ACH
- Non-depolarizing drugs competitively block these receptors and inhibit the recruitment of ACH at NMJ (this is why you see fade on TOF with non depolarizing agents)
- Sux does NOT block Presynatpic receptors- more recruitment of ACH= NO fade on TOF
Neuromuscular Electrical Stimulation
- In the absence of a NMBD when you apply an electric stimulus to a nerve, it will stimualte the muscle and cause a muscle twitch
- Current is the determining factor as to whether a muscle will twitch or not!
- If the nerve is stimulated with enough current all of the muscle fibers will contract= maximal current ~ 30mA
- usually a current just above the maximal current is used to assure that all muscle fibers will fire ~ 50-60 mA (this hurts!)
- do a baseline test to determine what the supramaximal current is as it may be more for those with diabetes or edema, or if there is increased resistance such as hair or poor skin contact (most machines do this)
Visual or Tactile Monitoring
- is qualitative or subjective
- you could just stimulate a nerve and then look at or feel if hte muscle contracts
- NOT reliable to assess detection of residual blockade
- patient may still have up to 70-80% of their nicotinic receptors blocked and you may still feel or see a “strong” twitch
Quanitative NMT
-is objective and numerical
Twitch patterns: SIngle twitch
-single twitch: no longer of ANY clinical relevance
Twitch patterns: Double Burst
- two short bursts of stimuli separated by 0.75 miliseconds
- better able to “feel” a fade as the second response if weakre than the first response with residual blockade
- first twitch can be detected a deeper block levels than the first twitch on TOF
Twitch patterns: Sustained Tetany
- rapid repeated stimulus at 50-200 Hz, results in more Ach release
- you look for fade with a non depolarizing block
- with a depolarizing block the response will be weak but sustained
Twitch Patterns: Post Tetanic count
-deep muscular blockade
Train of Four (TOF)
- 4 single pulses of equal intervals at 0.5 sec intervals
- Should not be repeated more frequently than 10-12 seconds
- TOF count= how many twitches are present
- TOF ratio= compare the first twitch to the fourth twitch
- Assessment of the quantity of a residual non depolarizing block
*Have to have four twitches to get a percent
TOF Residual Blockade with NonDepolarizing
- residual NMB postoperatively has been known for more than 35 years, and occurs commonly despite reversal with neostigmine with a reported incidence of 4-50%
- studies prior to 2005 suggested residual neuromuscular block should be definied by a train-of-four ratio (TOFR) of <.7
- However, subsequent studies have discovered blockade can occur at TOFR >0.9, as per the review by Murphy and Brull in 2010
TOFR=1.0 is basline
TOFR=0.5, Patient is able to lift head off table for 5 sec
TOFR=0.8, swallowing impaired, increased aspiration risk, decrease hypoxic ventilatory response
Depolarizing Blockade TOF
- Succinylcholine: produces an initial depolarization of hte muscle end plate region (resulting in muscle fasiciculations), followed by a flaccid musle paralysis (within less than 1 minute after IV administration) due to depolarization-induced Na Channel inactivation
- Phase 1 block: No fade will be seen on TOF
- with continuous administration of high concentrations or in the presence of atypical cholinesterase this can eventually be followed by the development of receptor desensitization
-Phase 2 Block: Fade present on TOF (sux starts to work as competitive NMB)
Common TOF Guidelines
TOF 0.15-0.25: indicates adequate surgical relaxation
TOF >0.9 needed for safe extubation and recovery after surgery
What nerve to stimulate?
-Best nerve to use is the ulnar nerve
-stimulus nerve and the response of the adductor pollicis muslce of the thumb is assed for adduction
- easily located at the distal ulna
- low risk of mistaking direct muscle stimulus as nreve generated stimulus (as nerve is far from the site of muscle contraction)
- Nerve is medial side of the arm, thumb is lateral side of teh arm
Place negative electrode OR black on the distal patch
What nerve to stimulate- posterior tibial nerve
- May be used when hands or arms are not accessible
- Easily located along the region of the medial malleolus
- asess the response of the flexor hallucis brevis (big toe flexion)
Facial Nerve
- Cranial Nerve 7
- observe the corrugator supercilli muscle which causes the brow to furrow
- or observe the orbicularis occuli that encircles the eye and causes the lido to close
- NMB is similiar to the adducotr pollicus at the orbicularis occuli, but the supercilli muscle is much more resistant to NMDAs
- much less current is needed (use ~ 20mA)
Relative sensitivities
Which stimulus patterns work well to assess the quality of a deep motor blockade? (Quiz question)
- TOF
- Post tetanic count (PTC)
Accelomyography
- measures how fast the muscle moves
- better than visual or tactile
- thin piezeoelectric transducer is fixed to the digit, when the digit moves a voltage proportional to the acceleration of the movement is generated
- easy to use, better than tactile estimate of the twitch
-hands NEED to be immoblized
Kinemyography
- measures muscle movement
- flexible piezoelectric film sensor
- when teh material changes shape, the electrical charges change
- the flow of charges is directly proportional to the amoutn of distortion
- hand does NOT need to be immobilized
- can measure TOF and anesthesia machines have the capability to display the results on monitor
- can ONLY be used for ulnar nerve at the thumb
Electromyography
- measures electrical current (and AP)
- most sophisicated
- requires 3 electrodes over the insertions and/or the belly of the muscle
- used for research purposes
- susceptible to artifact
- more complex to use
*pulse ox will not interfere but OR cautery will
Use of NMTs
- connect device before inducation so you can get a baseline after the patient is asleep
- dry electrode site free of hair
- during maintenance you can use the NMT monitor to dose your muscle relaxants (if paralysis needed, keep TOF 1-2)
- you may see breathing with 1-2 as the diaphragm is the most resistance muscle
-at end of case stop giving relaxant and see fade decrease
- recovery of pharynglea muscles does not come until TOFR is >0.9 or 90%
- clinical criteria, sustained head lift, do not exclude significant paralysis
Clinical Tests corresponding to various TOFR:
TOFR <0.7
0.7 < TOFR < 0.9
TOFR > 0.9
TOFR <0.7= MIP >25 cmH2O, vital capacity > 15 ml/kg tidal volume, sustained eye opening, hand grip, tongue protrusion
0.7 < TOFR < 0.9= MIP > 50 cmH2O, head-life test, leg-lift test, tongue depressor test, handgrip
TOFR > 0.9= non clinical test available
