Neuromuscular conduction Flashcards
Peripheral nerve stimulator
Function, components
Stimulates a major motor nerve. Muscle is indirectly stimulated
Components:
* Battery operated stimulator
* 2 surface ECG electrodes connected via leads to the nerve stimulator
* Ball electrodes connected to nerve stimulator casting can be directly applied to skin, or intradermal needle electrodes can be used to overcome high skin resistance
Use
* Silver/ silver chloride surface electrodes positioned on clean skin over peripheral or central nerves
* Negative (black) electrode positioned distally over superficial part of nerve. Positive (red) electrode placed more proximally to avoid direct muscle stimulation.
Locations for testing with peripheral nerve stimulator (5)
Nerve, location, movement observed
- Ulnar (wrist or elbow) -> thumb adduction and 5th digit movement. Note contraction of adductor pollicis is only stimulated by nerve transmision, not directly, because is a deep muscle. Movement of fingers may be caused by direct stimuation of forearm flexor muscles.
- Posterior tibial (posterior to medial malleolus) -> plantar flextion of big toe
- Peroneal (lateral to neck of fibula) -> foot dorsiflexion
- Facial (near earlobe where nerve emerges form stylomastoid foramen) -> orbicularis oculi contraction
- Mandibular (at the condyle and ramus angle)-> jaw closure
Peripheral nerve stimulator: characteristics of stimulus used
- Supramaximal stimulus (25-50% above that needed to produce a maximum response) used to ensure all motor fibres depolarize simultaneously
- Current of 15-40mA used for ulnar nerve (50-60 may be needed in the obese).
- Note whether a nerve depolarizes or not depends on size of current: so constant current more important than constant voltage. Placing the positive electrode proximally and negative electrode distally minimizes the current required (produces the greates neuromuscular response at a given current)
- Stimulus lasts under 0.2-0.3ms (>0.5ms can cause direct muscule stimulation or repetitive firing).
- Stimulus should be monophasic square wave to avoid repetitive nerve firing and muscle fatigue (fade)
Modes of stimulation for non-depolarizing NMBs (4)
- Single twitch
- ToF
- Double burst stimulation
- Tetanic stimulation
Single twitch stimulation
Stimulus used, pattern seen with NDNMB
- Short (0.1-0.2ms) stimulus delivered at 0.1-1Hz (i.e. one stimulus every 1-10 seconds)
- Size of twitch produced is compared to that evoked before muscle relaxation
- Gives crude indication of neuromuscular blockade
- Twitch magnitude starts to decrease with 75% of post synaptic ACh receptors are occupied. No twitch is seen when 100% of receptors are occupied
ToF
Stimulus used, patterns with increasing NDNMB
Four stimuli are delivered at 2Hz
ToF ratio = ratio of 4th to 1st twitch
Effect of NDNMB:
* Reduce size of T1 compared to pre-relaxation stimulus
* Progressive reduction in size of T1-T4 i.e. fade
* Twitches dissapear at following receptor occupancies: T4-75%, T3-80%, T2-90%, T1-100%
ToF: suitable results for intubation/maintenance/reversal with NDNMB
ToF ratio = ratio of 4th to 1st twitch
* T4/T1 ratio should be >70% for adequate respiration
Suitable ToF values:
* 1 twitch for tracheal intubation
* 1-2 twitches during established anaesthesia (one for upper abdominal surgery)
* 3-4 before routine reversal of neuromuscular blockade
Double burst stimulation
Stimlus used, advantages
Two bursts of three stimuli at 50Hz
Each burst separated by 750ms
Appear visually as two stimuli
Enables more accurate visual appreciation of fade than ToF
Tetanic stimulation
Stimulus used, uses following NDNMB
Stimulus of 50-100Hz applied for 5s
Uses:
* Detects any residual neuromuscular block i.e. low receptor occupancies: Fade can be seen even with a normal response to a twitch. No fade: no neuromuscular block
* Post tetanic count: tetanic stimulus can be followed by 1Hz twitch stimulation. Post tetanic count = number of twitches seen - indicates degree of neuromuscular blockade
* Post tetanic facilitation: can be used when no twitches are visible on ToF (i.e. profound neuromuscular block). Tetanic stimulus causes mobilization of presynaptic ACh
Reponse to peripheral nerve stimulation following depolarizing NMB
Single twitch, ToF, tetanic
Single twitch and ToF -> equal but reduced twitches (T4:T1 ratio =1)
Tetanic stimulation -> reduced but sustained contraction. NO tetanic fade. NO post tetanic stimulation
Assessing muscle contraction when using a peripheral nerve stimulator
5 methods
- Visual and tactile - easiest, least accurate
- Electromyography (EMG) - records a compound muscle action potential. e.g. when stimulating ulnar nerve, electrodes placed over addulctor policis.
- Acceleromyography: acceleration of the digit is inversely proportional to neuromuscular blockade
- Mechanomyography (MMG): small weight suspended from muscle to produce isometric contraction, tension produced on nerve stimulation is measured. Mostly used in research.
- Phonomyography (acoustic myography) - low frequency sound generated when skeletal muscle contracts recorded by a microphone
Overall: EMG is more specific than acceleromyography and mechanomyography.
However, underestimates non-depolarising blockade and overestimates depolarizing blockade compared with MMG
Sensitivity to NMB drugs of dirrerent muscle groups
Adductor pollicis, diaphragm, larynx, clinical implications
Sensitivity varies beween muscle groups
From most -> least sensitive:
* Adductor policis (one of most sensitive)
* Larynx
* Diaphragm (one of most resistant): requires 1.4-2x as much drug as adductor pollicis to achieve same degree of neuromuscular blockade. Onset of neuromuscular block and recovery is also quicker in the diaphragm
Therefore:
* Patient may cough when all responses in adductor pollicis have been abolished
* Recovery of the diaphragm can be assumed if adductor pollicis has recovered
Clinical signs of recovery from neuromuscular blockade
Clinically significant neuromuscular blockade can exist even if no fade is detectable on ToF or double burst stimulation –> should use clinical signs of recovery in addition to responses from nerve stimulator
Reliable tests of recovery:
* Sustained head or leg lift for 5s
* Sustained hand grip for 5s
* Maximum inspiratory pressure** over or equal to 40-50cmH20**
Unreliable tests of recovery:
* Sustained eye opening
* Tongue protrusion
* Normal vital capacity
Complications of neuromuscular conduction monitoring
(4)
Rare
- Burn injuries when diathermy used concurrently and after intermittent nerve stimulation with ball electrodes
- Ulnar nerve palsy if arm is poorly positioned and surface electrode causes pressure on the ulnar groove
- Thumb parasthesia follwoing use of thumb twitch force transducer
- Can interfere with function of permanent pacemakers
