Case 23 - Monitoring at NMJ Flashcards
Why is monitoring of neuromusclar blockade important whenever a NMBD is given?
NMBDs are used to facilitate tracheal intubation and surgical exposure.
Monitoring neuromuscular blockade allows:
1) titration of NMBD doses to the desired effect.
2) Adequate relaxation can be achieved without administering unnecessary doses of NMBD.
3) Recovery from relaxation becomes more predictable.
4) monitoring helps ensure adequate recovery and therby prevent respiratory failure after extubation
What are the basic principles of neuromuscular blockade monitoring?
Monitoring of neuromuscular tranmission requires supramaximal stimulation of a motor nerve and measurement of the respone evoked in innervated muscule.
NMBDs have NO direct effect on muscle –> direct stimulation of a muscle results in contracture despite complete blockade at NMJ.
Goal: Stimulate a motor nerve in a location where only indirect evoked potential can be elicited (via NMJ activation)
What is a site of neuromusuclar blockade monitoring?
Two common sites to monitor neuromuscular blockade: at the ulnar nerve and facial nerve.
Ulnar nerve - stimulation causes adduction of thumb by adductor pollicis brevis muscle. May cause direct stimulation (finger and wrist flexion, which you should ignore).
Facial Nerve - stimulation causes eyebrow contraction via indirect activation of obicularis occuli muscle.
Which site is better to monitor, facial nerve or ulnar nerve?
Ulnar nerve – adductor pollicis brevis muscle - is more appropraite to monitor as muscle twitch recovery from this site ensures that the larynx and diaphragm has recovered.
Facial Nerve - obicularis oculi - more resmenbles diaphragm and laryngeal neuromuscular blockade; however, reversing NMBD using TOF at this location does not ensure that upper airway muscles/pharyngeal muscles has recovered, thus risking post-op failure after extubation.
Monitoring NMBD at these locations act as a surrogate monitoring neuromuscular blockade at the muscles of respiration and airway patency (diaphragm, laryngeal muscles)
Why is a supramaximal stimulus used for neuromuscular blockade monitoring?
motor nerves consist of numerous nerve fibers, each innervate a motor unit in muscle. To asses muscule’s total respone, you need to stimulate all the nerve fibers.
Providing supramaximal stimulus ensures that all nerve fibers are stimulated, thereby allowing one to conclude that a decrement in muscle contraction is due to neuromuscular junction blockade and NOT DUE to failure to stimulate all the fibers in the motor nerve.
What is maximal stimulus?
In a normal unparalyzed patient, providing increasing current over a peripheral nerve, such as the ulnar nerve, will result in increased intensity of muscle contraction until a point where further increases in stimulus does not change the force of muscle contraction. At this point, all nerve fibers are activated.
Describe Peripheral Nerve Stimulators
peripheral nerve stimuators deliver a constant current with square wave pulse of 0.2 msec in duration. This pulse width stimulates motor nerves (sensory nerves require a longer pulse duration (1 msec) ).
Provies a subjective measurement based on observing or tactile feel of muscle contraction with obvious considerable interobserver variability.
What is Electromyography (EMG)?
EMG records compound muscle action potential in response to peripheral motor nerve stimulation.
Stimulating electrodes placed over a peripheral motor nerve, sensing electrodes placed over innervated muscle.
Sensing electrodes measure magnitude of evoked response after stimulation.
Can be used to measure neuromuscular blockade. Of note, EMG response must first be calibrated to baseline which is unparalyzed patient (this calibration number is considered 100% in an unparalyzed patient)
What is Accelerography
Accelerography is based on principle that Force is product of mass times acceleration (F = m * a)
if mass is constant, then force is proportional to acceleration.
transducer is attached to patients thumb, which measures the angular acceleration of the muscule contraction in response to evoked stimulus.
Provides a simple and objective method of asessing neuromuscular blockade.
What is Single Twitch Height?
a single supramaximal stimulus is appled for 0.2 msec, and the magnitude of the twitch is measured and compared to baseline.
Twitch height can remain at baseline until 75% of neuromuscular junction receptors are occupied by NMBD
Twitch heigh completely disappears when 90-95% NMBD blockade at receptors occur
What is the single twitch height after depolarizing vs NMBD blockade
Depoloarzing blockade - twitch height is uniformaly smaller and increases with recover
NMBD - twitch height progressively decreases (fade)
*return to control twitch height after NMBD does not mean 100% of neuromuscular receptors are unoccupied; recall 75% of receptors could still be blocked with NMBD and have a normal twitch height*
What is Tetanic Stimulation?
stimulation applied at frequency of 50Hz (ie 50 stimuli/sec) for 5 sec.
Unparalyzed patients - sustained muscle contraction with force greater than that achieved by a single stimulus
Depolarzing (Sux) - sustained contraction of smaller force (no fade in response)
NMBD - non-sustained contraction response, ie FADE
What is Train of Four (TOF)?
TOF - pattern of stimulation of a motor nerve that involves four successive supramaximal 0.2 msec stimuli delivered at 2 Hz. The ratio of T4 to T1 (T4/T1) is then measured.
Unparalazyed patient - TOF is 1
NMBD: 70-75% receptors blocked - T4 is lost
NMBD: 95% receptors blocked - all twitches disappear
TOF > 0.9 = good indicator of adequate: respiratory muscle function, laryngeal muscle function (airway patency and protection), VC. Also correlates with strong handgrib and 5-sec headlift.
What is Double-Burst Stimulation?
TOF can be quantified (objective) and qualtativiely assessed (subjective). We use the latter = manual detection of fade. However, this can lead to poor accuracy.
DBS is a method suggested to improve manual detection of fade.
two bursts, each consisting of three brief 50Hz tetanic stimuli are used. Fade more easily appreciated because only two responses, one to each burst, are compared.
What is posttetanic facilitation?
PTF is characteristic of NMBD
patients recieving NMBD will have decrease single stimuli response, TOF ratio decrease, and fade in response to tetanic stimulation.
PTF = first apply single stimuli (and evoke small twitch response), then give tetanic stimulation (50Hz for 5 sec), wait 3 seconds and deliver 20 single stimuli at 1 Hz (posttetany stimuli). Posttetany stimuli will produce a greater response than pretetanic one (they are potentiated), and then fade.
Degree and duration of PTF depend on degree of blockade
What is Posttetanic count?
Apply posttetanic facilitation, and count the number of posttetanic twitches (after tetany stimulation).
PTC of 0-1 = profound neuromuscular blockade
PTC increases as recovery increases, and tihs correlates with the time to reapperaance of the first twitch (T1) in TOF.
PTC 0 = recovery is indeterminate.
