Muscle Relaxants & Nerve Monitoring

1. Muscle Relaxants & Nerve Monitoring

2. Overview of Nervous System

3. Motor Neuron

4. Nerve Conduction • More Na+ outside membrane than inside at rest (15x).Na+ > K+ • More K+ inside membrane than outside at rest (30x)

5. Synaptic Cleft • Acetylcholine stored in vesicles at motor end plate • Nerve impulse ends with Ach released into cleft from vesicles

6. Acetylcholine • Synthesized from choline and acetate • Stored in vesicles at motor end plate • Quanta contain 5-10 Thousand ACh • 2-4 hundred Quanta(1-4 million Ach) with each action potential • Some quanta are released in absence of nerve stimulation • Ach metabolized by Cholinesterase within thousandths of a second

7. Nicotinic Acetylcholine Receptor • Consists of 5 glycoprotein subunits to form an ion channel. • 2 alpha, 1 beta,1 delta,1 gamma(epsilon) • Ach binds to the 2 alpha subunits opening the channel • 1-10 million receptors across cell membrane • Only 1 alpha subunit needs NDMR attached to block • Both alpha subunits need SCh attached to block

8. MUSCLE RELAXANTS • Depolarizers Succinylcholine Decamethonium • Non-Depolarizers Curare Pancuronium Vecuronium Rocuronium Atracurium Cis-Atracurium Mivacurium

9. ! Depolarizers Succinylcholine is 2 Ach molecules acting like Ach at post-synaptic receptor. Not fully understood exactly how it exerts action • CHARACTERISTICS: • Phase I block • Decrease single twitch • No FADE (decrease but Sustained response) to continual stimulus • TOF ratio greater than 0.7 • Potentiation by Anticholinesterase drugs • Cause fasiculations • Phase II block • Resembles non-depolarizer blockade characteristics

10. ! Non-depolarizers ...act on at least one alpha subunit to block conduction. Competitive antagonist • CHARACTERISTICS: • Decrease twitch response to a single stimulus • Produce FADE (unsustained response) to continual stimulus • TOF ratio less than 0.7 • Post tetanic facilitation • Potentiation by other non-depolarizers • Antagonism by anticholinesterase drugs

11. Why relax or paralyze Muscles? • To obtain adequate intubating conditions • To facilitate surgical exposure or manipulation • To improve mechanical ventilation

12. Nerve Stimulators • Many styles and sizes

13. Nerve Monitoring • Single twitch 0.1-0.15 Hz • Train of Four 2Hz (0.5secs apart) • Tetanus 50Hz 100Hz both 5 secs apart • Double Burst Stimulation 50Hz (0.2 ms,750ms apart) 2 sets of 3 bursts appear as two Twitches • Post-tetanic count stim. 50Hz, 5 secs • Impulse durations are 0.2 sec. • PURPOSE: To evaluate degree of muscle paralysis or recovery from paralysis

14. Nerve MonitoringTrain of Four Single Twitch Train of Four FADE on Train of Four

15. Non-Depolarizers • Exhibit Fade on Train of four. • Fade may progress to no twitches if enough NDMR is given or or or (none) Increasing NDMR dose

16. ! Nerve MonitoringTrain of Four R4 decreases at 75% receptors blocked R3 decreases at 85% receptors blocked R2 decreases at 90% receptors blocked R1 disappears at 95% receptors blocked What is your degree (% receptor blockade) of paralysis shown to the right? R1 R2 R3 R4 R=Response

17. ! Clinical Application • Surgical relaxation appreciated at >90% • Intubation facilitated at 95% • Total Flaccidity at 99%

18. Nerve MonitoringDouble Burst Fade on Double Burst Equal Double Burst

19. Phase 1 and 2 Block Seen with depolarizing blockade • Phase 1 block • Phase 2 block typical non-depolarizing TOF Baseline TOF high or repeated doses of depolarizer- 0/4 TOF

20. REVIEW • NDMR have FADE • Depolarizers have: Phase 1 block seen as 4/4 TOF but with LESS amplitude or Phase 2 block seen as 0/4 TOF That is NO TWITCHES

21. Placement of Nerve Monitor Leads THERE ARE MANY NERVES YOU CAN MONITOR: Most frequently used: ULNAR-adductor pollicis FACIAL-orbicularis oculi POSTERIOR TIBIAL-flexor hallucis *We stimulate a nerve to observe a muscle response

22. Ulnar nerve monitoring Note proper lead placement

23. Proper Twitch evaluation • Adduction of thumb byUlnar nerve stimulation causing ADDUCTORPOLLICIS muscle contraction

24. Improper Twitch evaluation • Median nerve and / or direct muscle stimulation(note finger contraction, not thumb contraction)

25. Direct muscle and Ulnar nerve (Adductor Pollicis) stimulation

26. Facial Nerve Monitoring Lead Placement

27. Facial Nerve Twitch Twitch of ORBICULARIS OCULI

28. Posterior Tibial Nerve monitoring Placement- Behind the external malleolus. Stimulation causes flexion of the big toe by contraction of the flexor hallucis

29. ! Which nerve is better to Monitor? • Good Question! • Orbicularis Oculi-relatively resistant to blockade. Similar to Diaphragms resistance • Go to sleep closing your eyes, wake up thumbs up. Monitor Facial nerve for intubating conditions and Adductor Pollicis ( Ulnar nerve) for extubation adequacy

30. Which nerve is better to Monitor? continued • Adductor Pollicis-easily blocked • Ulnar nerve monitoring beneficial for two main reasons: Ease of placement and access. And contralateral innervation of the monitored muscle. This allows discrimination between direct muscle stimulation and direct nerve stimulation. • Note-Ulnar nerve also innervates hypothenar muscles of 5th digit though these muscles are relatively resistant to blockade.

31. Which nerve is better to Monitor? continued • Posterior Tibial nerve- Allows monitoring on patients who we have limited access to. • Sensitivity of flexor hallucis undetermined

32. ! Residual Muscle Paralysis Evaluation • TEST: RECEPTORS OCCUPIED: • Normal tidal vol. Up to 80% may be blocked • Holds tetanus 50Hz Up to 75-80% may be blocked • Equal TOF, DBS Up to 75-80% may be blocked • Holds tetanus 100Hz Up to 50% may be blocked • Head lift x 5secs Up to 33% may be blocked • Remember 0% UP TO the above number % ie. Normal tidal vol.represents 0% or anywhere up to 80% of ACh receptors competitively blocked with muscle relaxant.

33. What do each of these modes mean? • Single twitch- allows continual evaluation of Depolarizing block (must know baseline) • Train of Four (TOF) - Allows estimation of DEGREE of non- depolarizing block • Double Burst Stimulation (DBS)- Easier visual evaluation of fade. Gives same info. As TOF

34. What do each of these modes mean? continued • Tetanic stimulation- More detailed assessment of degree of non-depolarizing block • Post Tetanic Stimulation Count: 1)Allows quantification of block if NO TOF. 2) Represents what chemical reversal drugs will achieve for you • Post Tetanic facilitation- release of large amount of ACh floods the NMJ and accentuates subsequent twitch responses

35. Reversal of muscle relaxants • Anti Cholinesterases ie.Neostigmine • Cholinergic/(muscarinic) side effects: Bradycardia • Give with Anti-Cholinergic / (anti-muscarinic)

36. Additional Readings • Appliguide Neuromuscular monitoring, Datex-Ohmeda pp.1-21, 31-35. Available in Anesthesia Office.

37. REVIEW • How are impulses conducted along the axon? • How are impulses conducted to the motor cells? • What is the resting phase of the motor neuron • Where is most potassium in the body? • In what part of the peripheral nervous system designations do we find the majority of motor neurons? • What compartment do muscle relaxants work in? • Explain the anatomy and function of an acetylcholine receptor • How do muscle relaxants effect ACh receptors • What is the duration of ACh? • How is ACh metabolized • Why do we need to paralyze muscles during surgery? • What is a nerve stimulator/monitor? • How does a nerve stimulator work? • What is Ohm’s law and how does it apply to nerve monitoring? • What is a Hertz? • What is Fade? • Which muscle relaxants cause Fade? Which do not? • What is a Twitch,Train of Four, Double Burst Stimulation,Tetanus and what do these represent? • What degree of paralysis is needed before we see any appreciable fade on train of four? • What is the degree of paralysis needed monitoring • Which Muscle (nerve) correlates best with the diaphragms resistance to paralysis? • Which Muscle (nerve) correlates best with the adequacy of intubating conditions? • What factor may give a false twitches in the presence of complete paralysis? • What degree of residual paralysis may be present despite a normal tidal volume? • What degree of residual paralysis may be present despite holds tetanus? • What is the best indicator of reversability after muscle relaxant use? • What is post tetanic facilitation? • Between what percentages of blockade do we maintain and monitor muscle relaxant paralysis?