NMBA

Question 1

Where do NMB work and what is their effect?

Answer 1

They interrupt transmission at the nicotinic Ach receptors in the NMJ

• Cause paralysis (skeletal muscle only); do not have any analgesic, sedative or hypnotic effects!

Question 2

What are examples of intermediate acting non-depolarizing NMBAs?

Answer 2

• Atracurium
• Rocuronium
• Vecuronium

Question 3

What are examples of a long acting non-depolarizing NMBA?

Answer 3

Pancuronium

Question 4

Name a depolarizing NMBA

Answer 4

Succinylcholine

Question 5

Describe non-depolarizing NMBAs

Answer 5

• Competitive antagonists of NAchR = bind to receptor and prevent interaction w/ Ach => FLACCID PARALYSIS

Question 6

How are the effects of non depolarizing NMBAs terminated?

Answer 6

• redistribution, metabolism, excretion
• administration of a reversal agent
  
  • Can be reversed by increasing the concentration of Ach!

Question 7

Describe atracurium

Answer 7

• Unique metabolism
  
  • ​Hoffman elimination, ester hydrolysis
• No accumulation
• Hoffmann elimination breakdown product = Laudanosine
  
  • ​liver metabolism = accumulates; crosses BBB = CNS stim => seizures
• H release

***temperature and pH dependent

Question 8

Describe cis-atracurium

Answer 8

• Causes most of atracurium NM effect
• 4x more potent vs. atracurium
• less H release
• less laudanosine produced
• no ester hydrolysis, 77% HE

***temperature and pH dependent

Question 9

Which two depolarizing NMBAs are steroid compounds?

Answer 9

Vecuronium and Rocuronium

Question 10

Compare and contrast vercuronium and rocuronium

Answer 10

• Both are metabolized by the liver
• Rocuronium is derived from vecuronium
  
  • ​less potent —>higher dose and faster onset

Question 11

Describe AchE inhibitors

Answer 11

• Most commonly used reversal agent for NMBAs
• Increased [Ach] competes w/ NMBA at NAchR
• NMBA needs to be partially gone (metabolized) for these to work
  
  • Cannot reverse a deep block!
• Agents: Neostigmine and Edrophonium

Question 12

Why should you be sure to administer AchE inhibitors slowly while all of the monitors are still connected?

Answer 12

• Ach also act on the MAchR
• severe bradycardia/bradyarrhythmias up to cardiac arrest can result
• can potentially pre-treat with an anticholinergic

Question 13

How do the novel NMBA reversal drugs, Sugammadex and calabadion work?

Answer 13

• Encapsulate the NMBA - can reverse a deep block
• No increase in Ach concentration - no CV side effects
• very expensive, not widely used in vet med just yet

Question 14

When should you not re-dose an NMBA?

Answer 14

• If the procedure is almost over
  
  • unless it is absolutely necessary, then use a very low dose titrated to effect (just enough to centralize the eye)
• otherwise you will need to keep the patient anesthetized until it is partially recovered and then reverse
  
  • at least three twitches with a TOF
  • or when patient breathes spontaneously

Question 15

Describe depolarizing NMBAs (succinylcholine)

Answer 15

= two Ach molecules put together

• Depolarizes synaptic membrane - repolarization cannot occur as long as the agent is bound
• fasciculations followed by flaccid paralysis
• Metabolized by butyrylcholinesterase (pseudocholiesterase) -low concen in NMJ
• No antagonist - cessation of effect due to diffusion in ECF

Question 16

What are the side effects of succinylcholine?

Answer 16

• CV
  
  • MAchR: bradycardia, sinus arrest
  • ANS ganglia: tachycardia/hypertension
• HyperK
• Myoglobinuria
• Increase IOP/ICP
• Malignant hyperthermia (potent trigger)

Question 17

What procedures might you use NMBAs for?

Answer 17

• ocular surgery (centralize eye)
• fracture reduction
• improvement of surgical field (relax thoracic/abd mm)
• preventing cough during intubation or one lung ventilation
• facilitate mechanical ventilation
• C-section (doesn’t cross placenta, lowers inhalant dose)
• As part of balanced anesthesia
• rapid sequence anesthesia

Question 18

What must you make sure to do with NMBAs?

Answer 18

• ALWAYS use them in conjunction with a hypnotic and an analgesic
• patient will be paralyzed
  
  • so IPPV is necessary!
  • cannot move in response to pain

Question 19

Why is monitoring a patient extremely closely after NMB administration important?

Answer 19

• To assess when patient is adequately relaxed
• Know when to re-dose
• Know when reversal can be attempted
• To exclude residual paralysis

Question 20

Why is residual paralysis a problem?

Answer 20

• Associated with post-operative respiratory complications
  
  • Airway/pulm collapse, blunted response to hypoxia, acute resp failure, aspiration
• causes increased morbidity and mortality

Question 21

How do we monitor NM function?

Answer 21

• Indirectly
• Subjectively: visual, tactile (cannot detect mild degree of residual paralysis)
• Objectively:
  
  • MMG: measures force of contraction (gold standard)
  • EMG: measures action potential
  • AMG: measures acceleration

Question 22

What is the most commonly used stimulation pattern used to monitor NMB?

Answer 22

The train of four (ToF) twitches

• T₄ : T₁ = ToF_r = 1 (100%) in the absence of paralysis
• essentially if a dog starts twitching after at least 3 impulses (ToF count is _>_3), reversal with an AchEi can be achieved
• also, if the patient is spontaneously breathing, ToF is most likely _>_3

Question 23

NMB is affected by many variables - name a few

Answer 23

• Age
• temp
• NM disorders
• antibiotics/other drugs
• Impaired metabolism/excretion
• anesthetic drugs
• acid/base disturbances
• electrolyte abnormalities (Mg, Ca, K)