Surgical Muscle Relaxation

Intra-operative movement
1. Undesirable
  1. Complicates surgery
  2. May be difficult to prevent
  3. Jeopardizes sepsis
  4. Wastes time & Money
  5. Damage & injury
2. Desirable
  1. Breathing
  2. Nystagmus (Except during ocular surgery)
  3. Reflex withdrawal to noxious stimuli
1) Deep GA + Alpha 2 - Agonists
1. Ethers > Alkanes
2. Propofol Vs. Ketamine
  1. Ketamine
    1. Poor Muscle relaxant !
      1. Myotonia
      2. Catatonia
        Not anaesthesia in general sense (Dissociative Anaesthesia)
3. Alpha 2 Agonists
  1. Drug - dependent effect
  2. Synergism + "Anti" - Ketamine effect
    1. => Muscle relaxation
4. Suppressing reflexes (Ex. scratch reflex) with high levels of GA incurs the risk of excessive Medullary depression
  1. Endangers unhealthy animals
  2. Healthy animals: High anaesthetic doses given over prolonged periods will delay recovery
2) "Spinal" Acting drugs
1. Guaiphenesin, Glyceryl guaiacolate ether (GGE)
  1. Horse, Cow, Pig
  2. Induction for animals on verge of shock
    1. X3 (Triple) Drip
      1. GGE, Xylazine & Ketamine
        Operations </= 45min
  3. Disadvantages
    1. Paradoxial Signs in OD
      1. Rigidity of muscle
    2. Irritant extravascularly
      1. Must use cannula to inject into Jugular
2. Blocks internuncial neurons (connecting neurons) in the spinal cord
3. Benzodiazepines
  1. Relieves muscle spasm
  2. Midazolam, Diazepam
  3. Sedative (?) muscle relaxants
    1. GABA receptors
  4. Don't use in Horses by itself !
    1. + Ketamine
  5. "Anti" - Ketamine effect
    1. Uses
      1. Induction (Horses)
        Top-ups (Horses)
        IM sedation (+ Ketamine)
3) Local Anaesthetics
1. Applied to peripheral nerve trunks (Conduction blocks)
  1. Blocks activity in the A alpha motor neurons (before motor nerve terminal)
2. Extradural => Profound muscle relaxation
3. Ex. Lidocaine
4. Ex. Brachial plexus block
4) Neuromuscular blocking agents
1. Predictable Muscle relaxation during clinical anaesthesia
2. Reversibly bind Nicotinic Cholinoreceptors at the motor end plate & prevent neuromuscular transmission
  1. Block access of Acetylcholine
3. No CNS depression
  1. Do NOT cross BBB
4. Effects can be anatagonized by Anticholinesterase drugs
5. Non- Depolarizng Drugs
  1. Commonly used
  2. Benzylisoquinolinums
    1. d-tubocurarine
      1. No longer used
      2. Injection causes Histamine release in Dogs
        Vasodilation
        Hypotension
        Tachycardia
        Bronchial spasm
    2. Mivacurium
      1. Very rapid onset
        Short duration
      2. New
    3. cis-Atracurium
      1. Unevaluated in animals
  3. Amino-steroids
    1. Pancuronium
      1. Used to be the most popular relaxant
      2. Intermediate onset
        Long duration of action (> 30 min)
      3. Causes modest Tachycardia after injection
        Still used (Pigs)
      4. Vecuronium
        Commonly used
        Derived from Pancuronium
        Intermediate duration of action (20 - 30 min)
        Little cumulative effects after repeated doses
        Few cardiovascular effects
    2. Atracurium
      1. Intermediate duration of action
        Rapid onset
        Hofmann elimination
        Drug molecule spontaneously degrades
        Good for animals with renal & hepatic disease
    3. Rocurium
      1. New
      2. Very rapid onset
        limited advantage in animals
  4. No Effect on cardiac or smooth muscle
6. Depolarizing Drugs
  1. Depolarize the post-synaptic membrane before stabilizing its RMP to cause paralysis
    1. Rigidity => Flaccid paralysis
  2. Suxamethonium
    1. Rapid onset (secs)
      1. Short acting (3 -5min)
    2. Horses, pigs, cats, primates & humans
      1. Facilitate endotracheal intubation
    3. 2 combined molecules of Ach ( drug exerts cholinergic behavior)
      1. Initial sarcolemmal depolarization (w/ muscle faciculations) & some ANS effects
    4. Increases intragastric & introcular pressure
  3. Decamethonium
  4. Not used in animals much anymore
7. Advantages
  1. Predictable total relaxation for surgeon & anesthetist
    1. Reduces anaesthetic requirement
      1. Reduces surgical traction
        Facilitates surgery
        Reduces wound margin trauma
        Facilitates PPV
        Facilitates ETT
        May be Antagonized (Reversible)
8. Disadvantages
  1. Controlled ventilation vital
    1. Difficulty in monitoring anaesthesia
      1. Absence of motor response to nerve stimulation
        Complexity of blockade monitoring
        Post- op weakness
        Zero motor response to Nn stimulation
        Ex. spinal surgery
        No twitch response when near motor nerves
      2. Muscle paralyzed, so can't indicate consciousness
      3. Depth monitoring complicated
        Can't use eye positioning
  2. No guarantee of unconsciousness
    1. Do NOT cross BBB
      1. Do NOT effect consciousness
9. Indications
  1. High risk cases
    1. Reduce anaesthetic requirement => Cardiopulmonary function is preserved
    2. Thoracic surgery & Diaphragmatic Hernia Repair
      1. Laparotomy
        Microsurgery
        Orthapedics
        Inefficient ventilatory pattern
        Insuppressible spinal reflexes
        Ex. Chronic otitis externa
        Scratch reflex
        Head shaking
        Race Horses
        Endotracheal intubation
        Facilitated in species with laryngeal reflexes resistant to GA
        Primates, Pigs & Cats
        PPV without animal "fighting the ventilator"
        Intraocular & neurological surgery (performed under microscopes)
        Less surgical traction required => less tissue trauma on wound margins
      2. Reduced rigidity in the thoracic cage => lower inflation pressure for PPV
10. Contraindications
  1. Neuromuscular blocking drugs must NOT be used if there is any doubt that the animal is adequately anaesthetized
    1. These drugs eliminate some of the obvious signs of inadequate anaesthesia (Movement, ocular position, cranial nerve reflexes)
11. Muscle Sensitivity to Blockers
  1. Muscles of expression > Neck, distal limb > Proximal limb> Abdominal muscles > Intercostal muscles > Diaphragm
  2. Muscles affected in this order during onset, sequence is reversed during offset
  3. Consequences:
    1. 1) If neuromuscular blockade is monitored at a resistant unit then overall overdose may occur with prolonged recoveries
    2. 2) Monitoring at a sensitive site may mean muscle relaxation is inadequate at the operating site
    3. 3) Attempting to capitalize on selective sensitivities ( Low doses to block sensitive muscles while preserving resp. muscle function) => Very Dangerous!
      1. Sensitivity hierarchy is very variable, as is response of all animals to these drugs
12. Monitoring Neuromuscular Blockade
  1. Peripheral nerve stimulators: stimulating a peripheral nerve & measuring the strength of the evoked response (or twitch)
    1. Muscle units for monitoring neuromuscular transmission:
      1. Dorsal buccal branch of facial nerve
        Dilator nasi lateralis (Dogs)
      2. Ulnar nevre
        Carpal flexors (Dogs & Pigs)
      3. Deep peritoneal
        Hind-limb digital extensor muscles (Horses & Dogs)
    2. 'Train of 4' stimulation pattern
      1. Delivery of 4 stimuli (at 2 Hz) every 12 secs
        Counting the # of twitches in each train can indicate the extent of block
13. Monitoring Consciousness
  1. Neuromuscular blockers paralyze facial skeletal muscle & eliminate normal reflexes
    1. After paralysis: Eyelids are open, eye is central; No corneal or palpebral reflexes
  2. Paralysed animals that are NOT unconscious:
    1. Paradoxial Jaw tone
      1. Paradoxial tongue twitching
        Mydriasis
        Lacrimation
        Salivation
        Tachycardia
        Hypertension
        Arrhythmias
14. Neuromuscular Blockade Antagonism
  1. Anti-cholinesterase Drugs
    1. Edrophonium
      1. + Atropine
    2. Neostigmine
      1. + Glycopyrrolate
    3. Increase Ach levels at the neuromuscular junction
      1. Additional Ach competes w/ neuromuscular blocking agent effectively for binding sites on the Nicotinic receptor
        => Restores neuromuscular transmission
      2. Rising Ach throughout the body activates Muscarinic receptors present on target organs of the Parasympathetic NS
        Anti-cholinesterases given alone => Bradycardia, Bradyarrhythmias (asystole), Bronchoconstriction, Bronchosecretion, GI Hypermotility & Hypersecretion
        Similar to OP toxicity
        + Antimuscarinic Drugs
5) Combinations

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Surgical Muscle Relaxation

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	Created by melian.yates about 11 years ago