Term
| Clinical indications of NMJ blockade |
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Definition
-relax skeletal muscles for surgical access
-facilitate control of ventilation – esp large dogs – tend to R IPPV
-opthalmic surgery – need eye in central position – otherwise can only be achieved in light or deep anaesthesia, neither of which is ideal
-assist reduction of dislocated joints and fractures? – when fresh before scar tissue formed
-decr. the amount of anaes. agent required |
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Term
| What must be considered when using NMJ blockade? |
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Definition
-Facilities for endotracheal intubation and IPPV must be available
-remember no analgesia and anaesthetic effects |
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Term
| Different sensitivities of muscles to NMJ bloackade |
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Definition
| -diaphragm most R - intercostals - larynx - peripheral muscles most susceptible |
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Term
| depolarising muscle relaxants |
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Definition
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Term
| Suxamethonium - mode of action |
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Definition
| -depolarising (non-competitive) muscle relaxant - a nicotinic ACH-R agonist, activates R but to a lower degree than ACh and not broken down by AChE
-broken down by plasma cholinesterases, which is much slower than AChE |
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Term
| Suxamethonium pharmacokinetics |
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Definition
| -fast onset
-initial muscle fasiculation / twitch
-only one dose or get phase II block
-no antagonist available
-short duration - 3-5mins in cats, 20mins in dogs |
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Term
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Definition
| -aid intubation in cats and pigg |
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Term
| Suxamethonium CI / side effects |
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Definition
-may trigger maliganant hyperthermia in susceptible pig breeds
-incr. serum K+ levels
-burns - incr, the no. of R and so exaggerated response |
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Term
| Non-depolarising (competitive) muscle relaxants drugs |
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Definition
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Term
| Non-depolarising (competitive) muscle relaxants basic mechanisms |
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Definition
| -compete w. ACh for post junctional binding sites
-no initial muscle fasiculation
-rel. slow onset (3-5 mins)
-can be topped up w. 1/3 initial dose
-can be antagonised |
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Term
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Definition
| -a non-depolarising (competitive) muscle relaxant
-consists on 10 isomers, only one active - cisatracurium |
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Term
| Atracurium pharmacokinetics |
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Definition
| Hoffman elimination i.e. T dependent degradation in the plasma |
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Term
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Definition
| agent of choice for animals w. renal / hepatic compromise (broken down in the plasma) |
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Term
| Atracurium side effects / CI |
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Definition
| -histamine release therefore needs to be given slowly
-breakdown product Laudanosine can cause seizures in some dogs - only at v. v. high doses so unlikely |
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Term
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Definition
-active isomer of atracurium
-can give alone to avoid side effects of atracurium but very expensive so not realistic |
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Term
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Definition
-a non-depolarising (competitive) muscle relaxant
-steroid compound |
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Term
| Vercuronium pharmacokinetics |
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Definition
-a powder which has to made up and is then stable for only 24hours
-40-50% undergoes hepatic biotransformation |
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Term
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Definition
-not ideal if hepatic/renal compromised
-does not release histamine - cardiac stable - advanatage |
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Term
| Signs of inadequate depth of anaesthesia when using NMJ blockade |
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Definition
NB: muscles relaxed so can't use many reflexes e.g. palbepral / pedal
-incr pulse rate, BP
-saliavation, lacrimation
-vasovagal response - bradycardia, hypotension, pallor
-incr. end tidal CO2 (unrelated to change in ventilation - muscles trying to work so produce CO2!)
-slight muscle twitching
-pupillary dilation |
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Term
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Definition
| NB: animal could be very deep w. no NMJ blockade or vica versa!
-use a peripheral n. stimulator over ulnar, peroneal or facial nn. - 4 electrical impulses applied to a nerve over 2 second period
-asses 'train of four' - w/o blockade there would be 4 muscle twitches of equal strength
-as blockade increases, the twitches dec. in no. and strength - twitch 1 may be present, 2 and 3 may be decr. and 4 may be absent
-only monitors degree of NMJ blockade not anaes. depth |
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Term
| Factors influencing NMJ blockade |
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Definition
-volatile agent
-hypothermia - incr. duration of blockade (esp w. atracurium - metabolism is T dependent!)
-hepatic / renal insufficiency
-electrolyte and acid base abnormalities (affect ionisation of drug)
-muscular dz. e.g. myaesthenia gravis
-aminoglycoside antibiotics (incr duration of block due to affect Ca++ post-synaptic channels)
-dose administered |
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Term
| Signs of inadequate depth of anaesthesia when using NMJ blockade |
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Definition
NB: muscles relaxed so can't use many reflexes e.g. palbepral / pedal
-incr pulse rate, BP
-saliavation, lacrimation
-vasovagal response - bradycardia, hypotension, pallor
-incr. end tidal CO2 (unrelated to change in ventilation - muscles trying to work so produce CO2!)
-slight muscle twitching
-pupillary dilation |
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Term
| Antagonism of NMJ blockade |
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Definition
| -only possible in non-depolarising, competitive blockers and once 1 or 2 twitches return
-use AChE e.g. neostigmine
-ACh conc. increase and competes w. NMJ blockade |
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Term
| Side effects / considerations when using NMJ blockade antagonists |
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Definition
-bradycardia (can be severe enough to cause cardiac arrest), salivation, bronchospams, diarrhoea
-administer anticholinergic drugs w. AChE e.g. atropine, glycopyrrolate (both short acting) to counter the effects of the AChE - NOT to horses
-Ventilation must be supported until spontaneous vent. commences |
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Term
| Recent advances in NMJ blockade |
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Definition
-cyclodextrins - antagonise NMJ blockade - doughnut structure and encapsulates NMJ blockade drug so can't act
-New NMJ blockade drugs can be terminated by cysteine adduction |
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