Non-Depolarising Muscle Relaxants Flashcards
1
Q
Classification (chemical structure)
A
Aminosteroid:
- Pancuronium
- Vecuronium
- Rocuronium
Benzylisoquinolium:
- Atracurium
- Cisatracurium
- Mivacurium
2
Q
Classification (duration of action)
A
Long acting
- Pancuronium
Intermediate acting
- Atracurium
- Cisatracurium
- Rocuronium
- Vecuronium
Short acting
- Mivacurium
3
Q
Mechanism of action
A
- Occupation of one or both of the ACh receptor sites
- Prevents ACh from binding to both sites (i.e. they compete with ACh)
- These agents do not produce the conformational change necessary for channel opening. The ion channel therefore remains closed, ions do not flow and depolarisation does not occur
- Nondepolarising muscle relaxants are competitive antagonists at the AChR
4
Q
Side effects
A
Autonomic effects:
- d- tubocurarine may block autonomic ganglia, causing hypotension
- Pancuronium blocks vagal muscarinic receptors in the SA node, causing a tachycardia
- Atracurium, cisatracurium, mivacurium, vecuronium, and rocuronium do not have significant autonomic side effects
Histamine release:
- Benzylisoquinolones - precipitate bronchospasm, hypotension and skin flushing
- Cisatracurium has no direct histaminergic effects, nor do any of the aminosteroids
5
Q
Metabolism
A
Hepatic clearance:
- Vecuronium (significantly metabolised by the liver)
- Rocuronium (lesser extent of metabolism)
Renal excretion
- Pancuronium
- Rocuronium
Plasma pseudocholinesterase
- Mivacurium
- Atriacurium (lesser extent)
Hoffman degradation
- Atracurium
- Cisatracurium
6
Q
Drug interactions
A
Volatiles decrease requirements by 15% (Desflurane > sevoflurane >isoflurane >halothane >N2O)
The following may potentiate neuromuscular blockade:
- antibiotics
- antiarrhythmics
- antihypertensives
- dantrolene
- furosemide (<10µg/kg)
- ketamine
- magnesium sulphate
The following may cause resistance to neuromuscular blockade
- anticonvulsants
- cholinesterase inhibitors
- furosemide (1-4 mg/kg)
7
Q
Factors affecting neuromuscular blockade
A
- Hypothermia - decreases metabolism or delays excretion, prolonging duration of action
- Respiratory acidosis potentiates block and antagonizes reversal
- Electrolytes: hypokalaemia, hypocalcaemia and hypermagnesaemia potentiate block
- Age: neonates are more sensitive, but have an increased volume of distribution; therefore the dose is not necessarily changed
- Hypersensitivity to relaxants e.g. amyotrophic lateral sclerosis, autoimmune disorders, familial periodic paralysis, Guillian-Barré syndrome, muscular dystrophy, myasthenia gravis, myotonia
- Resistance may occur with other conditions e.g. burns, cerebral palsy, severe chronic infections (tetanus, botulism)
- Muscle groups: diaphragm, laryngeal and orbicularis occuli muscles are paralysed faster and recover faster
8
Q
Pancuronium
A
- Dose: 0.08-0.12mg/kg
- Onset: 3.5-4 minutes
- Duration: 70-120 minutes
- Metabolism & excretion:
- Metabolised by the liver to a small degree
- Primarily renally excreted
- About 10% excreted in the bile
- Side effects:
- Hypertension & tachycardia due to vagal block & release of catecholamines from adrenergic nerve endings
- Ventricular arrhythmias secondary to increased AV conduction and catecholamine release
- Allergic reactions in those sensitive to bromides
9
Q
Atracurium
A
- Dose: 0.5mg/kg
- Onset: 3-4 minutes
- Duration: 50-60 minutes
- Metabolism & excretion
- Independent of renal & hepatic function - metabolised by 2 processes
- Hoffman elimination, a spontaneous nonenzymatic chemical breakdown which produces laudanosine
- Ester hydrolysis by nonspecific esterases
- Side effects
- Histamine release (bronchospasm) - may be decreased by slow injection
- Cardiovascular effects unusual at normal doses
- Laudanosine has been associated with CNS excitation and seizures in animal studies
- Hypothermia and acidosis prolong the duration of action
- Contraindications - avoid in asthmatics
10
Q
Cisatracurium
A
- Dose: 0.15mg/kg
- Onset: 4-5 minutes
- Duration: 70-80 minutes
- Metabolism & excretion - Hoffman elimination
- Side effects:
- Does not release histamine
- No cardiovascular effects, even at high doses
11
Q
Vecuronium
A
- Dose: 0.08-0.12mg/kg
- Onset: 3 minutes
- Duration: 50-60 minutes
- Metabolism & excretion:
- Hepatic metabolism
- Primarily excreted in bile
- About 20% renal excretion
- Side effects:
- Devoid of cardiovascular effects
- Does not release histamine
- Females more sensitive than males
12
Q
Rocuronium
A
- Dose: 0.6-1.2mg/kg
- Onset: 1-1.5 minutes
- Duration: 60-70 minutes
- Metabolism & excretion: -
- Not metabolised
- Eliminated primarily by the liver
- Minimal renal excretion
- Side effects:
- At dose of 0.9-1.2mg/kg has onset similar to succinylcholine, but much longer duration
- Slight vagolytic tendencies
- No histamine release
13
Q
Agents used in reversal
A
- Ach esterase inhibitors - reversal
- neostigmine
- pyridostigmine
- edrophonium
*Neostigmine may prolong the effect of mivacurium through inhibition of pseudocholinesterase
Muscarinic receptor antagonists - symptomatic treatment
- atropine
- glycopyrrolate
14
Q
Muscarinic effects
A
- Diarrhoea
- Urinary incontinence
- Miosis
- Bronchoconstriction, bradycardia
- Emesis
- Lacrimation
- Salivation
15
Q
Monitoring reversal
A
- Nerve stimulator
- Single twitch
- Train of 4
- Double burst stimulation
- Tetanic stimulation & posttetanic count
- Clinically
- 5 second head lift, 5 second leg lift
- Effective swallowing
- Patent airway without jaw lift
- Glottic closure against valsalva to oesophageal pressure of +30cm H2O
- Vital capacity >30% of control
