Anaesthetics Flashcards
Malignant Hyperthermia
Malignant hyperthermia (MH) is a rare, serious side effect of volatile liquid anaesthetics (isoflurane, desflurane, sevoflurane), which cause all skeletal muscle to rapidly contract, including during a neuromuscular blockade. MH is a genetic disorder, manifesting due to calcium overload in the skeletal muscle causing sustained muscular contraction and rhabdomyolysis, resulting in excess anaerobic metabolism causing acidosis. End-tidal CO2 increases as a result, along with body temperature which causes diaphoresis (excess sweating).
LA: Lidocaine
An amide
Local anaesthetic and a less commonly used antiarrhythmic (affects Na channels in the axon)
Hepatic metabolism, protein bound, renally excreted
Toxicity: due to IV or excess administration. Increased risk if liver dysfunction or low protein states. Note acidosis causes lidocaine to detach from protein binding. Local anesthetic toxicity can be treated with IV 20% lipid emulsion
Drug interactions: Beta blockers, ciprofloxacin, phenytoin
Features of toxicity: Initial CNS over activity then depression as lidocaine initially blocks inhibitory pathways then blocks both inhibitory and activating pathways. Cardiac arrhythmias.
Increased doses may be used when combined with adrenaline to limit systemic absorption.
Plain dose: 3mg/kg
With Adrenaline: 7mg/kg
LA: Cocaine
Pure cocaine is a salt, usually cocaine hydrochloride. It is supplied for local anaesthetic purposes as a paste.
It is supplied for clinical use in concentrations of 4 and 10%. It may be applied topically to the nasal mucosa. It has a rapid onset of action and has the additional advantage of causing marked vasoconstriction.
It is lipophillic and will readily cross the blood brain barrier. Its systemic effects also include cardiac arrhythmias and tachycardia.
Apart from its limited use in ENT surgery it is otherwise used rarely in mainstream surgical practice
LA: Bupivicaine
Bupivacaine binds to the intracellular portion of sodium channels and blocks sodium influx into nerve cells, which prevents depolarization.
It has a much longer duration of action than lignocaine and this is of use in that it may be used for topical wound infiltration at the conclusion of surgical procedures with long duration analgesic effect.
It is cardiotoxic and is therefore contra indicated in regional blockage in case the tourniquet fails.
Levobupivicaine (Chirocaine) is less cardiotoxic and causes less vasodilation.
Plain dose: 2mg/kg
With adrenaline: 2mg/kg
LA: Prilocaine
Similar mechanism of action to other local anaesthetic agents. However, it is far less cardiotoxic and is therefore the agent of choice for intravenous regional anaesthesia e.g. Biers Block.
Plain dose: 6mg/kg
With adrenaline: 9mg/kg
Effect of adrenaline on LA drugs
Adrenaline may be added to local anaesthetic drugs. It prolongs the duration of action at the site of injection and permits usage of higher doses (see above). It is contra indicated in patients taking MAOI’s or tricyclic antidepressants. The toxicity of bupivacaine is related to protein binding and addition of adrenaline to this drug does not permit increases in the total dose of bupivacaine, in contrast to the situation with lignocaine.
