Propofol
Propofol is one of the most commonly used injectable anaesthetic agents in small animal practice. It is a very short acting alkylphenol. It is generally used as a induction agent but can be used for maintenance of anaesthesia via a total intravenous anaesthesia (TIVA) protocol.
Mechanism of Action
Propofol binds to and activates GABA-A receptors, similarly to thiopental, benzodiazepines and alcohol, although it binds at a different site on this complex multi-subunit receptor. GABA is a major inhibitory neurotransmitter in the CNS. Activation of the GABA-A receptor causes increased chloride conductance leading to hyperpolarisation of the postsynaptic neuronal cell membrane, thus inhibiting conductance producing depression of the CNS.
Pharmacological Considerations
Propofol is highly lipid soluble and insoluble in aqueous solution so is formulated as a 1% oil-in-water emulsion. Once a vial has been opened it should be refrigerated and then discarded after 24 hours. Administration of the agent is via an intravenous route and should be given slowly to effect (over 2 minutes usually), with onset of action between 60-90 seconds. Administration of premedications such as alpha-2 agonists or opioids can significantly reduce the amount of propofol required to anaesthetise a patient.
Propofol is highly distributed meaning that blood levels significantly fall rapidly. It is metabolised by the liver and conjugates are excreted in urine. It is this hepatic metabolism that allows for repeated dosing as this rapid metabolism that means there is little accumulation of the drug, making it a suitable agent for TIVAs in some species.
Contraindications and Side Effects
Cardiovascular Effects
- Decreased myocardial contractility without reflex tachycardia.
- Greatest hypotension compared to any other injectable agents. However, it is of short duration.
- Occassionally profound bradycardia may be seen.
Respiratory Effects
- Most common side effect seen is apnoea upon induction. This is thought to be related to dose and rate of administration.
Propofol in Cats
In cats, glucuronide conjugation is reduced, which is require for phenolic compound metabolism. This means that recovery from propofol TIVAs is prolonged in cats and has been known to reduce the packed cell volume (PCV) in some patients. Repeated dosing in cats is also not advisable as it causes significant Heinz body formation as well as anorexia, diarrhoea, facial oedema, and depression.
Uses
As previously mentioned, propofol can be used as both induction and maintenance agents. It is useful in patients undergoing caesarean section as it has mininal foetal depression. It also has anticonvulsant properties and so can be used to anaesthetise a seizuring animal when other treatments, such as diazepam , have been ineffective. Similarly to thiopental, propofol reduces intracranial pressure (ICP), so it can also be used in patients with increased ICP. It may be beneficial to use propofol in patients with upper airway obstructive disease due to rapid recovery meaning that airway reflexes are more rapidly recovered.