- Many of the advancements in surgery and outcomes have only been possible due to the advances in anaesthesia and critical care.
The phases of an anaesthetic can be subdivided into the following:
- Preoperative visit, planning and pre-medication
- Induction of anaesthesia and airway control
- Maintenance of anaesthesia.
- Emergence from anaesthesia
- Postoperative recovery
- General anaesthesia encompasses the triad of analgesia, anaesthesia and muscle relaxation. Rather than using a large dose of a single drug to establish the clinical triad, a combination of drugs is used in smaller doses, thus avoiding dose-related adverse effects, and is termed “balance anaesthesia.”
- Intravenous and inhalation induction agents, and muscle relaxants will be summarised in the tables below.
Intravenous induction agents
Drug
|
Mechanism of action
|
Effects
|
Thiopentone (barbiturate)
|
Increases the conductance of chloride ions in nerve cells, mediated by GABA channels causing hyper depolarisation and neuronal inhibition
|
Short acting 5–10 minutes
Dose-dependent respiratory depression
Reduction in CO and SVR
|
Propofol
|
Reduced the opening time of sodium channels inhibiting depolarisation
|
Obtunds upper airway reflexes (useful for LMAs)
Hypotension and decrease in SVR
Respiratory depression and apnoea
Pain at injection site
|
Ketamine
|
Antagonist effect of the excitatory neurotransmitter glutamate via the NMDA receptor
|
Potent analgesic
Stimulates sympathetic nervous system therefore increases HR, BP and CO
|
CO = cardiac output
SVR = systemic vascular resistance
LMA = laryngeal mask airway
HR = heart rate
BP = blood pressure
NMDA = N-methyl-d-aspartate receptor
Inhalation agents
Agent
|
Benefits
|
Limitations/side effects
|
Nitrous oxide
|
Analgesic
Used as a carrier for the other volatiles
|
Not potent enough to be used as sole anaesthetic agent
|
Halothane
|
Non-irritant to upper airways
Used in paediatrics
|
Hepatitis and cardiac arrhythmias
|
Sevoflurane
|
Non-irritant to upper airways
Non-arrythmogenic
|
Renal toxicity
|
Isoflurane
|
Cheaper
|
Upper airway irritant
|
Desoflurane
|
Rapid recovery
|
Upper airway irritant
|
Muscle relaxants
- Muscle relaxants are used to facilitate tracheal intubation and provide optimal operating conditions. Muscle relaxants target the neuromuscular junction (NMJ). At the end of the procedure muscle relaxants can be rapidly reversed with the use of neostigmine, which has a plasma half-life of 60 minutes, longer than all the commonly used muscle relaxants. Neostigmine is an antagonist to acetylcholine (ACH) esterase (the enzyme that breaks down ACH) resulting in a flux of ACH at the NMJ.
- A list of commonly used muscle relaxants with their benefits and side effects are shown below:
Drug
|
Mode of action
|
Benefits
|
Side effects
|
Suxamethonium
|
Mimics ACH, persistent depolarisation of NMJ causes muscle relaxation
|
Rapid onset and short acting
|
Myalgia and hyperkalaemia, anaphylaxis and suxamethonium apnoea
|
Atracurium
|
Competetive inhibitor of ACH
Non-depolarising
|
Intermediate acting spontaneously breaks down (Hofman elimination) useful in hepatic and renal impairment
|
Histamine release can cause brochospasm and vasodilation
|
Rocuronium
|
Competetive inhibitor of ACH
Non-depolarising
|
Intermediate acting, rapid onset useful in patients at risk of aspiration
|
|
NMJ = neuromuscular junction
ACH = acetylcholine
- Adjuncts to anaesthesia include anti-emetics, which aim to reduce the incidence of postoperative nausea and vomiting. Local anaesthetic and peripheral blocks used in conjunction with general anaesthesia have led to improved postoperative pain relief and aided postoperative physiotherapy.