Anaesthetics
Anaesthesia is a controlled temporary loss of awareness, with varying degrees of pain relief (analgesia), loss of memory (amnesia) and muscle paralysis. Involves 3 agents:
1) General Anaesthetic
Believed to potentiate GABA receptors and K+ channels and inhibit excitatory NMDA receptors.
2) Opioid pain relief
e.g. fentanyl + sufentanil – Provides very fast acting pain relief
3) Muscle relaxant
e.g mivacurium or succinylcholine – Blocks the NMJ to stop muscle reflexes
Inhalation Anaesthetics
These are complete anaesthetics as they cause loos of consciousness + analgesia/amnesia + immobility
– The potency is measured by the Minimum Alveolar Concentration (MAC) = Minimum concentration of anaesthetic in alveoli that results in immobility in 50% of patients when exposed to noxious stimuli
– As the potency of the agent increases, the lower the MAC value will be.
For inhalation anaesthetics, the solubility of the gas in air vs blood is measured by blood-gas coefficient:
For low solubility gases, it means few molecules need to be present in blood to increase partial pressure.
– Therefore, this gives rapid equilibrium in CNS with rapid onset action + rapid elimination
– Increasing cardiac output decreases induction rate, as presents larger volume into which gas dissolves
– For high solubility, means slower induction + slower elimination as it redistributes into fat.
Nitrous Oxide (N20)
This is often useful as a combination drug, as it increases uptake rate of gas anaesthetics
– It lacks potency for surgical anaesthesia + no muscle relaxant properties
Side effects
- Postoperative nausea
- Vomiting
Isoflurane/Desflurane/Halothane
These are inhalation anaesthetics which give rapid induction and emergence
Sevoflurane
This gives rapid induction with rapid recovery, widely used in paediatrics
– Also causes bronchodilation, useful for patients with respiratory difficulties.
General Side effects
- Agents cause a reduction in the BMR and the set point for thermoregulation, giving vasodilation and hypothermia
- Malignant hyperthermia –> a rapid increase in muscle metabolism due to mutations of the ryanodine receptor, occurring after use with GA halothane usually
Symptoms:
- Tachycardia
- Muscle spasm
- Hyperthermia
Treatment:
- Dentrolene (stops Ca2+) release from SR
IV Anaesthetics
e.g. Propofol
This is a commonly used agent for quick sedation, given IV and has a short duration of efficacy.
– Widely used for many surgical procedures giving minimal side effects
Side effects
- Systemic hypotension from vasodilation
Etomidate
This is an alternative to Propofol and thiopental for rapid-onset, short-duration anaesthesia
Ketamine
This is an antagonist of NMDA receptors, so decreases glutamergic transmission
– It has similar MOA to Memantine – NDMA antagonist used to treat Alzheimer’s Disease
– Produces a special type of anaesthesia giving analgesia and amnesia without the loss of consciousness
– Also, it is a cardiovascular stimulant for people in shock
Side effects
- Distortions of reality
- Hallucinations
LOCAL ANAESTHETICS
These drugs cause a transient reversible loss of sensation without a loss of consciousness.
– They are classified as either esters or amides.
– They both act by blocking voltage-gated sodium channels in the inactivated state.
– All local anaesthetics are vasodilators, and so they are given with adrenaline to reduce systemic absorption
Amides – Lidocaine (suffix = -caine)
Lidocaine has a rapid onset and intermediate duration of action (1-2 hours)
– It is are used for minor procedures, epidurals and spinal anaesthesia (+anti-arrhythmic)
Esters – Procaine + Tetracaine + Benzocaine
Procaine has a short duration of action and so is used for infiltration anaesthesia
– Tetracaine is long acting and so is used for spinal anaesthesia and ophthalmology use.
General Side effects
- CNS (dizziness, sedation, tremor)
- Cardiovascular (bradycardia + hypotension)
Sources
Image 1: John D’Orazio, Stuart Jarrett, Alexandra Amaro-Ortiz and Timothy Scott / CC BY (https://creativecommons.org/licenses/by/3.0) 2.
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