Anaesthesia
the loss of sensation/consciousness
- local: allow maintenance of consciousness (eg: lidocaine)
- general: reversible loss of consciousness (eg: isofluourance)
Analgesia
loss of pain
Routes of administration
- Epidural (space around spine)
- Intrathecal (subarachnoid space)
- nerve block
- infiltration (subcutaneous cut and intradermal application)
- topical
Stages of anaesthesia
- Analgesia
- Excitement due to inhibition of inhibitory reticular neurons
- Surgical anaesthesia
- medullary depression (cessation of spontaneous respiration that without support causes death)
Overton-meyer correlation
The more lipid-soluble an anaesthetic is, the greater the potency
General mechanism of anaesthesia
- Lipid-solubility
- Stimulation of GABAa
- inhibition of glutamate receptors
MAC
minimum alveolar concentration
A metric used to compare the potency of various inhalation agents
- 1 MAC = concentration necessary to prevent responding in 50% of the population
- related to lipid solubility, whereby a smaller MAC represents a greater potency, and thus a greater lipid-solubility
General anaesthetics
Depress all excitable tissues (CNS neurons, cardiac muscle, smooth and striate muscle) with differing sensitivities
- allows anaesthesia given at concentrations that allow loss of consciousness without cardiac and respiratory depression
Classification of general anaesthetics
- Inhalation: further divided into N2O and volatile liquids such as sevofluorane
- Intravenous: induction and maintenance agents such as propofol
Stages of general anaesthetic
Premedication
Induction
Maintenance
Elimination
Premedications (4 types with examples)
Medication used to prevent parasympathetic effects or anxiety
Anxiolytics
- BZDs (diazepam/lorazepam)
- reduce anxiety, thus mitigate interference with surgery efficacy
Muscarinic antagonists
- hyoscine
- protect heart from arrhythmias and prevents bronchial secretions
Analgesics
- fentanyl
- supplement N2O anaesthesia
Anti-emetics
- ondansetron (a 5HT3 antagonist)
- prevents nausea and vomiting
Induction agents (3)
(generally) i.v. agents given for rapid induction of anaesthesia
Barbiturates
- thiopental
- induce anaesthesia in under 30 seconds due to high lipid solubility
- high overdose potential, which varies vastly between populations
Non-barbiturates
- propofol
- avoids dangers with barbiturates
- allows rapid recovery without nausea, but rarely causes convulsions
Ketamine
- given to children
- can cause hallucinations
Maintenance agents (6)
Gases used for maintaining anaesthesia
- rate of equilibrium determined by oil:gas partition co-efficient
- potency determined by lipid solubility
Nitrous oxide
- used as a carrier
- minute effect on cardiac and respiratory systems
Halothane
- high potency
- myocardial depression causes hypotension
- respiratory depression
- arrhytmias and cardiac arrest
- can cause severe hepatotoxicity due to high liver metabolism
Enflurance
- similar to halothane, but less metabolised
- can cause seizure activity and muscle twitching
Isofluorane
- similar to halothane
- less cardiac depression
- dose-related hypotension due to reduction of vascular resistance
- less hepatotoxicity risk
Desflurane
- rapid recovery
- requires high inhalation concentrations, thus can cause respiratory tract infection
Sevofluorane
- rapid recovery
- may require post-operative anaesthesia
Elimination of general anesthesia
Agents with low lipid solubility are eliminated quickly through the lungs
- minimal metabolism, with some occurring in liver
Malignant hyperthermia
A rare, inherited, and potentially lethal syndrome associated with general anaesthesia (mainly maintenance agents)
Characterised by hyper-metabolic activity, marked CO2 production, altered skeletal muscle tone and metabolic acidosis
Treated with dantrolene, a Ca2+ channel blocker
