General anaesthetics Flashcards
Uses
Abolish awareness and response to Pain
Used in surgery; loss of consciousness (including memory),supression of pain (analgesic), supression of skeletal muscle reflexes and tone.
Disadvantages
Low TI - low margin for error
Must be treated with absolute caution due to respiratory and bp depression (especially in elderly and infants)
Liver/kidney damage - rare, due to toxic metabolites forming.
4 stages of general anaesthesia
Stage I - analgesia
Stage II - excitement ( dangerous stage: reflexes, coughing, vomiting, choking, irregular respiration
Stage III - surgical anaesthesia (regular respiration, patient unconcious and cannot feel pain)
Stage IV - medulary paralysis ( overdose, CVS and resp shutdown)
Analgesia
Suppression of pain inputs at spinal level ( substantia gelatinosa)
Unconsciousness
Reticular activating system (thalamocortical tract)
Muscle reflexes
Decreased transmission at spinal interneurones
Amnesia
During and after surgey due to affect on hippocampus
No ideal GA
Often given in combinations as well as premeds. Using combination also allows you to use less of each, which is better for the patient, and have more fine control.
Pharmacokinetics of GA’s
Dependant on blood gas partition coefficient.
Low value = rapid induction, short acting (less soluble in blood so easily removed from blood into tissues)
Potency
Dependant on oil-gas partition coefficient
High value =highly lipid soluble, high potency
Main routes of administration
I.V - short acting, mainly used for induction anaesthesia, not essily reversible, can storein fat and be released at random times, diffuses away from cns quickly into other body parts
Inhalation - longer acting, used for maintenance anaesthesia, much more easily controlled, action ends on exhalation. Some yraces found in other parts of the body. Pulse, respiration, pupil dilation and reflxes used to monitor level of anaesthesia.
Structures of inhalation anaesthetics
Mainly hydrocarbons and ethers
N2O - gas and air, analgesic, childbirth and suplements others in surgery
Halothane - commonly used,rare problem of hepatotoxicity due to tri-fluoro-acetic acid
Xenon - expensive so not really used
I.V anaesthetics
Etomidate - hangover effect due to metabolism slowly working to get rid of lingering stores, less cardiorespiratory depression.
Propofol - rapid metabolism and recovery, use in day surgery
Ketamine - dissociate anaesthesia, often used in paediatric surgery
Midazolam/Diazepam - low dose anaesthesic, hypnotic, unconscious, inducer.
Lipid theory
Increasing size and lipid solubilitu increases potency but there is a cut-off point.
Protein theory
GA’s bind to intramembrane proteins I.e ion channels.
Most anaesthesics potentiate action of GABA on chloride ion channels causing hyperpolarisation and therefore inhibition.
