General Anaesthetics Flashcards
What is an anaesthetic?
Anaesthesia = “without sensation”
reversible loss of awareness
What is local anaesthetic?
Local
- patient remains conscious
- cheaper, safer
What is a regional anaesthetic?
larger area of the body involved
What is general anaesthetic?
- loss of consciousness (central effect)
- major surgery can take place
How many stages of anaesthesia are there?
STAGES dependent on dose and with some MOA, the dose will not ramp up instantly into the range that we need for full surgical ANAETHESHIA
What is stage 1 of anaesthesia:
- Pt is conscious but drowsy
- Reduced responses to pain
- Amount of time that patient will stay in stage 1 depends on agent used
What is stage 2 of anaesthesia:
Dangerous phase - paradoxical things happening
- gag reflex, coughing increased
- Responses to pain preserved
- Loss of response to non painful stimuli
Concerns:
- Choking
- Breath holding
- Talking
- Vomiting
- Movement
What is stage 3 of anaesthesia:
The desired phase for surgery:
- Surgical anaesthesia
- Regular respiration
- Possibly some reflexes, muscle tone preserved
- Movement ceases
- Progressive shallowing of breathing
What is stage 4 of anaesthesia:
- Unless you do something about it your patient will die
- Overdose
- Medullary paralysis
- Respiration and vasomotor control ceases
Pharmacokinetics of General Anaesthesia:
- We would like rapid induction and rapid recovery
- We would prefer to avoid stage 2
- We would prefer the patient not to die in stage 4
- We would prefer to avoid side-effects
is there one single drug for anaesthesia?
Therefore, drugs often used in combination
- different anaesthetics
- analgesics, muscle relaxants, anxiolytics
Stages of anaesthesia become less apparent
Modern General Anaesthesia drugs used:
- rapid induction of unconsciousness; i.v. propofol
- maintenance of unconsciousness and production of anaesthesia; inhaled N2O/halothane
- supplementary i.v. analgesic e.g. morphine
- neuromuscular blocker e.g. atracurium
- Fast induction and recovery (anxiety, hangover), reduces stage II, homeostatic reflexes remain intact, amnesia
How do general anaesthetics work?
- Alter function of neurones
- Protein theories
- lipid theory
- Structures very diverse, argues against unified theory
What are the volatile anaesthetics?
- N2O
- Xe
What are the iv anaesthetics:
- Propofol
- Sodium thiopental
- Etomidate
What is the protein theory:
- GABAA receptors (inhibitory) (many volatile, IV anaesthetics. Not: xenon, ketamine)
Function POTENTIATED
- NMDA receptors (Ketamine, xenon, volatiles)
Function INHIBITED
- Two pore potassium channels
Function POTENTIATED
- We have targets on membrane proteins. The binding sites for the anaesthetic located in lipid bilayer and an.
- May work by dissolving in the bilayer, accessing the binding site on their protein and changing the behavior of the protein.
- Since these ideas started to crytallise we’ve actually identified. We now know on which proteins some of these drugs work.
Types of anaesthetic:
- Inhalational
- Intravenous
- Neurolept
- Dissociative - ketamine
Inhalation anaesthetics – pros and cons:
Easy to maintain degree of anaesthesia
(fast air:blood equilibration)
+Rapid emergence from anaesthesia
- Cumbersome and expensive apparatus
- Administered via a mask – psychological effects
- Atmospheric pollution (scavengers) – operating staff exposed all day everyday
Inhalation anaesthetics – metabolism and toxicity:
- Metabolism unimportant for elimination BUT! Toxic metabolites
- Fluranes generate fluoride, causes renal toxicity
- -> Halothane converted to bromide and TFA, hepatotoxic
- Problem for theatre staff
- -> liver disease, leukaemia, abortion, birth defects
- Scavenger systems required
How do inhalation anaesthetics enter and leave the body:
Enter and leave the body via the lungs
Metabolism generally unimportant for recovery
Pass from gas phase to blood, then to tissues
What is the Minimum Alveolar Concentration (MAC)?
The concentration in the alveoli required to produce anaesthesia in 50% of
patients
Measures the potency of the anaesthetic
What is Blood-gas partition coefficient?
- A measure of how well the drug dissolves in blood
- Determines rate of induction and recovery
- We want it to be low
Oil-gas partition coefficient:
- High oil-partition coefficient confers high POTENCY (see previous slide)
- BUT! Lots of the anaesthetic will dissolve in fat
- Fat is poorly vascularized
- Anaesthetic will take a long time to leave this tissue
- Patient will have a slowly resolving “hangover” as lots of the drug will dissove in fat
- This will be worse the fatter the patient (and the more fat soluble the drug)
Recovery from anaesthesia:
Recovery from anaesthesia often has two phases.
The first, rapid phase will depend on the blood:gas coefficient as for induction: the lower the blood solubility, the faster the recovery. However, there is a slow phase too. this is due to anaesthetic dissolving in fatty tissues. The patient is likely to be conscious but “groggy” during this part of recovery.
