General Anaesthetics Flashcards
What is a GA?
an agent used as an adjunct to surgical procedures which renders the patient unconscious or unaware of, and unresponsive, to simulation that would normally lead to pain, stress and/or discomfort
What 4 things do GAs produce?
- loss of sensation
- loss of consciousness
- loss of memory (amnesia) for the period under the influence
- loss of most movement (muscle relaxation)
What are GAs not?
analgesics or hypnotics
Why is alcohol not a good GA?
it does not allow for rapid induction, smooth maintenance and control or a rapid recovery (hangover effects)
What are the 4 stages of older versions of anaesthesia?
- analgesia without amnesia
- excitement - loss of consciousness, no response to non-painful stimuli but reflex responses to painful stimuli
- surgical anaesthesia - spontaneous movement ceases and respiration is regular
- medullary depression/paralysis - respiration and vasomotor control cease and death ensues in minutes
Why is stage 2 the danger stage of older versions of anaesthesia?
the subject may move, talk incoherently, hold their breath, choke or vomit
How does modern amnesia avoid the stages?
balanced anaesthesia
What is balanced anaesthesia?
the use of a combination of agents to take the patient to the desired state as rapidly and smoothly as possible
What are the 2 classes of GAs in human pharmacology?
inhalation and intravenous anaesthetics
Give examples of common inhalation anaesthetics
- ether
- nitrous oxide
- halothane
What is the partition coefficient of inhalation anaesthetics?
the solubility in different media defined as the ratio of the concentration of the agents in two phases at equilibrium
What is the main factor of inhalation anaesthetics?
the blood-gas partition coefficient
What does the blood-gas partition coefficient determine?
the rate of induction and recovery; the lower the coefficient, the faster the induction and recovery
What inhalation anaesthetics have high and low blood-gas partition coefficients and what does this mean?
- nitrous oxide - low i.e. fast acting
- ether - high i.e. slow acting
What is the oil-gas partition coefficient?
a measure of fat solubility that correlates with the potency of the anaesthetic
What does a high oil-gas partition coefficient indicate?
high potency but slow kinetics and a long recovery (hangover)
What inhalation anaesthetics have high and low oil-gas partition coefficients and what does this mean?
- halothane - high i.e. highly potent but can accumulate in fat
- nitrous oxide - low i.e. low potency but rapidly eliminated
What is the minimum alveolar concentration (MAC)?
a measure of the concentration of anaesthetic required in the lungs to induce anaesthesia
What is the relationship between MAC and potency?
inversely proportional
What inhalation anaesthetics have high and low MACs and what does this mean?
- halothane - low i.e. low concentrations required to induce anaesthesia
- nitrous oxide - high i.e. high concentrations required to induce anaesthesia
Overall, what does nitrous oxide have?
a fast induction/recovery, a low potency and an excellent analgesic property
What are adverse effects of halothanes?
- malignant hyperthermia in genetically susceptible individuals
- hangover due to lipid solubility and accumulation in fat (worse for halothane than other halogenated anaesthetics)
- liver toxicity, especially with repeated use
Why is nitrous oxide often used for induction?
due to its speed of action and strong analgesic effect
What is the lipid theory (Meyer-Overton correlation)?
the greater the lipid solubility of a compound in olive oil, the greater its anaesthetic potency
What do many inhalation anaesthetics do?
- enhance inhibitory GABAA receptor currents
- inhibit excitatory receptors e.g. nitrous oxide interacts with NMDARs and nAChRs
How do non-immobilisers (e.g. hexafluorethane) break the Meyer-Overton correlation?
they should be potent anaesthetics according to their oil-gas partition coefficients, but they appear to interact with hydrophobic sites to produce amnesia but not immobility in response to stimuli
What are intravenous anaesthetics usually used for?
the induction of GA, followed by an inhalation agent
Why are intravenous anaesthetics used?
they have a fast onset of action (20-30s) and are less stressful than a facemask
Why are intravenous anaesthetics rarely used for maintenance?
they have a slow elimination compared to inhalation anaesthetics, therefore have a slow recovery and often produce hangover effects
What is the primary mechanism of action of intravenous anaesthetics?
enhancement of GABAA currents/activity
What are the properties of propofol?
- rapid onset (~30s) due to its rapid transition of the BBB
- rapid rate of distribution
- short action and little hangover effect due to rapid metabolism to its inactive form
- less nausea and vomiting than inhalation anaesthetics
What is the exception to intravenous anaesthetics being used for maintenance?
propofol
What is the mechanism of action of propofol?
- potentiates GABAA receptor activity
- sodium channel blocker
- possible involvement in endocannabinoid system (regulates cognitive and emotional processes in CNS)
What are adverse effects of propofol?
- cardiovascular depressant – can cause hypotension and bradycardia
- occasionally can cause respiratory depression
- pain at site of injection
- propofol infusion syndrome
What can cause propofol infusion syndrome?
prolonged administration of high doses (especially in children)
Give examples of symptoms of propofol infusion syndrome
- metabolic acidosis
- skeletal muscle necrosis
- hyperkalaemia
- lipaemia
- hepatomegaly
- renal failure
- arrythmia
- cardiovascular collapse
