General Anaesthetics Flashcards
Uses of GA:
- Produce unconsciousness and lack of responsiveness to all painful stimuli – inhibit sensory and autonomic reflexes
- causes hypnosis, amnesia and analgesia
- allow for skeletal muscle relaxation
GA stages:
- Pre-assessment/premed
- Induction of anaesthesia
- Airway management
- Maintenance of anaesthesia
- Reversal/emergency
- Post-operative care
Traits of ideal GA:
- Unconsciousness
- Analgesia
- Muscle relaxation
- Amnesia
- Brief and pleasant
- Depth of anesthesia can be increased or decreased with ease
- Minimal adverse effects
- Large margin of safety
Drugs commonly used in combination in GA:
- inhalation and intravenous anaesthetics
- Short-acting barbiturates (induction)
- NM blocking agents (muscle relaxation)
- Opioids and nitrous oxide (analgesia)
What is MAC:
index of inhalation anaesthetic potency (the lower the MAC, the more potent it is)
- Defined as minimum concentration of drug in alveolar air that will produce immobility in 50% of patients exposed to painful stimulus
- Values alter with age, condition, concommitant administration of other drugs etc.
Examples of inhalation anaesthetics:
volatile liquids: halothane, enflurane, desflurane, isoflurane, sevoflurane
Gases: nitrous oxide
Possible MoA of Inhaled GA:
- enhance neurotransmission at inhibitory synapses via allosterically increasing GABA receptor sensitivity to action by GABA itself (positive allosteric modulator)
- Depress neurotrransmission at excitatory synapses via blocking glutamate neurotransmitter acting on NMDA receptor – prevent activation (negative allosteric modulator)
Factors affecting absorption of Inhaled GA:
- Concentration of anaesthetic in inspired air
- Solubility of GA in blood
- Blood flow through lungs
Factor affecting distribution of inhaled GA:
Regional blood flow – determines which tissues receive GA – highly perfused tissues will have the anaesthetic levels equilibrating with those in blood quickly after administration
mechanism of elimination of inhalation anaesthetics:
exported in expired breath
eliminated almost entirely via lungs
minimal hepatic metabolism
Metabolites of inhalation anaesthetics
- Inorganic fluorides(isoflurane and enflurane) – nephrotoxic
- Halothane – hepatotoxic
Adverse effects of halothane:
- Respiratory depression (dose-dependent)
- decreased BP due to depression of cardiac output
- Arrhythmia
- halothane-associated hepatitis
- (May be a use as well) relaxes skeletal muscle and potentiates skeletal muscle relaxants
Adverse effects of isoflurane:
- dose-dependent respiratory depression
- decreased BP due to decrease in systemic/vascular resistance
- less hypotension and arrhythmia compared to halothane
MAC of inhaled anaesthetics:
Halothane: 0.75%
Isoflurane: 1.4%
Sevoflurane: 2%
Nitrous oxide: 105-110%
mechanism of nephrotoxicity in sevoflurane:
metabolised in liver to release inorganic fluoride
Rank the time needed for onset and recovery of inahled anaesthetics:
- Isoflurane
- Halthane
- Nitrous Oxide
- Sevoflurane
nitrous oxide < sevoflurane < isoflurane and halothane
limitations of use of nitrous oxide in GA: :
give analgesia and amnesia but not complete unconsciousness/surgical anaesthesia
- used to supplement analgesic effects of primary anaesthetic
Major concern in use of nitrous oxide
post-operative nausea and vomiting
Things to take note of in the use of sevoflurane:
degrade to compound that is potentially nephrotoxic when exposed to carbon dioxide absorbants in anaesthetic machines
Types of intravenous anaesthetics
thiopentane, etomidate, propofol, ketamine, midazolam
Purpose of IV anaesthetics. Comment on its duration of action
- induce unconsciousness
- does not keep you asleep for very long – effects on its own is not very long