General Anaesthetics Flashcards
What are general anaesthetics used for?
- To produce unconsciousness and lack of responsiveness to all painful stimuli (inhibition of sensory and autonomic reflexes)
- i.e. hypnosis, amnesia, analgesia
- Provide condition for interventions (e.g. surgery to occur)
- Control of physiology (elevated heart rate, control body temperature)
What are the different stages of general anaesthesia?
- Pre-medication assessment (PREMED)
- Induction of anaesthesia
- Airway management
- Reversal/emergency
- Post-op care
What are the ideal properties of general anaesthesia?
- unconsciousness
- analgesia
- amnesia
- muscle relaxation
- brief and pleasant
- depth of anaesthesia can be adjusted
- minimal adverse effects
- large safety margin
What are the different elements needed for anaesthesia to be balanced?
Possess optimal:
- muscle relaxation properties
- pain relief properties
- sedation properties
Most commonly used anaesthesia
- Barbiturates (provide rapid onset of action)
- Neuromuscular blocking agents (relax muscles)
- Opioids and nitrous oxide (for analgesia)
What is the main concept behind issue distribution of inhalant GA
The more soluble the drug, the faster its onset of action.
Halothane vs nitrous oxide (states of anaesthesia)
Halothane - volatile liquid
Nitrous oxide - gases
Halothane is much more soluble in blood / has a faster onset of action than nitrous oxide.
What are the proposed mechanism of actions of general anaesthetics?
- enhance neurotransmission at inhibitory synapses via allosterically increasing GABA receptor sensitivity to action by GABA itself (positive allosteric modulator)
- also depressing neurotransmission at excitatory synapses via blocking glutamate neurotransmitter acting on NMDA receptor, thus preventing NMDA receptor activation (negative allosteric modulator)
What is the minimum alveolar concentration (MAC)?
The minimum concentration of gaseous anaesthetics required to achieve immobility in 50% of patients exposed to a painful stimulus.
“MAC is a fixed concept.” True or false?
False. Concept of MAC varies with age, condition and concomitant administration of medications
Absorption of volatile liquids anaesthetics
- concentration of anaesthetics in air
- solubility of anaesthetics
- airflow in lungs
Distribution of volatile liquids anaesthetics
Determined by regional blood flow –> which tissue receive GA
Metabolism of volatile liquids and anaesthetics
- Metabolites can be toxic
- Halothane: hepatotoxic, isoflurane + enflurane (nephrotoxic)
Elimination of volatile liquids and anaesthetics
Mainly via the export of expired breath
Halothane characteristics
- Volatile liquid, non-flammable, non-irritating
- Little or no analgesia until unconsciousness sets in.
- Decreases BP due to depression of cardiac output. Bradycardia and arrhythmia can lead to hypotension and dysrhythmia
- May cause halothane-associated hepatitis
Isoflurane characteristics
- MAC potency (1.4%)
- Medium onset of recovery
- Decreases BP due to reduced systemic vascular resistance
Sevoflurane characteristics
- Potent (MAC 2.0%)
- Metabolised in the liver to release inorganic fluoride, also nephrotoxic
Nitrous oxide characteristics
- Non-flammable
- Nitrous oxide alone gives analgesia and amnesia, but not unconsciousness and surgical anaesthesia
- When used alone: as analgesic agent
Dosing of different IV general anaesthetics
Thiopentone: 4-7mg/kg Etomidate: 0.2 - 0.3 mg/kg Propofol: 2-4mg/kg Ketamine: 1.5mg/kg IV Midazolam: 0.02mg/kg
Characteristics of general anaesthetics
- induction agent is a substance that induces unconsciousness
- most agents depress respiration: you will need to take over ventilation of patients
- may be used to supplement the effects of inhalation agents
Advantages of administering inhaled and intravenous anaesthetics together?
- Permit dosage reduction of the inhaled anaesthetic
2. More effective than administering inhaled anaesthetic alone
thiopentone characteristics
- enters the brain easily and rapidly, rapid onset of action (~10-20s after administration)
- multiple doses/infusions: duration of action depends on clearance
- extensively bound to plasma protein, eliminated renally via glomerular filtration and tubular reabsorption
thiopentone MOA
allosterically modifies GABAa receptor so as to allow more GABA molecules to bind to it, depressing CNS effects.
Propofol characteristics
- induction rate is similar to thiopentone, recovery is rapid
- rapid onset (kicks in after t60s)
- extensively used in day surgery
- significant cardiovascular effects (decreases b.p. and negative inotropic) –> hypotension
Ketamine characteristics
- can cause dissociative anaesthesia
- rapid induction
- large volume of distribution, rapid clearance –> allows for continuous infusion while lengthening the duration of action
- risk of psychologic adverse effects associated with diazepam and midazolam
What are the functions of anaesthetic adjuncts?
Augment GA –> sedation, analgesia, amnesia
Lower GA doses used –> reduce risk of adverse effects
Benzodiazepine: Midazolam IV characteristics
- used for sedation during procedures not requiring GA
- metabolised in the liver (elderly are more sensitive, slower recovery)
- side effects are compounded by concurrent usage of other agents
Alpha-adrenergics: dexmedetomidine IV
- short term sedation (<24 hours)
- little respiratory depression
What are the different types of analgesics (NSAIDS)?
- Opioids (fentanyl, morphine): used in the perioperative period
- Relative potency to morphine [duration of action]:
» Sufentanil, remifentanil, fentanyl, alfentanil
» Remifentanil (ultra-short), the others are intermediate acting - Metabolised in liver
Neuromuscular blockers characteristics
Non-depolarising (e.g. vecuronium)
Aids many surgical procedures and provides additional insurance of immobility
