General Anaesthetics Flashcards
What is GA used for?
- Produce unconsciousness and lack of responsiveness to all painful stimuli
- Balanced anesthesia:
~ Triad of hypnosis/unconsciousness, amnesia and analgesia/pain relief
~ Inhalation + IV used together
What are the stages of GA?
1) Pre-assessment
2) Induction of amnesia
3) Airway management
4) Maintenance of analgesia
5) Reversal/emergency
6) Post-op
What are the most common drug types for each point of balanced anaesthesia?
Induction:
- Short acting barbiturates
Muscle relaxation:
- Neuromuscular blocking agents
Analgesics:
- Opioids and nitrous oxide
What is the tissue distribution for gaseous anaesthesia?
Gas -> Lung -> Blood -> Brain and other tissues
Higher the solubility in blood = slower onset
- If it is not very soluble in blood, it accumulates in the brain quicker
~ Nitrous oxide has low solubility
~ Halothane has high solubility
~ NO < Sevo < Iso < Enflurane < Halothane
What are the classification of gaseous anaesthesia?
Gases:
- NITROUS OXIDE
Volatile liquids (need to be vaporised)
- HALOTHANE
- Enflurane
- Desflurane
- ISOFLURANE
- SEVOFLURANE
What is the MOA of GA?
1) Increases GABA receptor sensitivity to action
- Enhances neurotransmission at inhibitory synapses
- Positive allosteric modulator
2) Blocks glutamate neurotransmitter acting on NMDA receptor
- Depresses neurotransmission at excitatory synapses
- Prevents NMDA receptor activation
- Negative allosteric modulator
What is minimum alveolar potency?
- Anesthetic potency
~ Low MAC = high potency
~ Min. conc. of a drug in the alveolar air that will produce immobility in 50% of px exposed to a painful stimulus - Nitrous oxide has the lowest potency
~ Des < Sevo < Isoflurane (highest potency)
What are the pharmacokinetics of gaseous anesthetics?
- Must reach CNS conc. sufficient to suppress neuronal excitability
Absorption:
- Conc. of anesthetic in inspired air
- Solubility of GA
- Blood flow through lungs
Distribution:
- Determined by regional blood flow
- In highly perfused organs, tissue anesthetic levels quickly equilibrate with blood leveks after administration
~ Brain, liver, lungs, heart
Elimination:
- Almost entirely by lungs/exhalation
- Minimal hepatic metabolism
- Factors that determine uptake also determine elimination
Metabolism:
- Isoflurane and enflurane fluorides are nephrotoxic
- Halothane is hepatotoxic
What are the traits of halothane?
- Potent
- Medium rate of onset and recovery
- Analgesia
~ Little to no UNTIL unconsciousness occurs - Relaxant
~ Relaxes skeletal muscle and potentiates relaxants - Not compatible with epinephrine
Adverse effects
- Causes respiratory depression dose-dependently
- Decreases BP due to depression of CO
- Bradycardia or arrhythmia
~ Leads to hypotension and dysrhythmia
- May lead to halothane-associated hepatitis
What are the traits of isoflurane and sevoflurane?
Isoflurane:
- Pungent
- Potent
- Medium rate of onset and recovery
- Similar to halothane with less hypotension and arrhythmia
- Decreases BP mainly due to dec in systemic vascular resistance
Sevoflurane:
- Potent
- More rapid rate of onset and recovery
- Metabolized in the liver
~ Nephrotoxic
- Unstable
~ When exposed to CO2 absorbents in anesthesia machines
~ Degrades into a nephrotoxic compound
What are the traits of nitrous oxide?
- Odorless
- Rapid-onset and recovery
- Lacks potency
- Gives analgesia and amnesia
- Does not give complete unconsciousness or surgical anesthesia
~ Supplements the analgesic effects of primary anesthetic instead (eg NO + halothane)
~ If used alone, only as an analgesic agent in situations where px does not need to be unconscious (eg dentistry) - Not much effects on BP and respiration
Side effects:
- Major postop N/V
What are IV GA used for?
- Mostly as an induction agent to induce consciousness
~ But does not necessarily keep asleep for long - Depresses respiration
~ Manual respiration needed - May be used alone or to supplement gaseous agents
- THIOPENTONE
- PROPOFOL
- KETAMINE
- Etomidate
- Midazolam
What are the advantages of using gaseous + IV anesthetics together?
- Permits dosage of gaseous agent to be reduced
- Produces effects that cannot be achieved with inhalation alone
What are the traits of thiopentone/sodium thiopental?
- Barbiturate with high lipid solubility
~ Enters brain easily and rapidly
~ Within 10-20 sec after IV - Potentiates action of GABA on GABA ion channels
~ Causes CNS depression (due to hyperpolarization of cell) - Single dose
~ Re-distributes to less vascularized tissues (ultra-short duration) - Multiple doses
~ Duration of action depends on clearance - Slow elimination
- Large volume of distribution
~ Easily goes to peripheral organs (especially fatty ones) - Active metabolite (pentobarbital)
~ Complicates liver cirrhosis - Extensively bound to plasma protein
~ Small amount of free drug can be excreted by kidneys
What are the traits of propofol?
- No need to reconstitute
- Induction rate similar to thiopentone, but more rapid recovery rate
- Used both for induction and maintenance
- Short duration of action
~ Due to rapid redistribution from brain to other tissues
~ Needs continuous, low-dose infusion for extended effects - Has reduced postop vomiting
Side effects:
- Decreased BP, hypotension during induction
~ To be used w caution in elderly px, cardiac dysfunction px and hypovolemic px
