GA Flashcards
What is General Anaesthesia used for?
To produce
1. unconsciousness
2. lack of responsiveness
to all painful stimuli (inhibition of sensory or autonomic reflexes)
General anaesthesia stages (6)
- Pre-assessment/Premed
- Induction
- Airway management
- Maintenance of anaesthesia
- Reversal/emergency
- Post-operative care
Ideal properties of GA (8)
- unconsciousness
- analgesia
- muscle relaxation (inhibit reflex)
- amnesia
- brief & pleasant
- depth of anaesthesia can be altered easily
- minimal adverse effects
- margin of safety large
Triad for balanced anaesthesia
- pain relief
- reflex inhibition
- unconsciousness (induction)
Why is there a need for balanced anaesthesia?
To ensure smooth and rapid induction & that analgesia and muscle relaxation are adequate
Types of GA (2)
- Inhalation anaesthetics
2. IV anaesthetics
Inhalation GA (3+1)
Volatile liquids
- Halothane
- Isoflurane (pungent)
- Sevoflurane
Gasesous
1. Nitrous oxide
MOA of inhalation GA (2)
- Enhance neurotransmission at inhibitory synapse via allosterically, increasing GABA receptors sensitivity to GABA itself (positive allosteric modulation)
- Depress neurotransmission at excitatory synapse via blocking glutamate neurotransmitter acting on NMDA receptors, prevent activation of NMDA receptors (negative allosteric modulation)
MAC
Minimum Alveolar Concentration
- index of INHALATION anaesthetic potency
- minimum concentration in alveolar air that will produce immobility to 50% of patients exposed to a painful stimulus
- decrease MAC = increase potency
- MAC alters with age, condition and concurrent drug use
Factors affecting absorption of inhalation GA
- conc of anaesthetic in inhaled air
- blood perfusion of lungs
- solubility of anaesthetic in blood
increase means increase absorption
BUT
increase solubility in blood means slower onset of action
Factors affecting distribution of inhalation GA
- perfusion of organ/tissue
highly perfused organ (eg brain, lungs, liver & heart) will achieve equilibrium quickly after administration
Factors affecting elimination of inhalation GA
- conc of anaesthetic in inhaled air
- perfusion of lungs
- solubility of anaesthetic in blood
- increase means increase elimination
- most inhaled GA are eliminated through expired breath
Halothane
- first modern inhaled anaesthetic
- potent (MAC 0.75%)
- medium rate of onset & recovery
- little or no analgesic until unconsciousness supervenes
- relaxes muscles & potentiates skeletal muscle relaxants
Halothane ADR (3)
- respiratory depression (dose dependent)
- bradycardia & arrhythmias (hypotension & dysrhythmia)
- halothane-associated hepatitis
Isoflurane
- pungent smell
- potent (MAC 1.4%)
- medium rate of onset & recovery
Isoflurane ADR (2)
- bradycardia & arrhythmias
- similar to halothane but reduced hypotension & arrhythmias
- hypotension due decrease systemic vascular resistance - metabolism of isoflurane produce inorganic F (nephrotoxic)
Sevoflurane
- potent (MAC 2%)
- more rapid rate of onset & recovery than halothane
- metabolised by liver
Sevoflurane ADR (2)
- metabolism of sevoflurane produce inorganic F (nephrotoxic)
- usage of CO2 absorbent in vaporiser produce nephrotoxic agent
Nitrous oxide
- aka dinitrogen oxide
- rapid onset & recovery but lack potency (MAC 110%)
- analgesia & amnesia but no complete consciousness/surgical anaesthesia
- only can used to supplement analgesic effect for primary anaesthetic
- used alone for analgesic effect (eg dentist & labour)
Nitrous oxide ADR
post-operative N/V
propofol have less post-operative N/V
IV GA (3)
- Thiopentone
- Propofol
- Ketamine
Advantages of using a combination of inhaled + IV (2)
- reduce dose of inhaled GA
2. produce additional effects that cannot be achieved with an inhalation alone
Thiopentone / thiopental
- high lipid solubility
- MOA : enhance GABA binding to GABA receptors
- rapid onset & recovery (high lipid solubility)
- extensive plasma protein binding (mainly hepatic elimination)
- duration of action depends on :
1. elimination rate
2. hepatic function (cirrhosis)
3. Vd
4. active metabolite (pentobarbital)
Propofol
- rapid onset & faster recovery
- used for both induction & maintenance (continuous infusion of low dose)
- used for day surgery
Propofol ADR
- N/V (but lesser than nitrous oxide, anti-emetic)
- Cardiovascular effect (bradycardia & hypotension)
- to be used in caution in elderly, compromised cardiac function & hypovolemic
Ketamine
- racemic but S- > R+
- produce dissociative anaesthesia
- produce sedation, amnesia, analgesia & immobility
- rapid induction and recovery
- used for both induction & maintenance (continuous infusion of low dose)
- hepatic metabolism to less active metabolite
Ketamine ADR
- psychological reactions (hallucination, delirium)
- treat with premedication of diazepam & midazolam
- dissociative anaesthesia
Blood solubility and onset of action
Increase blood solubility, longer time taken for onset of action
Affect onset only, not efficacy
Halothane vs Isoflurane in BP reduction
Halothane
- due to depression of cardiac output
Isoflurane
- due to decrease in systemic vascular resistance
Advantages of Ketamine (4)
- Analgesia
- Sedation
- Amnesia
- Immobility
GA with analgesia
- Ketamine
2. Nitrous oxide
Opioid agonist used together GA
Remifentanil
- 10min duration
- 300x more potent than morphine
