Anaesthetics Flashcards
What are the broad categories of anasthesia?
- General
- Local
What are the methods of administration of general anasthesia?
- Inhalation, or volatile
- Intravenous
What is the difference in onset of action between inhalational and intravenous general anaesthesia?
Intravenous is quicker
What is conscious sedation?
The use of small amounts of anaesthetics or benzodiazepines to produce a ‘sleepy-like’ state
What are the stages in general anaesthesia?
- Premedication
- Induction
- Intraoperative
- Muscle paralysis
- Maintenenance
- Reversal of muscle paralysis and recovery, which includes post-operative analgesia
- Provision for PONV
What is often used in the premedication stage of anaesthesia?
A hypnotic, typically benzodiazepine
How is anaesthesia induction normally performed?
Usually IV, but may be inhalational
Give an example of a drug used in anaesthesia induction
Barbituates
What class of drug is usually used for intraoperative analgesia?
Opioids
What is the purpose of muscle paralysis in surgery?
To facilitate intubation/ventilation/stillness
Why is intubation important in anaesthesia?
Analgesia can cause respiratory depression, so you need a relaxant to allow airway access
How is maintenance anaesthesia administered?
Intravenous and/or inhalation
What is used for postoperative analgesia?
- Opioids
- NSAID
- Paracetamol
How is volatile general anaesthesia administered?
You fill a reservoir with the liquid drug which then evaporates. Fresh gas flows across evaporated liquid and the patient inahles it
What molecules will act as anaesthetics?
Any molecule with a volitile compononet
Describe the use of xenon as an anaesthetic
It is a very good anaesthetic in very high concentrations, and is good in neonates to prevent cerebral ischaemia
Give four examples of intravenous anaesthetic agents
- Propofol
- Barbiturates
- Etomidate
- Ketamine
What are the stages in Guedel’s signs?
- Stage 1 - Analgesia and consciousness
- Stage 2 - Unconscious, breathing erratic. Delirium could occur, leading to an excitement phase
- Stage 3 - Surgical anaesthesia
- Stage 4 - Respiratory paralysis and death
Describe the muscle tone in Guedel’s stage 1
Normal
Describe the eye movements in Guedel’s stage 1
Slight
How long does Guedel’s stage one last?
Only the first few breaths
Describe the muscle tone in Guedel’s stage 2
Normal to markedly increased
Describe the eye movements in Guedel’s stage 2
Moderate
Describe the muscle tone in Guedel’s stage 3
Progresses from slightly relaxed to markedly relaxed
Describe the eye movements in Guedel’s stage 3
Progresses from slight to none
Describe breathing in Guedel’s stage 3
Decreases until significantly reduced respiratory effort
What are the options once a patient reaches Guedel’s stage 4?
- Wake patient
- Ventilate the patient
What is anasthesia a combination of?
- Analgesia
- Hyponosis
- Depression of spinal reflexes
- Muscle relaxation
What is the end point of anasthesia dependant on?
Concentration
In what order to functions go under anaesthesia?
- Memory
- Consciousness
- Movement
- Cardiovascular response
What measure is used to describe potency for volatile anaesthetics?
MAC - Minimum Alveolar Concentration
What is MAC?
The alveolar concentration of the anaesthetic at which 50% of subjects fail to move to surgical stimulus
What is the alveolar concentration of anaesthetic equal to at equilibrium?
The concentration of anaesthetic in the spinal cord
What is the anatomical substrate for MAC?
The spinal cord
What important factor affects induction and recovery?
The partition coefficients (solubility)
What are the potential partitions in anaesthetics?
- Blood:gas partition
- Oil:gas partition
What does the blood:gas partition coefficient measure?
The solubility of gas in the blood
What effect does a low blood:gas partition have on induction and recovery?
Means it will be fast
What does the oil:gas coefficient measure?
The solubility of gas in fat
What does the oil:gas partition coefficient determine?
The potency
Why does the oil:gas partition coefficient determine potency?
Because target sites are in membranes
What factors affect MAC?
- Age
- Body temperature
- Pregnancy
- Alcoholism
- Central stimulants
- Other anaesthetics and sedatives
- Opiods
How does age affect MAC?
It is high in infants, and low in the elderly
How does body temperature affect MAC?
It is increased in hyperthermia and decreased in hypothermia
How does pregnancy affect MAC?
Increases it
How does alcoholism affect MAC?
Increases it
How do central stimulants affect MAC?
Increase it
How do other anaesthetics and sedatives affect MAC?
Decrease it
How do opiods affect MAC?
Decrease it
How does administering anaesthetic agents with nitrous oxide affect MAC?
Significantly reduced
What is the advantage of reducing MAC using nitrous oxide?
Can provide an extra window of safety
Other than lipid solubility, what does anaesthetic potency correlate to?
GABAA activity
What is GABAA?
A major inhibitory neurotransmitter
What kind of receptor is the GABAA receptor?
Ligand gated ion channel
What happens when GABAA binds to its receptor?
It causes the opening of the ion channel, allowing flow of Na+ and therefore the membrane becomes more difficult to excite, leading to CNS depression
What are the effects of GABAA receptor activation?
- Anxiolysis
- Sedation
- Anaesthesia
Which anaesthetics potentiate GABAA mediated Cl- conductance to depress CNS activity?
All except for Xe, N2O and ketamine
What receptors do Xe, N2O and ketamine act on?
Probably NMDA receptors
What balance exists in the conscious brain?
Excitation (glutamate) and inhibition (GABA)
What effect do anaesthetics have on the balance in the brain?
They modulate it
What effect do anaesthetics have on brain structures?
- Reticular formation (hindbrain, midbrain, and thalamus) are depressed
- Hippocampus depressed, reducing memory
- Brainstem depressed, reducing respiratory and some CVS functions
- The dorsal horn of the spinal cord is depressed in analgesia
- The motor neuronal activity of the spinal cord is depressed with MAC
What does the reticular system normally do?
Increase arousal, hence it is called the activating system
What does the thalamus normally do?
Transmits and modifies sensory information
What are the main intravenous anaesthetics?
- Propofol
- Barbiturates
- Ketamine
What is the use of intravenous anaesthetics?
- Induction
- Can be used as the sole anaesthetic in TIVA (Total Intravenous Anaesthesia)
What happens in TIVA?
An infusion pump continues adminsteration, typically using propofol
How is intravenous anaesthetic potency described?
As the plasma concentration to acheive a specific end point, e.g. loss of eyelash reflex, BIS value
What is the end point used to determine in mixed anaesthesia?
The point at which to switch to volatile anaesthesia after induction
How does TIVA determine what dose to be giving?
It uses a defined PK based algorithm to infuse at a rate to maintain the set point
What are the uses of local and regional anaesthetics?
- Dentisty
- Obstetrics
- Regional surgery
- Post-op
- Chronic pain management
Give 4 examples of local anaesthetics
- Lidocaide
- Bupivacaine
- Ropivacaine
- Procaine
What factors determine the characterstics of local anaesthetics?
- Lipid solubility
- Dissociation constant
- Chemical link
- Protein binding
What does the solubility of local anaesthetics determine?
The potency - the higher the lipid solubility, the higher the potency
What does the dissocation constant determine in local anaesthetics?
The time of onset - lower the pKa, the faster the onset
What does the chemical link determine in local anaesthetics?
The rate of metabolism - ester links don’t last as long, as tissues possess lots of esterases, whereas amides last longer
What does protein binding effect in local anaesthetics?
Duration - the more the binding, the longer the duration
What is regional anaesthesia?
Selectively anaesthetising a part of the body
What is regional anaesthesia often described as?
A ‘block’ of a nerve
What agents to regional anaesthetics use?
A local anaesthetic, with or without an opiod
What are the sites of regional anaesthesia in the upper extremity?
- Interscalene
- Supraclavicular
- Infraclavicular
- Axillary
What are the sites of reginal anaesthesia in the lower extremity?
- Femoral, sciatic, popliteal, saphenous
What regional anaesthetics are used in labour?
- Extradural
- Intrathecal
- Combined
What are the main side effects of general anesthesia?
- PONV (with opiods)
- Hypotension
- Post-operative cognitive dysfunction
- Chest infection
What increases the risk of post-operative cognitive dysfunction?
Increasing age
What do side effects of local and regional anaesthesia usually result from?
Systemic spread
What might result in systemic spread of anaesthetics?
- Wrong dose
- Wrong site of administration
Give an example of a systemic side effect of local anaesthetics
Cardiovascular toxicity, as locals are sodium channel blockers