Introduction Flashcards
What are the 4 main objectives of anaesthesia?
- To work with zero morbidity and mortality
- Make patient unaware of surgery and recover pain free
- Make some surgery posisble and all surgery as easy as possible
- To support psychological function and counter adverse effects of surgery
What are 7 differences between sleep and anaesthesia?
- Respond to pain in sleep, not in anaesthesia
- Move around in sleep vs. no movement in anaesthesia
- Can dream in sleep but not anaesthesia
- Maintain your airway in sleep (snoring/apnoea if not fully maintained), but lose this in anaesthesia
- Normal breathing in sleep, depressed ventilation and altered CO2 response in anaesthsia
- Can swallow in sleep, reduced swallowing under anaesthesia (pharyngeal secretions sometimes build up)
- Sleep EEG and anaesthesia EEG are very different
What are the 3 components of general anaesthesia?
- Hypnosis - ‘sleep’, lack of awareness
- Muscular relaxation - preventing movement in response to surgery
- Analgesia - obtund painful stimuli (note - can’t be in pain if anaesthetised but can respond - use analgesics to block response)
What is meant by a ‘balanced anaesthesia’?
Traditional name fo the combination of drugs: anaesthetic, aanlgesic and a if needed a muscle relaxant (this combo is given in addition to the large dose of IV or inhalational anaesthetic)
What are the 2 key things needed to achieve the components of anaesthesia?
- Sufficiently large dose of intravenous or inhalational anaesthetic
- Using a combination of drugs:
- anaesthetic at much lower dose
- analgesic
- muscle relaxant if needed
What are 2 broad approaches to anaesthesia?
- Single agent anaesthesia
- Balanced technique
What is the advantage of single-agent anaesthesia?
Very simple, only one delivery system needed
What is the key disadvantage of single-agent anaesthesia?
As such a large amount of one drug is needed, it can lead to respiratory and cardiac depression - “not taking the drug well” e.g. chloroform and ether in the past, can lead to death due to profound respiratory and cardiovascular depression
In modern times, what is single-agent anaesthesia used for?
Now used for short procedures only, e.g. resetting a wrist fracture: single shot of IV agent, IV agent infusion or inhaled in oxygen
What are 2 benefits of the balanced technique of anaesthesia?
- Each component contributes its effects
- Side effects are reduced
What is a key disadvantage of the balanced technique of anaesthesia? Give 3 examples.
Requires attention to detail
- Too little anaesthetic agent = awareness
- Too little analgesia = excess reponse to noxious stimuli
- Too little relaxant = unwanted movement, or unwanted muscle tone - e.g. makes abdominal surgery different
What are 2 steps to general anaesthesia and how can they each be achieved?
- Induction: can be induced by intravenous or inhalational agents
- Maintenance: can be maintained by intravenous or inhalational agents
What are 2 ways that general anaesthetics can be combined with other drugs?
- Can be combined with analgesics and muscle relaxants as in balanced anaesthesia
- Combined with local or regional anaesthesia
What are 2 states of patient care that general anaesthesia can be administered in?
- Patient breathing spontaneously OR
- ventilation provided artificially
What is sedation?
Form of semi-hyponosis in which the patient is rousable, can talk, but may have no memory of events (e.g. midazolam especially good at blocking memory)
Why is it important that a sedated patient is able to talk?
Means you must be able to maintain your airway
Where does sedation turn into anaesthesia and why is it important?
Some deep sedation may be like anaesthesia, need to think about maintenance of airway in these patients
What are the 2 types of local anaesthesia?
- Topical e.g. cream
- Infiltration e.g. before putting in venflon, dentist
What are 5 types of regional anaesthesia?
- Peripheral nerve block
- Ganglion/ plexus block
- Epidural
- Spinal
- Intravenous regional
What 2 types of anaesthesia are being increasingly combined?
General and regional anaesthesia
How does general anaesthesia act on the body’s response to pain?
Prevents pain that is generated from being interpreted as pain by the central nervous system; doesn’t stop transmission of painful stimuli from the source of pain
How do local and regional anaesthesia act with regards to pain?
Prevent transmission of a painful stimulus reaching the central nervous system
Why might local anaesthesia be combined with general anaesthesia or sedation on some occasions?
- When combined with general anaesthesia, local anaesthesia used as the analgesic part of balanced anaesthesia
- When combined with sedation, can take advantage of the benefits of both approaches
What is the chemical structure of most local anaesthetics?
- Most have a lipid soluble, hydrophobic aromatic group and a charged, hydrophili amide group and a charged, hydrophilic amide group.
- There is an amide or ester bond between groups
What are 3 examples of amide-bonded local anaesthetics?
- Lignocaine
- Bupivacaine
- Prilocaine
What are 2 examples of ester-bonded local anaesthetics?
- Cocaine
- Amethocaine (used topically as lozenges)
What does the metabolism of ester-bonded LAs result in?
The production of para-aminobenzoate (PABA) = associated with reasonably high incidence of allergic reactions
How do local anaesthetic agents work?
- Inhibit sodium influx through sodium-specific ion channels int he neuronal cell membrane (voltage-gated sodium channels)
- Are weak bases (B), usually available as hydrochloride solutions
- At physiologic pH, both the ionised (BH+) and unionised forms (B) of the molecule exist; only the ionised form is able to block sodium channels
Why is pH important in the way local anaesthetic agents work?
pH is important in determining the ratio fo ionised to unionised forms of the LA molecules, as they’re an acid-based compound; only the unionised form diffuses readily across cell membranes
In what form is the LA agent given?
Weak base injected as a hydrochloride salt in an acid solution - tertiary amine group becomes quarternary and suitable for injection i.e. dissolves in solution
What happens once an LA agent has been injected?
pH increases (due to higher pH of the tissues, which is usually 7.4) and the drug dissociates, the degree of which depends on pKa, and free base is released
What happens once the LA agent has been injected and it has dissociated?
Lipid soluble free base (B), i.e. the unionised form, enters the axon. Inside the axon, pH is lower/more acidic, and re-ionisation takes place. The re-ionised portion enters the Na+ channels and blocks them, preventing depolarisation
Give the 5 steps to summarise how LA agents work.
- Weak base exists as hydrochloride salt in acid solution
- Injected and dissociates to B and BH+
- Unionised, free base form B crosses neuron cell membrane and enters axon
- Re-associates in axon due to lower pH
- Re-ionised portion enters Na+ channels and blocks them
By which route should local anaesthetic never be given?
Intravenously
How safe are local anaesthetic agents generally?
In right dosage and properly injected, they are very safe
What negative effects can occur as a result of local anaesthetics if injected IV, with increasing concentration?
- 4mg/ml: light headedness, tinnitus, circumoral and tongue numbness
- 6mg/ml: visual disturbances
- 8mg/ml: muscular twitching
- 10mg/ml: convulsions
- 12mg/ml: unconsciousness
- 15mg/ml: coma
- 20mg/ml: respiratory arrest
- 26mg/ml: cardiovascular collapse
What are the maximum safe doses of a) bupivacaine b) lignocaine and c) prilocaine?
Bupivacaine: 2mg/kg
Lignocaine: 3mg/kg
Prilocaine: 6mg/kg
What is the ratio of cardiovascular collapse : convulsion for bupivacaine vs lignocaine and what does this mean?
The ratio os 4 for bupivacaine, 7 for lignocaine; therefore lignocaine less likely to cause seizures than bupivacaine, bupivacaine carries higher risk if injected by accident (lower number = more cardiotoxic)
What are the 2 routes of administration of local anaesthetics?
- Topical application
- Local infiltration
What are 4 examples of local anaesthetics given via topical application?
- Eutectic mixture of local anaesthetic (EMLA) cream, lignocaine 2.5% and prilocaine 2.5% in an emulsion
- Amethocaine cream/ gel
- Lignocaine spray (10%) - sometimes used for vocal cords, strong
- Benzocaine lozenges - for back of throat
What are 2 types of local infiltration that can be done with local anaesthetics?
- Field block
- Wound infiltration
What are 4 examples of local infiltration used to give local anaesthetic?
- Cannulae
- Sutures
- Inguinal hernia - done using field block
- Post-operative pain relief - using wound infiltration
What is local anaesthetic used in wound infiltration often given with, and what should you be careful of?
adrenaline, to reduce absorption of drug (so it’s longer lasting): have to be careful of the total dose, however
At what level would regional spinal anaesthesia by put in, in an adult for transurethral resection of the prostate?
L3-L4, sometimes L2-L3
What is the advantage of putting the spinal anaesthesia at the level of L3-4?
The spinal cord is finished her so no risk of puncturing
What kind of needle is used to insert spinal anaesthesia (type of regional)?
25-27g needle i.e. very fine
What is the aim when inserting the needle to administer spinal anaesthesia?
Aiming to pass thorugh all layers of the dura into the CSF, which can be determined by CSF leaking back through the needle
How much drug and what types can be injected during spinal anaesthesia?
Small volumes of drugs, often bupivacaine and occasionally other types
How quikly does it take for spinal anaesthesia to work and what should be the effect?
Rapid onset if in the right place; total block: sensory and muscle block
What type of block is produced by spinal anaesthesia?
Total block: sensory and muscle
What can be added to spinal anaesthesia?
Opioids e.g. fentanyl (done by some anaesthetists)
In which specialty is epidural anaesthesia very popular?
Obstetrics
How is epidural anaesthesia given?
- Tuohy needle (pronounced too-ee) used through the ligaments int he back until there’s loss of resistance, into the epidural “space”, an area with fat and some vessels.
- Feel a loss of resistance, pop through the ligamentum flavum and can start to inject fluid.
- Pass catheter through after putting the needle in
At what level can epidural anaesthesia be given?
Commonly L2-3 or 3-4 for lower limb surgery in adults, or lower thoracic; occasionally cervical