13.2 Anaesthetics Flashcards
How can anaesthesia be used ?
General
Local
How is general anaesthesia delivered?
- Inhaled (volatile)
- IV
How can local anaesthesia be used?
Regionally -can block an entire part of the body
Sometimes used if haemodynamically unstable and cannot use general
What is conscious sedation?
Small amount of anaesthetic or benzodiazepines used to produce a sleep state
What are the practical steps of anaesthesia?
- Pre-medication to help anxiety- benzodiazepines
- Induction -IV or inhalation
- Intraop anaesthesia -opioids
- Muscle paralysis for intubation
- Reversal of muscle paralysis and recovery including post op analgesia
- Post-op anti-emetics
Describe the structure of general anaesthetics
Lots of different structures
May have hydrocarbon with fluorine ring
What are some examples of inhaled general anaesthetics?
- Nitric oxide (N2O)
- Chloroform
- Halothane
- Fluroxene
- Methoxyflurane
What are some examples of IV general anaesthetics?
- Propofol
- Barbiturates
- Ketamine
What are Guedel’s signs?
Level of anaesthesia from the presentation of the patient
Eye movement and muscle tone
What are the different stages of Guedels signs?
Stage 1: Analgesia
Normal muscle tone
Some eye movement
Conscious
Stage 2: Excitement
Muscle tone normal to increased
Moderate eye movement
Uconscious
Breathing erratic
Delirium could occur
Stage 3: Surgical anaesthesia - Muscle relaxedness (slight, moderate, markedly, markedly ) increases until breathing becomes weak
Stage 4: Respiratory paralysis - Muscle tone flaccid
No eye movements
Death
What is anaesthesia a combination of?
- Analgesia
- Hypnosis - loss of consciousness
- Depression of spinal reflexes
- Muscle relaxation
What is anaeastheisa end point of determined by?
Concentration dependent
How do we describe potency in volatiles/inhalational agents?
MAC
Minimum alveolar concentration (EC50)
What order do responses go with anaesthesia?
- Memory
- Consciousness
- Movement
- Cardiovascular response
What is potency?
EC50
Concentration of drug needed to elicit half of the maximal response
What is MAC or minimum alveolar concentration?
Alveolar concentration of drug (1 atmosphere) at which 50% of patients fail to move to a surgical stimulus
What is alveolar concentration equal with at equilibrium?
Alveolar concentration of drug equals spinal cord concentration of drug at equilibrium
What are the different types of MAC?
MAC
MAC-Bar - autonomic response
MACawake
What is a main factor (compartment wise) affecting induction and recovery from anaesthesia?
Partition co-efficients - measures of solubility
What are some examples of partition co-efficients that can effect anaesthesia induction and recovery?
Blood Gas partition
Solubility in blood
Oil Gas partition
Solubility in fat
What is the relationship of Blood:Gas partition and anaesthesia?
Low value (so low solubility)
Fast induction and recovery as it is not in tissues and blood for long
E.g. desflurane
What is the relationship of Oil:Gas partition and anaesthesia?
Determines potency and slow accumulation due to partition in fat e.g halothane
Higher value so higher potency as it accumulates in fat
What affects MAC?
- Age- high in infants lower in elderly
- Hyperthermia (increased), hypothermia (decreased)
- Pregnancy (increased)
- Alcoholism (increased)
- Central stimulants (increased)
- Other anaesthetics and sedatives (decreased)
- Opioids (decreased)
How does nitric oxide affect MAC?
Nitric oxide decreases MAC
Need less of if you add nitric oxide
What is the relationship of GABA and the potency of anaesthesia?
Increased interaction with GABA
Increases its inhibitory effects therefore increased potency of anaesthesia
What is the main receptor target for inhaled anaesthesia?
GABA- increases effects of GABA (potentiates, GABA still needs to bind)
More hyperpolarisation, therefore further from threshold
What is the exception to the GABA binding rule?
Xenon, Nitric Oxide and Ketamine
These work at NMDA receptors (glutamate receptors) and block them
What systems is the main target of anaesthesia?
Reticular formation- controls consciousness
What occurs to each part of reticular formation during anaesthesia?
Thalamus
Transmits and modifies sensory information before reaching PSC
Hippocampus
Depressed, memory loss
Brainstem
Depressed, respiratory and CV effects
Spinal cord
Depressed, analgesia ( dorsal horn) and Motor neurone activity decreased
Howe can we see effects of anaesthesia on the brain?
PET scan
Give examples of fast acting anaesthesia
Propofol
Barbiturates
Give an example of slow acting anaesthesia
Ketamine
What are IV anaesthetics often used for?
- Induction in surgery
- TIVA (total intravenous anaesthesia)
What can Ketamine cause when you wake up?
Odd emergence reactions
What is the main receptor target for IV anaesthesia?
GABA- potentiates
Not ketamine its binds to NMDA and inhibits GABA
How is IV anaesthesia potency described?
Plasma concentration
Achieve a specific end point such as eyelash reflexes loss
For induction with mixed anaesthesia
IV bolus used to end point, then switched to inhaled agent
What does TIVA use to calculate potency and anaesthesia end-point?
Total IV
Uses defined PK based algorithm to infuse at a rate to maintain a set point
Given a bolus before this
When is local anaesthesia used?
- Dentistry
- Obstetrics
- Regional surgery
- Post-op
- Chronic pain management
What are some examples of local anaesthetics?
Lidocaine
Bupivacaine
Ropivacaine
Procaine
-caine suffix
What is the common structure of local anaesthetics?
Aromatic ring with ester or amide connecting to amine group
What are some characteristics of local anaesthesia?
Lipid solubility
If it was higher they would have greater potency
Dissassociation constant
Lower pKa leads to a faster onset
Chemical links (ester vs amide) Determines metabolism
Protein binding
More protein binding causes longer duration
How do different bonds alter the duration of local anaesthetics?
Ester
Shorter acting, plasma contains esterases which break it down
Amide
Longer acting
How does local anaesthetic work in wound analgesia?
Buvipacaine
- Cocaine-like molecule
- Cocaine enters channel via two ways - hydrophobic vs hydrophilic
- Cocaine crosses plasma membrane if uncharged and picks up positiveve charge
- Blocks Na+ channels that are open (hydrophilic pathway) or closed (hydrophobic pathway)
- Na+ unable to travel through and depolarise resulting in decreased APs
What do molecules block in local anaesthesia?
Dependent block
Only blocks channels if there is lots of firing
Blocks small myelinated afferent nerves in preference
Nociceptive and sympathetic block
How does adrenaline affect local anaesthetics?
- Vasoconstriction
- Decreased blood flow
- Increased duration of anaesthesia
What is the difference between Bupivacaine and Procaine?
Bupivacaine
More potent
Longer duration
Procaine
Ester metabolised
Slower onset
What determines potency of local anaesthesia?
Determined by lipid solubility
The higher the solubility the more potent
Write out the strength of local anaesthetics from highest to lowest
Bupivacaine
Ropivacaine
Lidocaine
Procaine
What is the speed of acting from fastest to slowest of local anaesthesia?
Determined by pKa
Low pKa means a faster onset
- Lidocaine
- Bupivacaine
- Ropivacaine
- Procaine
Outline the duration of local anaesthesia from longest to shortest
Determined by protein binding
More binding causes a longer duration
- Bupivacaine
- Ropivacaine
- Lidocaine
- Procaine
What is regional anaesthesia?
Anaesthetising a part of the body
Described as a nerve block
Patient remains awake
Uses local anaesthesia or opioids
Where do you use regional anaesthesia?
Upper extremities
Supraclavicilar
Interscalene
Infraclavicular
Axillary
Lower extremities
Femoral
Sciatic
Poplitieal
Saphenous
Extradural
Intrathecal (spinal cord)
What are the side effcts of general anaesthesia?
- Nausea and vomiting acutely
- Hypotension
- Post operative cognitive dysfunction - increases with age and longer anaesthesia
- Chest infection
Why can you get a chest infection follow general anaesthesia?
Lack of coughing following surgery
Foreign tube with microorganisms directly into thoracic cavity
Main concern generally of anaesthesia from public too
Anaphylaxis/allergic reaction
