15 - Anaesthetics Flashcards
What are the different types of anaesthetics?
- IV: propofol, barbituarates, etomidate, ketamine (children)
- Gases: often contain a lot of Cl or F e.g nitrous oxide, chloroform, xenon, cyclopropane
What are the different stages of general anaesthesia?
- Premedication (hypnotic-benzodiazepene)
- Induction (usually IV but can be inhalational)
- Intraoperative analgesia (e.g fentanyle)
- Muscle paralysis to facilitate intubation
- Maintenance IV or inhalational
- Reversal of muscle paralysis and recovery
- Post operative analgesia
- Provision for PONV with antiemetics
What are Guedel’s signs?
- The level to which the anaesthetic is working on the CNS (read stages on image)
- Stage 2 is paradoxical excitment
- Want to be in stage 3 and as you go down breathing decreases and you lose muscle tone
What is MAC when we talk about anaesthetics?
Minimum alveolar concentration: alveolar concentration of the anaesthetic at which 50% of subjects fail to move to surgical stimulus. Used to compare potency between anaesthetics
[Alveolar] = [Spinal cord]
Can have MAC-BAR (autonomic respose) and MAC-awake
What factors affect induction and recovery of anaesthesia?
Blood:Gas Partition
Solubility of volatile anaesthetic in the blood. Low value means fast induction and recovery e.g desflurane
Oil:Gas Partition
Solubility in fat so determines potency. Greater ability to travel in fat, the higher the potency but the anaesthetic can accumulate in fat so can take patient a long time to wake up e.g halothane
What are some factors that affect MAC?
- Age (high in infants, low in elderly)
- Hyperthermia (increased) and Hypothermia (decreased)
- Pregnancy (increased)
- Alcoholism (increased)
- Central stimulants (increased)
- Other anaesthetics and sedatives (decreased)
- Opioids (decreased)
What effect does nitrous oxide have on MAC?
- Commonly added to other volatile anaesthetics to decrease the MAC so don’t need to use as much anaesthetic so less side effects
- Same with opiates and other anaesthetics
What is conscious sedation?
Conscious sedation is the use of small amounts of anaesthetic or benzodiazepines to produce a ‘sleepy-like’ state
What is the anatomical substrate for MAC?
Spinal cord
What is the relationship between anaesthetic potency, lipid solubility and GABAa activity?
- Graph on left x-axis is oil:gas partition
- The more lipid soluble a drug is, the more activity it has and the more potent it is
How are GABAA receptors involved in general anaesthesia?
- All anaesthetics (few exceptions) activate inhibitory GABAA causing Cl- conductance and hyperpolarisation so depressed CNS activity as increased negative charge so cannot produce an action potential
- Xe, N2O and Ketamine are the only ones that don’t do this
- GABAA activation therefore depresses reticular system which depresses hippocampus (memory), brainstem (CVS and resp) and spinal cord (analgesia)
What effects arise from potentiating GABAA receptors?
- Anxiolysis
- Sedation
- Anaesthesia
Explain how general anaesthetics modulate consciousness and anaesthesia in the brain?
- In the brain consciousness is a balance between excitation (Glutamate) and inhibition (GABA)
- Anaesthetics modulate this balance
How does ketamine work to induce sedation on a molecular level?
- NMDA glutamate receptor blocker
- NMDA glutamate agonists are used to wake the patient up as they excite
What are the main IV anaesthetics and how do they work?
- Propofol (MJ Juice) (rapid)
- Barbiturates (rapid)
- Ketamine (slower)
- All potentiate GABAA apart from ketamine which acts on NMDA
How do we describe the potency of an IV anaesthetic as we cannot use MAC?
- Plasma concentration to achieve a specific end point (e.g loss of eyelash reflex)
- Use bolus to induce then infuse a steady rate to maintain unconsciousness
- If using mixed anaesthesia (to lower dose of both for less side effects) use bolus to end point in IV then switch to volatile
What areas are local and regional anaesthetics used?
- Dentistry
- Obstetrics
- Regional surgery with patient awake e.g elderly patient with CVS issues
- Post op wound pain
- Chronic pain management
What are some examples of local anaesthetics?
- Lidocaine
- Bupivacaine
- Ropivacaine
- Procaine
What are some of the characteristics of local anaethetics?
- Lipid solubility-potency – higher so greater potency
- Dissociation constant (pKa) – lower pKa, faster onset
- Chemical link – metabolism (esterase in blood breaks esters in anaesthetic so short duration as break down fast)
- Protein binding – higher for longer duration
What makes a local anaesthetic long acting or short acting?
- Long acting: amide link
- Short acting: ester link
How does bupivacaine work for wound analgesia?
- Block voltage gated Na channels by going through membrane (as not charged) and get charged in the cell so can pass into channel and block it so no Na can enter and no depolarisation
- Block small myelinated (afferent) nerves in preferance hence nociceptive and symp block
- Adrenaline ↑ duration as vasoconstriction so less clearance
- Block is user dependent so more firing more goes in so neccessary to touch area to see if can feel it
How do each of the local anaestheticd vary?
- Bupivacaine is more potent and has a longer duration of action as highest protein bound
- Procaine is esterase metabolised and has a slower onset time
What is regional anaesthesia and what areas can you give it to?
- Selective anaesthetising of a part of the body
- Nerve block with local anaesthetic or opioid
- Upper limb: interscalene, supraclavicular, infraclavicular, axillary
- Lower limb: femoral, sciatic, popliteal, sciatic
- Extradural, Intrathecal and combined for labour
What are some side effects of general anaesthesia?
- PONV (opioids)
- CVS
- Hypotension
- POCD (post-op cognitive dysfunction - increases with age)
- Chest infection
- Allergic reactions/anaphylaxis
Get the right mix and dose to minimise side effects
What are the side effects of local anaesthetics?
Local and regional – depends on the agent used, locals are Na+ channel blockers so cardiovascular toxicity
Why are vasoconstricting agents used in conjunction with a local anaesthetic?
- Decrease peak plasma concentration of local anaesthetic
- Decrease minimum effective dose
- Increase duration of anaesthesia
