24. Anaesthetics Flashcards
WHat are the 2 types of anaesthesia?
General and local
What are the 2 types of anaesthesia used for general?
- inhalation (volatile)
- Intravenous
what is the type of local anaesthetic?
regional
Define conscious sedation.
- Conscious sedation: use of small amounts of anaesthetic or benzodiazepines to produce a ‘sleepy-like’ state. (Maintain verbal contact but feel comfortable)
what are the usual steps in anaesthetics?
- Premedication (Hypnotic-benzodiazepine).
- Induction (sleep) (usually intravenous but may be inhalational (needle phobics)).
- Intraoperative analgesia (usually an opioid e.g. fentanyl).
- Muscle paralysis-facilitate intubation/ventilation/stillness.
- Maintenance (typically inhalational).
- Reversal of muscle paralysis and recovery which includes postoperative analgesia (opioid/NSAID/paracetamol).
- Provision for PONV
Give examples of volatile anaesthetics.
- Nitrous oxide
- Desflurane
- Sevoflurane
- Isoflurane
- Enflurane
- Halothane
Give examples of IV anaesthetics.
- Propofol
- Barbiturates
- Etomidate
- Ketamine
What are the range of effects on the CNS produced during general anaesthesia described by?
Guedel’s signs
What are the stages of Guedel’s signs?
- Stage 1: analgesia and consciousness
- Stage 2: unconscious, breathing erratic but delirium could occur, leading to an excitement phase.
- Stage 3: surgical anaesthesia, with four levels describing increasing depth until breathing weak.
- Stage 4: respiratory paralysis and death.
What is anaesthesia a combination of?
- Analgesia
- Hypnosis (loss of consciousness)
- Depression of spinal reflexes
- Muscle relaxation (insensibility and immobility)
As anaesthetic concentration rises, what is lost?
- memory
- consciousness
- movement
- cardiovascular response
Define potency.
Concentrational dose range, over which a drug produces its effects
What is MAC?
Minimum Alveolar Concentration
Define MAC.
[Alveolar] at which 50% of subjects fail to move to surgical stimulus (unpremedicated breathing O2/air)
Are higher or lower value MACs more potent?
A lower MAC value represents a more potent volatile anesthetic
When is a volatile anaesthetic at equilibrium?
[alveolar] = [spinal cord]
WHat is the anatomical substrate for MAC?
Spinal cord
What factor affects induction and recovery?
Partition coefficients (solubility):
• Blood:Gas partition (in the blood)
• Oil:Gas partition (in fat)
What is the blood/gas partition coefficient?
Reflects solubility of the volatile agent in blood and is defined as the ratio of its concentration in blood to alveolar gas when their partial pressures are in equilibrium
WHat does a higher blood:gas partition indicate?
More soluble: leads to greater uptake by the pulmonary circulation, but a slower increase in alveolar partial pressure of the agent and therefore more prolonged induction and recovery from anaesthesia
WHat does a lower blood:gas partition indicate?
Less soluble: lesser uptake by pulmonary circulation, therefore faster increase in alveolar partial pressure
– Low value = fast induction and recovery e.g., desflurane
What is the oil:gas partition coefficient?
Determines potency and slow accumulation due to partition into fat - fat solubility
What does a higher oil:gas partition value indicate?
More potent
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)
What is the effect of nitrous oxide on MAC?
NO Typically mixed with other volatile agents to reduce their MAC.
- has little side effects, so can be used to reduce side effects of other agents
- reduced MAC means reduced dosing
what are the main rapid intravenous anaesthetics?
Propofol
Barbiturates
what is the main slow intravenous anaesthetic?
Ketamine
when are intravenous anaesthetic used as sole anaesthetic?
TIVA (Total IntraVenous Anaesthesia).
How Do We Describe Intravenous Anaesthetic Potency ?
- Plasma concentration to achieve a specific end point (e.g., loss of eyelash reflex).
- For induction in mixed anaesthesia – Bolus to end point then switch to volatile.
how is TIVA prescribed?
• TIVA uses a defined PK based algorithm to infuse at a rate to maintain set point. Pre-ceded by a bolus.
WHat is the critical site of action of volatile anaesthetics? Exceptions?
GABA-A receptors
- ligand gated ion channels
- Cl- conductance - Cl- flows in so inside more negative which inhibits transmission of action potential - depress CNS activity
Exceptions: Xe, N2O, ketamine - block NMDA receptors
What does potentiation of GABA receptors cause?
• Anxiolysis • Sedation • Anaesthesia
how does anaesthetic potency correlate to lipid solubility?
higher potency = higher lipid solubility
how does anaesthetic potency correlate to GABA-A interaction ?
higher the potency the greater the interaction
in the brain, what is consciousness a balance of?
between excitation (NMDA Glutamate) and inhibition (GABA). Anaesthetics modulate this balance.
What are the main systems of the CNS that are targeted by anaesthetics?
Loss of connectivity between systems:
- reticular formation (arousal -consciousness) depressed
- thalamus (sensory relay)
- hippocampus depressed (memory)
- brainstem depressed(resp and CVS)
- dorsal horn (pain) and motor neuronal activity depressed
When is local and regional anaesthesia used?
- Dentistry
- Obstetrics
- Regional surgery (patient awake)
- Post-op (wound pain)
- Chronic pain management (PHN)
What are the main local anaesthetics?
(most potent first)
Bupivacaine, Ropivacaine, Lidocaine and Procaine
What is the basic structure of local anaesthetics, how does this affect its metabolism?
- made up of aromatic ring and amine group
- linked by either a ester (short acting)or amide(long acting)
We have lots of esterases in plasma, so these are broken down quickly - short duration of action
What characteristics are important to consider in local anaesthetics?
- lipid sol (higher = greater potency)
- dissociation constant (pKa - lower = faster onset)
- chemical link (metabolism)
- protein binding (duration)
what is the mode of action of local anaesthetic?
block voltage gated sodium ion channels - block action potential propagation
• Block small myelinated (afferent) nerves in preference hence nociceptive and symp block
What is use dependent block (local anaesthetics)?
the degree of block is proportional to the rate of nerve stimulation
- more drug molecules enter sodium channels when they are open and cause block
how does adrenaline impact use of local anaesthetic?
• Adrenaline ↑ duration
which local anaesthetic is more potent and which has slower onset?
Compared to Procaine, Bupivacaine is more potent with a longer duration of action. Procaine is esterase metabolised and has a slower onset time but not much in it.
WHat is regional anaesthesia?
WHat is regional anaesthesia?
Selectively anaesthetising a part of the body
WHat does regional anaesthesia typically use?
Local anaesthetic and or an opioid
What are the upper extremity regional anaesthesai?
Interscalene, supraclavicular, infraclavicular, axillary
What are the lower extremity regional anaesthesai?
Femoral, sciatic, popliteal, saphenous
how are regional anaesthesia given?
Extradural / Intrathecal / Combined (labour).
What are the main side effects of general anaesthesia?
- PONV (postoperative nausea and vomiting) (opioids)
- CVS - hypotension
- PO cognitive dysfunction (increases with increasing age)
- Chest infection
general concern allergic reaction/anaphylaxis
What are the main side effects of local anaesthesia?
Depends on the agent used and usually result from systemic spread (Locals are Na+ channel blockers so cardiovascular toxicity)
