Anaesthetics Flashcards
What are the different types of anaesthesia?
General - Inhalational / volatile or IV
Local - Regional
What is conscious sedation?
Use of a small amounts of anaesthetic or benzodiazepines to produce a ‘sleepy-like’ state. -Common at the dentist
How does anaesthesia work form a practical viewpoint?
- Premedication (hypnotic - benzodiazepine)
- Induction (usually IV but can be inhalational)
- Intraoperative analgesia (usually opioid)
- Muscle paralysis -facilitate intubation / ventilation / stillness
- Maintenance (IV and/or inhalational -more common)
- Reversal of muscle paralysis and recovery which includes postoperative analgesia (opioid / NSAID / paracetamol)
- Provision for PONV (post operative nausea and vomiting)
Key example of poly pharmacy!
How many deaths occur from anaesthetics?
Very rare! 5 in 1million
In 1940s - 640 in 1million.
But, side effects are the risk.
What are the four 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 is weak.
Stage 4: Respiratory paralysis and death
According to Guedel, what is anesthesia a combination of?
Alagesia
Hypnosis (loss of consciousness)
Depression of spinal reflexes
Muscle relaxation (insensibility and immobility)
What is MAC?
Minimum alveolar concentration
It is the concentration of a vapour in the alveoli of the lungs that is needed to prevent movement (motor response) in 50% of subjects in response to surgical (pain) stimulus.
Volatile anaesthetic potency is described by MAC -Used to compare anaesthetics.
What partition coefficients (solubility) affect MAC?
Blood:Gas partition -a low value means fast induction and recovery
Oil:Gas partition -This determines potency and slow accumulation due to partition into fat.
What affects MAC?
Age (higher in infants, lower in elderly)
Hyperthermia (increased); hypothermia (decreased)
Pregnancy (increased)
Alcoholism (increased)
Central stimulants (increased)
Other anaesthetics and sedatives (decreased)
Opioids (decreased)
By adding a small concentration of another anaesthetic - you need smal amounts so could reduce side effects
What does anaesthetic potency correlate with?
Correlates with lipid solubility and GABAa activity.
What effect does anaesthetic have on GABA?
GABA receptors are critical targets Major inhibitor transmitter LGIC (Cl- conductance) Potential GABA activity -Anxiolysis -Sedation -Anaesthesia
With the exception of Xe, N2O and ketamine, all anaesthetics potentiate GABAa mediated Cl- conductance to depress CNS activity.
NMDA receptors probable other site.
How do anaesthetics works on the brain (simply)?
Consciousness is (simplistically) a balance between excitation (Glutamate) and inhibition (GABA)
Anaesthetics modulate this balance
What effect does anaesthetics have on the brain?
Reticular formation depressed. Connectivity lost.
Reticular system is often called the “activating system” due to its ability to increase arousal.
Thalamus transmits and modifies sensory information.
Hippocampus depressed (memory)
Brainstem depressed (respiratory and some CVS)
Spinal cord-depress dorsal horn (analgesia) and moor neuronal activity (MAC)
What are the main IV anaesthetics?
Propofol (rapid)
Barbiturates (rapid)
Ketamine (Slower)
Give intravenously for ‘induction’
Can be used as sole anaesthetic in TIVA (Total IntraVenous Anaesthesia)
Target sites are the same as inhalational
With the exception of ketamine, they all potentiate GABA.
Systems target as for inhalational.
How do we describe IV anaesthetic potency?
Plasma concentration to achieve a specific end point
For induction in mixed anaesthesia - Bolus to end point then witch to volatile
TIVA uses a defined PK based algorhythm to infuse at a rate to maintain set point. Pre-ceded by a bolus.
When is local and regional anaesthesia used?
Dentistry Obstetrics Regional surgery (patient awake) Post-op (wound pain) Chronic pain management (PHN)
What are the basic chemical structures in a local anaesthetic?
Aromatic ring - Amine
Ester link
Amide link
What are the characteristics of local anaesthetics?
Lipid solubility - potency (higher = greater potency)
Dissociation constant (pKa) - time of onset. Lower pKa = faster onset.
Chemical link - metabolism
Protein binding - duration (higher = longer duration)
How can you strengthen the effects of local analgesia?
Strengthen effects by adding vasoconstrictor (adrenaline)
Rate of working is proportional to rate of firing of neurone
How does Bupivacaine Infultration for wound analgesia work?
It is a cocaine archetypal
Esters are short acting and amides are longer acting.
Block is use dependant
Blocks small myelinated (afferent) nerves in preference so, there is a nociceptive and sympathetic block.
Adrenaline increases the duration but causing vasoconstriction.
Bupivacaine is an amend so it is more stable and longer lasting with a slow onset.
What is regional anaesthesia?
This is when you selectively anaesthetise a part of the body
Block inflow / outflow
Often described as a ‘block’ of a nerve and hence the patient remains awake.
Uses local anaesthetic or an opioid
Upper extremity - interscalene, supraclavicular, infraclavicular, axillary
Lower extremity - femoral, sciatic, popliteal, saphenous
Extradural / Intrathecal / Combined (labour)
What are the main side effects of anaesthetics?
General:
Postoperative Nausea and Vomiting - Very common
CVS - hypotension
Post operation cognitive delirium - more common with increasing age
Chest infection
Local and regional:
Depends on the agent used and usually results from systemic spread (locals are Na+ channel blockers so cardiovascular toxicity)
Increased general concern re: allergic reactions / anaphylaxis
