S7) Introduction to Anaesthetics Flashcards
Outline the different types of anaesthetic techniques
What is conscious sedation?
Conscious sedation is the use of small amounts of anaesthetic or benzodiazepines to produce a ‘sleepy-like’ state
In 7 steps, describe the process of administering anaesthesia
⇒ Premedication e.g. benzodiazepine
⇒ Induction (IV/ inhalation)
⇒ Intraoperative analgesia (opioid, usually)
⇒ Muscle paralysis (facilitate intubation, ventilation, etc)
⇒ Maintenance (IV/ inhalation)
⇒ Reversal of muscle paralysis (postoperative analgesia)
⇒ Provision for PONV (post-op nausea and vomiting)
why is pre medication given in anaesthetics
to reduce anxiety, control pain, decrease the risk of aspiration pneumonitis
Provide four examples of intravenous anaesthetics
- Propofol (rapid)
- Barbiturates (rapid)
- Ketamine (slower)
Provide four examples of inhalational anaesthetics
- Nitrous oxide
- Xenon
- Chloroform
- Cyclopropane
What is the term given to the range of effects on the CNS produced during general anaesthesia?
Guedel’s signs
Identify and describe the four stages in 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
General anaesthesia is a combination of which four events?
- Analgesia
- Hypnosis (loss of consciousness)
- Depression of spinal reflexes
- Muscle relaxation (insensibility and immobility)
How might one determine the volatile anaesthetic potency?
Volatile anaesthetic potency is described by the minimum alveolar concentration (MAC)
What is the anatomical substrate for MAC?
The anatomical substrate for MAC is the spinal cord
What is MAC?
- MAC is the [alveolar] at 1 atm, at which 50% of subjects fail to move to surgical stimulus (unpremedicated breathing air)
which is defined as the concentration of the anesthetic in the alveoli of the lungs required to prevent movement in 50% of patients in response to a standardized surgical stimulus.
- At equilibrium [alveolar] = [spinal cord]
Explain how partition coefficients (solubility) affect the induction and recovery of general anaesthesia
- Blood:Gas partition (in the blood) – low value fast induction and recovery e.g. desflurane
- Oil:Gas partition (in fat) – determines potency and slow accumulation due to partition into fat e.g. halothane
what is the blood:gas partition
→ solubility constant describing distribution of inhaled anaesthetic between blood and alveolar gas
the lower, the faster the anaesthetic moves between the alveolar and the blood to the brain
what is oil: gas partition
used to estimate the distribution of lipophilic drugs or compounds in biological systems.
ratio of the concentration of the drug or compound in oil to its concentration in gas at equilibrium.
A high oil-gas partition coefficient indicates that the drug or compound is more soluble in oil than in gas: so lipophilic drugs tend to be absorbed more readily across cell membranes, including the blood-brain barrier, and are more likely to accumulate in fatty tissues.
Identify five factors which affect MAC by increasing it
- Age (high in infants, low in elderly)
- Hyperthermia
- Pregnancy
- Alcoholism
- Central stimulants
Identify three factors which affect MAC by decreasing it
- Age (high in infants, low in elderly)
- Other anaesthetics and sedatives
- Opioids
Describe the relationship between anaesthetic potency, lipid solubility and GABAA activity
Anaesthesia potency correlates with lipid solubility and GABAA activity,
The more lipid soluble an anastehtic is the faster it will pass across the lipid membrane, this means it will enhance activity of GABA and so increase potency of anaesthetic
Describe the role of the GABA receptors in general anaesthesia
- GABAA receptors are a critical target and act as major inhibitory transmitter for Cl- conductance
- All anaesthetics (except for 3) potentiate GABAA mediated Cl- conductance to depress CNS activity
What are the three effects which arise from potentiating GABA activity?
- Anxiolysis
- Sedation
- Anaesthesia
Which three general anaesthetics do not potentiate GABAA activity?
- Xe
- N2O
- Ketamine
Briefly, explain how general anaesthetics modulate consciousness in the brain
- In the brain consciousness is a balance between excitation (Glutamate) and inhibition (GABA)
- Anaesthetics modulate this balance
In four steps, describe the effects of general anaesthesia on the brain circuity
⇒ Reticular formation (hindbrain, midbrain and thalamus) depressed and connectivity lost
⇒ Hippocampus depressed (memory)
⇒ Brainstem depressed (respiratory and some CVS)
⇒ Spinal cord – depressed dorsal horn (analgesia) and motor neuronal activity (MAC)
What is involved in regional anaesthesia?
- Local anaesthesia involves selectively anaesthetising a part of the body
- It is the ‘block’ of a nerve and hence the patient remains awake and uses local anaesthetic / an opioid
