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
Provide four examples of nerves blocked in the regional anesthesia in the upper extremity
- Interscalene nerve
- Supraclavicular nerve
- Infraclavicular nerve
- Axillary nerve
Provide four examples of nerves blocked in the regional anesthesia in the lower extremity
- Femoral nerve
- Sciatic nerve
- Popliteal nerve
- Saphenous nerve
Identify five circumstances where local and regional anaesthesia are used
- Dentistry
- Obstetrics
- Regional surgery (patient awake)
- Post-op (wound pain)
- Chronic pain management (PHN)
Provide four examples of local anaesthestics
- Lidocaine
- Bupivacaine
- Ropivacaine
Describe the characteristics of local anaesthestics in terms of the following:
- Lipid solubility-potency
- Dissociation constant
- Chemical link
- Protein binding duration
- Lipid solubility-potency – higher greater potency
- Dissociation constant (pKa) – lower pKa, faster onset
- Chemical link – metabolism
- Protein binding – higher for longer duration)
Describe the mechanism of action of wound analgesia
Bupivacaine infiltration for wound analgesia:
- Block small myelinated (afferent) nerves in preferance hence nociceptive and symp block
- Adrenaline ↑ duration
What are the side effects of general anaesthesia?
- PONV (opioids) - post op nausea and vomiting
- CVS
- Hypotension
- POCD (post-op cognitive dysfunction)
- Chest infection
What are the side effects of local anaesthesia?
Local and regional – depends on the agent used, locals are Na+ channel blockers so cardiovascular toxicity
why do you give a local anasthetic with a vasoconstrictor
- reduces peak plasma conc of local anasthetic
- increases duration of the anasthesia
- reduces minimum effective dose
what are important measurements you need for a patient
BMI
renal or liver function
