Anaesthetics

Question 1

What are the main inhaled anaesthetics?

Answer 1

Nitrous oxide, isoflurane, desflurane, sevoflurane

Question 2

Name the anaesthetics administered by IV

Answer 2

Propofol, ketamine

Question 3

What are the effects of anaesthesia?

Answer 3

Sedation to unconsciousness, muscular relaxation, reflex suppression, analgesia, amnesia, anxiolysis

Question 4

What are the 4 types of anaesthesia used in practice?

Answer 4

General - affects sensory, motor and sympathetic transmission over the whole body (unconscious)

Regional - inhibit transmission between specific part of the body and the spinal cord e.g. Epidural (conscious)

Local - peripheral nerve block e.g. Tooth extraction (conscious)

Dissociative - inhibit transmission between higher and lower centres of the brain to create a hallucinogenic state (more so used in children or the elderly)

Question 5

What parts of the CNS are targeted?

Answer 5

Reticular formation - consciousness

Thalamus - consciousness

Hippocampus - memory

Brain stem

Dorsal horn

Motor neurones

Question 6

Describe the effect of anaesthetic on inhibitory channels

Answer 6

GABAA activated chloride channels - positive allosteric regulation - increase sensitivity to GAGA (lower EC50 and increase efficacy) which increases the cl- current which hyper polarises the cell and decreases excitability

Glycine activated chloride channels - same mechanism

Question 7

Describe the effects of anaesthesia on excitatory ligand gates ion channels

Answer 7

Nicotinic ACh receptors - inhibits ACh binding this reduces Na+ currents and reduces excitability (once bound it in activates the receptor so reduces efficacy but not potency as there are remaining receptors)

NMDA receptors - responsive to glutamate, binding reduces Ca+ current involved in synaptic transmission

Question 8

How is an inhaled anaesthetic prepared and administered

Answer 8

Mixed with oxygen, air and often nitrous oxide and is inhaled through a mask

Question 9

What is a MAC?

Answer 9

The minimum alveolar concentration is the percentage anaesthetic in the mixture that removes the response to surgical incision in 50% of patients e.g. A lower MAC means the anaesthetic is more potent

Question 10

Between what percentages is the normal MAC value and when standardised how many MACs usually achieve surgical depth?

Answer 10

1-6%

1.2-1.5

Question 11

How can you reduce the MAC?

Answer 11

Use of adjuvants e.g. Nitrous oxide

Question 12

What determines the absorption of an inhaled anaesthetic?

Answer 12

The blood : gas coefficient which is the volume of gas that can divide in 1l of blood thus the higher the value the more readily it will enter the blood

Question 13

What affects the distribution of an anaesthetic?

Answer 13

Relative blood supply to each organ

Brain : blood coefficient

Muscle : blood coefficient

Oil : blood coefficient

Muscle and fat compartments take up proportionately more than the brain and can provide a reservoir than may redistribute back to the brain during recovery

Question 14

How are modern fluranes metabolised and eliminated?

Answer 14

They undergo little hepatic metabolism.

As the concentration drops it moves back out of the cells to the lungs to be exhaled (first by well perfused tissues e.g. Brain). Due to the slow movement from muscle and fat the recovery can take hours - days, especially as it can redistribute to the CNS

Question 15

Why is IV anaesthesia used during induction)

Answer 15

It reaches anaesthetic depth quicker and bypasses the stage II

Question 16

Describe the distribution of IV anaesthetic

Answer 16

Heavily protein bound

After a few minutes it redistributes from the CNS to muscle and fat which have higher capacities

Question 17

Name the major adjuvants for anaesthesia in surgery

Answer 17

Benzodiazepines - agonist at GABA receptors (anxiolysis and amnesia)

Propofol - rapid induction and reduces MAC

Nitrous oxide - acts on NMDA receptors to induce analgesia and reduce flurane MAC

Opioids - analgesia e.g. Morphine, fentanyl

Neuromuscular blocking agents - removes reflexes and induces muscular relaxation e.g. Tubocurarine, succinylcholine

Question 18

What are the main ADRs for anaesthetics?

Answer 18

Cardiovascular and respiratory depression (acts on medullary centres)

Arrhythmia and hypotension (acts on anterior smooth muscle to decrease resistance)

Increased cranial blood flow which can lead to raised intracranial pressure (reduced vascular resistance)

Airway irritation

Malignant hypothermia

Bronchodilation (good ADR)

Diffusion hypoxia where nitrous oxide diffuses out too quickly and lowers the oxygen concentration in alveoli

Question 19

Outline the review steps of anaesthetic procedure

Answer 19

Pre-surgical review - age, BMI, medical and surgical history, history of drug abuse, fasting time, airways assessment

Peri-surgical review - mixture calculation

Peri-surgical monitoring - cardiovascular, respiratory and thermoregulatory function (ECG, BP, pulse oximetry), EEG can also look at CNS activity

Question 20

Outline the stages of anaesthetic procedure

Answer 20

Induction - propofol administered and starting of inhalation and adjuvants

Maintenance - keep adjuvants in balance to maintain adequate depth

Recovery - agents withdrawn and functions monitored

Question 21

Outline the stages of anaesthetic depth

Answer 21

Stage I - analgesia (effects on spinothalamic tract)

Stage II - excitement (can also experience delirium or aggression)

Stage III - surgical anaesthesia (ideal)

Stage IV - medullary depression and death