Anaesthetics - principles + pharmacology

Question 1

• Understand the mechanism, action and pharmacological kinetics of: local anaesthetic agents, general anaesthetic agents, opiates and muscle relaxants.
• Describe the “triad of anaesthesia” and discuss how this relates to the concept of balanced anaesthesia.
• Discuss the physiological effects of general and regional anaesthesia and how these may interact with patients’ underlying illness.

Answer 1

. Pharmacokinetics - ADME, how the body deals with the drug

Question 2

Purpose of general anaesthetic

Answer 2

Produces insensibility in the whole body, usually causing unconsciousness

Question 3

Purpose of regional anaesthetic

Answer 3

The agent is applied to the nerve and insensibility is produced in a distal area, remote from the injection, e.g. spinal or epidural block

Question 4

Purpose of local anaesthetic

Answer 4

Agent applied directly to tissue and insensibility is produced in only that part

Question 5

Types of anaesthetic drugs (5)

Answer 5

Inhalational anaesthetics, i.e. general
Intravenous anaesthetics, i.e. general
Muscle relaxants
Local anaesthetics
Analgesics, e.g. opiates

Question 6

What is the triad of anaesthesia

-an anaesthetic may consist of varying contributions from all 3 but doesn’t need to have all 3

Answer 6

Analgesia
Hypnosis
Relaxation

Question 7

Describe hypnosis

Answer 7

Unconsciousness.

Necessary component of any general anaesthetic

Question 8

Describe analgesia in terms of anaesthesia

Answer 8

Removal of perception of unpleasant stimulus

Question 9

Describe relaxation in terms of anaesthesia

Answer 9

Refers to skeletal muscle relaxation necessary to provide immobility for certain procedures

Question 10

Which components of the triad of anaesthesia do the following have

• GA
• LA
• Opiates
• Muscle relaxants

Answer 10

GA - all 3
LA - analgesia + relaxation
Opiates - analgesia + hypnosis
Muscle relaxants - relaxation

Question 11

What is balanced anaesthesia

Answer 11

Means that you can control the individual components of the triad and allow different drugs to do different jobs, e.g. can use less GA by adding a muscle relaxant

Essentially allows doses of individual drugs to be minimised

Question 12

Problems with muscle relaxants

Answer 12

Means that artificial ventilation is needed to maintain the airway

Question 13

Patients can be awake but paralysed and unable to communicate due to the separation of hypnosis (unconsciousness) from muscle relaxation

What is this called

Answer 13

Awareness

Question 14

Mechanism of GA agents

-what ion channels are opened

Answer 14

Suppress neuronal activity by hyperpolarising neurones (so they’re less likely to fire) - hyperpolarised by opening chloride channels or suppressing excitatory synaptic activity

Question 15

How do inhalational GA agents hyperpolarise neurons

-what ion channels opened

Answer 15

Dissolves in the membrane and causes the Cl channel to open

Question 16

How do IV GA agents hyperpolarise neurones

-what ion channels opened

Answer 16

Allosteric binding to GABA receptors which stimulates Cl channels to open

Question 17

Action of GA

• what is lost
• what is spared

Answer 17

Removes cerebral function

• complex processes lost first, primitive functions later
• remove consciousness

Reflexes are spared (e.g. spinal reflexes) and other automatic functions

Question 18

Cautions during GA use (2)

Answer 18

Need to maintain airway

Control breathing

Question 19

Name some IV anaesthetic agents

Answer 19

Propofol

Thiopentone

Question 20

How do drugs like propofol cause rapid onset unconsciousness and rapid recovery

Answer 20

Fat soluble so cross BBB very quickly and cause unconsciousness as soon as it hits the brain

Also leave the circulation very quickly

Question 21

Pharmacokinetics of IV anaesthetics

• absorption
• distribution
• metabolism
• excretion

Answer 21

Aborsbed into blood so blood conc. of the agent high initially then falls as it moves into highly perfused tissues

Distributed to organs, muscle and fat

Metabolised by liver

Excreted by kidney

Question 22

Does muscle or fat absorb IV anaesthetics quicker

Answer 22

Muscle, but the effect is large because of huge mass of skeletal muscle

Fat can store large amounts of the agent due to to high fat solubility of the drug

Question 23

Inhaled anaesthetics are what kind of compounds + name on

Answer 23

Halogenated hydrocarbons, e.g. sevoflurane

Question 24

How do inhalational anaesthetics work once inhaled

Answer 24

The gas moves down the conc. gradient into the blood (from the lungs) then to the brain to acheive a high enough partial pressure to produce unconsciousness

Continuously breathed during the procedure

Question 25

Inhalational drugs have a MAC (minimum alveolar conc.) which is a measure of What would a drug with a low MAC mean?

Answer 25

the concentration of the drug required in the alveoli in order to produce any anaesthetic effect A low MAC means the drug is potent as less of it is needed

Question 26

Is induction of inhalational anaesthetics fast or slow

Answer 26

Slow

Question 27

Main role of inhalational anaesthetics

Answer 27

To prolong or continue anaesthesia, i.e. MAINTAIN ANAESTHESIA (usually to maintain IV induction)

Question 28

How are inhalational anaesthetics reversed

Answer 28

Machine is switched off and patient is given a gas mixture with no anaesthetic agent in it to reverse the conc. gradient and decrease the alveolar conc. therefore decreasing the blood conc. of the drug and subsequently the brain

Question 29

Most common sequence of GA uses what agents

Answer 29

IV induction followed by inhalational maintenance

Question 30

Physiological effects of GA on the CVS -central (3) -peripheral (3)

Answer 30

Central effects - reduce sympathetic outflow activity - reduce contractility (negative inotropic effect) - vasodilation Peripheral - reduce contractility of vessels (negative inotropic) - vasodilation leading to reduced peripheral resistance - venodilation --> decreased VR --> decreased CO

Question 31

Physiological effects of GA on resp function (4)

Answer 31

Reduce hypoxic and hypercarbic drive Reduce tidal volume so increase RR Paralyse cilia Decrease FRC (i.e. decrease lung volumes so interferes with V/Q matching)

Question 32

GA agents decrease lung volumes so interfere with V/Q matching in the lungs and this effect persists post-op, so patients should be given what post-op

Answer 32

Oxygen (probably several days)

Question 33

Action of muscle relaxants

Answer 33

Paralyse skeletal muscle in an indiscriminate way, therefore airway muscles are affected too even though you don't want to

Question 34

Indications for muscle relaxant use (3)

Answer 34

Ventilation & intubation When immobility is essential (e.g. microscopic surgery, neurosurgery) Body cavity surgery

Question 35

Disadvantages of muscle relaxants (3)

Answer 35

Loss of awareness - awake but paralysed Incomplete reversal - airway obstruction/resp insufficiency persists in immediate post-op period (i.e. at the end of the anaesthetic) but unlikely to persist when they're back on the ward Apnoea - hence the need for airway + ventilation support during op

Question 36

If the muscle relaxant is given systemically (orally) then also must provide which triad of anaesthesia

Answer 36

Hypnosis (unconsciousness)

Question 37

Analgesia is most commonly used in what way in terms of anaesthetising?

Answer 37

In conjunction with unconsciousness (i.e. GA) as part of 'balanced anaesthesia'

Question 38

Does analgesia always have to be accompanied by GA ?

Answer 38

No, can use it as regional anaesthesia where it's used itself, e.g. spinal or epidural analgesia

Question 39

What's the point of intra-operative analgesia for someone who's unconscious? + benefit of opiate analgesics

Answer 39

Prevents arousal from the pain Opiates have direct sedative effect, contributing to GA Suppress reflex responses to pain, e.g. tachycardia, hypertension

Question 40

Benefits of local + regional anaesthesia (4)

Answer 40

Retain awareness Complete analgesia with no hypnotic effect Can allow lower levels of GA to be used if GA is needed Less physiological effects than GA, mainly effects on CVS

Question 41

Mechanism of local anaesthetic

Answer 41

Block Na+ channels so prevent AP from propagating

Question 42

Disadvantages of local anaesthetic

Answer 42

Lots of side effects and toxic if delivered wrongly

Question 43

Local + regional anaesthesia mainly have effects on which organ system

Answer 43

CVS, resp function spared