Module 3C: Overview of Inhalational Anaesthetic Agents

Question 1

What determines speed of induction of anaesthetic agent?

Answer 1

concentration in the brain

Question 2

Name 3 things that affect the concentration of volatiles in the brain

Answer 2

Delivery of the agent to the lungs

Uptake of agent from lungs to bloodstream:
- solubility of agent in blood
- Cardiac output

Uptake of agent into other tissues(besides brain)

Question 3

What governs the partial pressures of anaesthetic in all body tissues

Answer 3

alveolar partial pressure

Question 4

True or false: alveolar concentration of an agent is proportional to the alveolar partial pressure

Answer 4

True, factors decreasing the alveolar concentration slow down induction of anaesthesia

Question 5

What 2 things determines the delivery of an inhalational anaesthetic to the lungs?

Answer 5

Inspired concentration of the agent: higher concentration->faster induction

Alveolar ventilation: increased ventilation=increased induction + visa versa

Question 6

What determines the uptake of the inhalational from the lungs

Answer 6

solubility in blood:
- Agent that dissolves easily= SLOW INDUCTION
- Agent insoluble in blood= FASTER induction

Question 7

Why is the rate of induction SLOWER if the agent is more soluble in blood?

Answer 7

The circulation constantly carries agent away-> alveolar concentration not have a chance to build up-> concentration in brain rises slowly.

Less soluble agent will be carried to brain ready to bind to fatty tissue

Question 8

Why does a faster cardiac output lead to slower rate of induction?

Answer 8

blood needs enough time to travel through capillary/alveolar interface to pick up agent. HIGHER CO-> blood will be LESS saturated

Question 9

What 3 things affect uptake of inhalational by the tissues?

Answer 9

1. Tissue solubility
2. Tissue blood flow
3. Concentration gradient between blood and tissues

Question 10

Where is the greatest uptake of inhalational anaesthetics?

Answer 10

Vessel rich organs (heart, brain, lungs, kidneys)->reaches equilibrium

Second phase uptake by muscle group-> equilibrium is reached within 1-3 hours

Last phase uptake by poorly vascularized tissues (fat,bone)-> equilibrium takes many hours

Question 11

Name the factors that cause faster induction?

Answer 11

High inspired concentration of agent
Increased ventilation
Less soluble agent (in blood)
Decreased cardiac output
Decreased shunting

Question 12

Name the factors that lead to slower induction

Answer 12

Lower inspired concentration of agent
Decreased ventilation
More soluble agent (in blood)
Increased cardiac output
Increased shunting

Question 13

True or false: recovery from anaesthesia is slow in soluble agents.

Answer 13

true, recovery is slow in soluble agents and fast in poorly soluble agents

Question 14

What is the major route of removal of volatile agents

Answer 14

1. Via lungs
2. varying % by liver

Question 15

Which drug is mostly metabolised by liver and should not be given in liver impairment

Answer 15

Halothane 20% metabolization in liver

Question 16

What are some harmful metabolites

Answer 16

CF3 causes halothane hepatitis

Fluorides from methoxyflurane and enflurane causes renal impairment

Question 17

What is potency?

Answer 17

measure of a drug’s efficacy at a given dose
A more potent agent requires a lower dose to achieve the same effect; a less potent drug requires more

Question 18

What is MAC

Answer 18

Minimum Alveolar Concentration (in %) required to prevent
50% of patients from moving in response to a standard surgical
stimulus (incision) at sea level.

HIGHER MAC= LESS POTENT

LOWER MAC= MORE POTENT

Question 19

What factors decreases MAC?

Answer 19

Sedatives
N2O
Analgesics
Increased age
Hypotension
Hypothermia
Myxoedema

Question 20

What factors increase MAC

Answer 20

Alcoholism
Children
Hyperthermia
Thyrotoxicosis