Inhalational Agents

Question 1

What are the PHYSICAL properties of an ideal inhalational agent?

Answer 1

Liquid at room temp 🌡
Non-flammable or explosive 🔥💥
High saturated vapour pressure (easy vapourisation)
Low latent heat of vapourisation
Low specific heat capacity
Chemically stable in light and heat
Inert when in contact with metal, rubber and soda lime 🍋 
Long shelf-life
Inexpensive 💰 
No additives or preservatives 
Environmentally friendly 🌎 
Non-irritant, pleasant smelling 👃🏻

Question 2

What are the PHARMACOKINETIC properties of an ideal inhalational agent?

Answer 2

Low blood:gas partition coefficient (fast onset)

High oil:gas partition coefficient (low MAC)

Minimal metabolism; No fluoride ion production

Excretion via the lungs

Question 3

What are the PHARMACODYNAMIC properties of an ideal inhalational agent?

(Think in terms of systems)

Answer 3

CNS: 
Smooth, rapid induction
Only affects the CNS
Rapidly reversible Anaesthetic effects 
No increase in Cerebral blood flow or intracranial pressure
Some analgesic properties 
Not epileptogenic

CVS: ❤️
No CV depression
No sensitisation of myocardium to catecholamines
No decrease in coronary flow

Resp: 💨
No breath-holding, laryngospasm, coughing, or increase in secretions
Bronchodilation
No resp depression

MSK: 💪🏻
Skeletal muscle relaxation
Non-trigger of malignant hyperthermia

GI:
Anti-emetic

Renal/hepatic/haematological:
Uneffected
No decrease in renal or hepatic blood flow

Other:
No effects on graced uterus
Not teratogenic/ carcinogenic
No interactions

Question 4

Which properties of an ideal inhalational agent determine the onset and potency?

Answer 4

The ONSET is determined by the BLOOD:GAS partition coefficient. The lower the blood gas solubility the faster the onset of action

The POTENCY is determined by the OIL:GAS partition coefficient. The higher the oil:gas partition coefficient the more potent the agent

Question 5

What is the significance of the BLOOD:GAS partition coefficient?

Answer 5

The BGPC is the ratio of the amount of anaesthetic in the blood to that in gas when the two phases are of equal volume and in equilibrium with each other at 37•C

There is a paradoxical relationship assoc between the BGPC and the onset of induction; the lower the BGCP the fast induction

Poorly soluble agents exert high partial pressure in the blood and have fast onset

The Anaesthetic effects are related to the partial pressure in BLOOD and consequently the BRAIN and not the absolute amount present.

Question 6

In ascending order give the BGPC for each inhalational agent.

Answer 6

Xenon 0.17

Desflurane 0.42

Nitrous Oxide 0.47

Sevoflurane 0.68

Isoflurane 1.4

Enflurane 1.9

Halothane 2.3

Question 7

What factors effect the onset of action?

Answer 7

BGPC

MAC

Alveolar ventilation

Respiratory irritability

Example Desflurane has a low BGPC but is of limited value as it is a resp irritant

N.B: an agent with rapid onset has also got a rapid recovery (off set)

Question 8

Draw a graph to represent the relationship of between the inspired conc and the alveolar conc for different inhalational agents

Answer 8

Needs graph inserting here

After prolonged administration of inhalational agents, the PP in the alveoli equilibrates with that in the arterial blood subsequently the brain.

This is steady state but for most agents is rarely achieved as the process takes many hours.

The time taken to reach this or a ratio of alveolar conc to inspired conc of 1 varies between agents and their BGPC

Agents with low BGPC reach equilibrium more quickly.