3.7 Comparing Volatile Anaesthetics

Question 1

What is the volatile agent you use for inhalational induction and why?

Answer 1

Sevoflurane

Non-irritant,
sweet smelling,

has optimal
oil:gas
blood:gas
coefficients

Question 2

compare and contrast the three commonly used volatiles:

1. isoflurane,
2. sevoflurane,
3. Desflurane.

Physical

Answer 2

1. Irritant
   Coughing and breath holding
2. Non-irritant
   Sweet smelling
3. Laryngospasm
   Excessive secretions

Question 3

compare and contrast the three commonly used volatiles:

1. isoflurane,
2. sevoflurane,
3. Desflurane.

Pharmacokinetic

Answer 3

1. B:G—1.4
   O:G—98
   MAC—1.1
2. B:G—0.7
   O:G—80
   MAC—1.8
3. B:G—0.45
   O:G—29
   MAC—6.6

Question 4

compare and contrast the three commonly used volatiles:

1. isoflurane,
2. sevoflurane,
3. Desflurane.

Pharmacodynamic

Answer 4

1. 0.2% metabolised
2. 3%–5%
3. Resistant to metabolism

Question 5

What is Blood:Gas solubility coefficient?

What does it explain?

can you draw a graph to explain? see Figure 3.3.

Answer 5

Ratio of the amount of anaesthetic in blood and gas when the two phases
are of equal volumes and pressure and in equilibrium at 37°C.

High B:G—
The gas is more soluble in the blood,
so low partial pressure in blood leading to
low partial pressure in brain;
slow onset.

And vice versa

Question 6

Graph Blood Gas

Answer 6

Question 7

What do you understand by oil:Gas partition coefficient?

Again, draw a graph to show its importance. see Figure 3.4.

Answer 7

O:G—link between lipid solubility and potency.

High o:G—
Higher lipid solubility; more gas reaches brain.

Hence, the drug has got good potency.

Question 8

Oil Gas Partition

Answer 8

Question 9

What factors influence the speed at which inhaled agents attain equilibrium?

Answer 9

To achieve equilibrium the gas must be at the same concentration

in the brain as in the delivered gas flow.

The rate at which this occurs depends upon:

1. Drug factors
2. Ventilation factors
3. circulation factors

Question 10

Drug factors

Answer 10

• Dilution within existing gases

Question 11

Ventilation factors

Answer 11

1. • Inhaled concentration
2. • Alveolar ventilation
3. • Diffusion
4. • Blood/gas partition coefficient.
     A low b/g partition coefficient indicates
     low solubility so equilibrium will be reached
     with relatively small transfers
     of gas, and therefore will be rapid
5. • Pulmonary blood flow
6. • V/Q matching
7. • Concentration effect:
     More of the gas means greater concentration which
     equates to a quicker attainment of equilibrium
8. • Second gas effect:
     As nitrous is absorbed it
     increases the concentration of the volatile

Question 12

Circulation factors

Answer 12

1. • Cardiac output
2. • Distribution to other tissues:
     Uptake in tissues is related to their blood
     flow, solubility and arterio-venous difference