eLFH - Gases in Solution

Question 1

Liquid at equilibrium

Answer 1

In a closed container, a liquid exists in 2 phases - liquid at bottom and vapour at top

Equilibrium occurs when evaporation rate = condensation rate

Question 2

Saturated Vapour Pressure

Answer 2

Partial pressure exerted by the vapour molecules at the point of equilibrium

Question 3

Henry’s Law

Answer 3

At a fixed temperature, the amount of a gas dissolved in a given liquid is directly proportional to the partial pressure of the gas in equilibrium with the liquid

I.e the higher the pressure, the more dissolved gas

Question 4

What causes the ‘bends’ when diving

Answer 4

Henry’s Law
Rapid decompression and decrease in pressure as re-surfacing
Nitrogen comes out of solution and forms bubbles in joints

Question 5

Effect of temperature on solubility

Answer 5

Solubility decreases as temperature increases

Question 6

Describing solubility

Answer 6

Described as volume of gas dissolved in a volume of liquid

2 ways of describing this are:
- Bunsen solubility coefficient
- Ostwald solubility coefficient

Question 7

Bunsen solubility coefficient

Answer 7

Volume of gas is corrected to standard temperature and pressure dissolved in a unit volume of a liquid at the temperature concerned where the partial pressure of the gas above the liquid is 1 atmosphere

Question 8

Ostwald solubility coefficient

Answer 8

The volume of gas dissolved in a unit volume of liquid at the temperature concerned

Preferred by anaesthetists as not corrected to standard temperature and pressure, but measured at a known temp and pressure

Question 9

Partition coefficient

Answer 9

Ratio of the amount of a given substance present in one phase compared with another at equilibrium

Higher number = more soluble

Similar to Ostwald coefficient but need to specify the order of the phases being compared (i.e blood-gas coefficient is different to gas-blood coefficient)

Question 10

Gas tension in solution

Answer 10

Same as partial pressure of the gas in equilibrium with it

Question 11

How does solubility of gas affect speed of anaesthesia induction

Answer 11

Lower solubility gases induce anaesthesia faster

Less soluble agent achieves equilibrium in the alveoli faster than more soluble agents