Gases in Solution Flashcards
What is the relationship between solubility of anaesthetic gases and speed of induction?
The less soluble the gas the faster the induction as it equilibrates in the alveoli more quickly.
What is the saturated vapour pressure?
The saturated vapour pressure refers to the pressure exerted by the molecules of a vapour at the point of equilibrium.
Equilibrium occurs when dissolved gas molecules leave the liquid at the same rate at which others dissolve.
State Henry’s Law?
At a constant temperature the amount of gas dissolved in a liquid is directly proportional to the partial pressure the gas is exerting on the liquid.
Aka the greater the pressure the more gas will be dissolved.
Note Henry’s Law applies to a given gas and liquid (as different gases have different solubilities)
Describe a clinical condition related to Henry’s Law?
Decompression sickness (the bends)
During scuba diving the pressure increases and therefore more nitrogen is dissolved in the blood. If a diver ascends too quickly then the nitrogen will form bubbles in the blood and joints leading to decompression sickness.
What is the relationship between temperature and solubility of gas in a liquid?
As temperature increases solubility of a gas decreases.
Conversely as temperature decreases solubility of a gas increases.
How do you describe a solution of a gas?
Solutions of gases are described as volumes of gas dissolved in a volume of liquid.
What is Bunsen’s solubility coefficient?
Bunsen: the volume of gas at standard temperature (0C) and pressure (1atm) dissolved in a unit volume of liquid.
What is Otswald’s solubility coefficient?
Ostwald: the volume of gas dissolved in a unit volume of liquid at the measured temperature and pressure.
Otswald solubility coefficient is preferred by anaesthetists as it uses measured temp and pressure.
Define the term partition coefficient?
The partition coefficient is the ratio of the amount of a given substance present in one phase compared with another, both phases being of equal volume and at equilibrium.
More simply it refers to the ratio of a substance in 2 phases.
E.g the blood gas partition coefficient of N2O is 0.47
Note the temperature must be specified. Need to know temp when running a blood gas.
The order of the coefficient is important e.g blood:gas coeffiecient for N2O will be different to the gas blood coefficient for N2O.
Partition coefficients can be between 2 liquids or a liquid and a gas.
What is the gas tension?
The term ‘gas tension’ can be used instead of ‘partial pressure’. The tension of a gas in solution is the partial pressure of the gas in equilibrium with the liquid.
When a blood warmer is used explain why bubbles form in the line?
As the temperature increases gas solubility decreases therefore less gas is dissolved, therefore gas bubbles form in the line.
A patient with rib fractures and a small pneumothorax is given entonox for analgesia, what are the risks of this and why?
The risk is expansion of the pneumothorax.
Nitrous oxide is more soluble than nitrogen therefore it will diffuse into the closed cavity (pneumothorax) faster than the nitrogen will diffuse out, potentially leading to expansion of the pneumothorax.
