SOLUBILITY

BYDR ABDUL JALIL

ASSISTANT PROFESSORDEPARTMENT OF ANAESTHESIA

AND ICULAHORE GENERAL HOSPITAL/PGMI

LAHORE

Solubility

Vapour Pressure Equilibrium State Saturated Vapour Pressure

Factors Affecting Solubility Partial Pressure Temperature Gas Concerned Liquid Concerned

Effect of Partial Pressure Effect of partial pressure on solubility

can be well understood by taking into account the statement of Henry’s Law:

“AT A PARTICULAR TEMPERATURE THE AMOUNT OF A GIVEN GAS DISSOLVED IN A GIVEN LIQUID IS DIRECTLY PROPORTIONAL TO THE PARTIAL PRESSURE OF THE GAS IN EQUILIBRIUM WITH THE LIQUID”

Applied aspect of effect of pressure

Decompression Sickness or ‘The Bends’

Effect of Temperature

The temperature of the liquid has an inverse relationship to the solubility of

gas in it.

Applied aspect of effect of temperature

IV Fluid in OT (bubbles of air may form in saline in an infusion line which has passed through a blood-warming coil).

Effect of Gas Concerned

Different gases are found to have different solubilities.

Effect of Liquid Concerned

The solubility of a given gas varies with the type of the liquid present.

Summary

To summarize, the solubility of a gas depends on:

The Partial Pressure The Temperature The Gas and The Liquid Concerned

Solubility Coefficients

Solubilty of solids (moles, millimoles)

Solubility of Gases (in terms of volume; at constant temperature and pressure)

Bunsen Solubility Coefficient

“Volume of a gas corrected to STP, which dissolves in 1 unit volume of the liquid”

Usually quoted in: Scientific Tables Text Books

Ostwald Solubility Coefficient“The volume of a gas which dissolves in

1 unit volume of the liquid at the temperature concerned”

Volume of gas is not corrected to STP in this case but, instead, is measured at a known temperature and pressure.

Pressure does not modify the

dissolved volume, provided that the volume is measured at the ambient pressure

Partition Coefficient

“The ratio of the amount of substance present in one phase compared with another, the two phases being of equal volume and in equilibrium”

Partition Coefficient Temperature (37°C) Phases (blood and gases) Relative order of the phases (N2O

blood- gas partition coefficient 0.47 and N2O gas- blood partition coefficient 2.1)

Partition Coefficient

It can be applied to two liquids, in contrast to Ostwald solubility

coefficient.

Tension of a Gas Tension is used in place of partial

pressure.

“The tension of a gas in solution is the partial pressure of the gas which would be in equilibrium with it”

Solubility and Uptake of Anaesthetics

Clinical applications of

solubility

Solubility coefficient of different anaesthetics determines their solubility in blood.

The greater the solubility of anaesthetic, the greater is its transportation across alveolar wall and the lesser is the build up of its concentration in the alveolar air.

From the above mentioned facts it is concluded that during the course of an inspiration of a gas mixture containing N2O, N2O is absorbed into

the blood stream faster than O2 or N2, so at peak inspiration, when the pressure in the alveoli has equalized with ambient pressure, there is net surplus of N2 and O2 molecules; this phenomenon is referred to as the

“second gas effect”

Diffusion Hypoxia Excretion of N2O at the end of anaesthesia

N2O from blood to alveoli

Gases in the alveolar air are diluted

Resulting into fall of O2 concentration; known as “DIFFUSION HYPOXIA”

Oil Solubility And Anaesthetic Potency

Oil is used in measurements because: Fat and Oil are similar Fat is a normal constituent of cell

membranes and neurons in particular.

MAC

“MAC (Minimum Alveolar Concentration) refers to the concentration at which 50% of the patients cease to move in response to a stimulus”

Anaesthetics with high solubility in oil have a low MAC value and hence, are more potent. This effect is due to:

Mild attraction between anaesthetic molecule and fatty acid chains, interference in molecular configuration of long chain fatty acids in the brain, the attachment being readily reversible and loose with van der Waal type bonds .

(Long chain fatty acids)

Long Carbon Chain Molecules Anaesthetics are attracted to long carbon

chain molecules in rubber and plastics. Substantial quantities of anaesthetics may

be absorbed into the tubing used in breathing system and may later be released and administered to patients even if no anaesthetic vaporizer is turned on.

The anaesthetic released in this way might be small but the quantity can be sufficient to trigger the onset of “Malignant Hyperthermia”.

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