SOLUBILITY IN ANAESTHESIA PRACTICE

1. SOLUBILITY
MOHAMED ANWER
RIFKY
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2. 1-THE CONCEPT OF SOLUBILITY:
At equilibrium >> dissolved gas molecules leave the liquid at the same rate at which others dissolve>>a saturated
solution.
As an example>>water and
nitrogen.
N.B Saturated vapour
pressure.
A-Henry's law states that 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.
Twice as many molecules of gas above the
water>>
{200 kPa (2
bar)}
Temperature must be kept
constant-
relevant to deep-sea divers>>
nitrogen and other gases, if breathed under pressure and,if a return
to atmospheric pressure is made
rapidly>>
decompression sickness or the 'bends’>>bubbles in the joints and
elsewhere.
It also can occur in hyperbaric chambers.
B-A left cylinder is at a
room temperature of 20°C, the right one at body temperature,
37°C.
**pressure of the nitrogen in both cases is 100 kPa.
At room temperature volume of nitrogen
is 0.016 litre, while at body temperature it is only 0.014
litre.
As a kettle boils, the simmering >>release of bubbles>>
uair come out of solution . Similarly, in the operation room
bubbles come of the infusion line through a blood –warming coil.
C- Different gases with with differerent
solubility,
At equilibrium ,nitrous oxide dissolved >> 0.39 litre
compared with 0.014 litre of nitrogen.Temp.at
37degree.
D-The liquid too must be specified when considering
solubility.
0.39 litre nitrous oxide dissolves in 1 litre water at body
temperature, whereas 1 litre blood dissolves 0.47 litre. Temp.
37 degree.
2-SOLUBILITY COEFFICIENTS:
A-The solubility of solids =number of moles
or of millimoles of the solute which dissolve in 1 litre of the solvent, but
for gases =its volume as in the previous examples. (gas is normally corrected to s.t.p).
.
B-The Bunsen solubility coefficient is the volume of gas, corrected
to s.t.p., which dissolves in one unit volume of the liquid at the
temperature concerned, where the partial pressure of the gas
above the liquid is one standard atmosphere pressure.
(Quoted in scientific
tables)
C-The Ostwald solubility coefficient is the volume of gas which
dissolves in one unit volume of the liquid at the temperature concerned..
(The volume of gas is not corrected to
S.T.P).
It is independent of
pressure.
The volume of nitrogen dissolved measured at the ambient
Pressure remains constant even the number of nitrogen molecules in solution is also doubled >>(Boyle's law).
3-THE PARTITION COEFFICIENT: The partition coefficient is defined as the ratio of the
amount of
substance present in one phase compared with another,
the two
phases being of equal volume and in equilibrium.
Difference between the partition coefficient and the
Ostwald
solubility coefficient:
a-The relative order of the phases of the partition
coefficient must be clearly identified>> (gas-blood instead of the blood-gas). N2O (2.1 instead of 0.47.for
blood/ gas ).
b-Can be applied to two liquids>>1.4 litres nitrous oxide dissolves in 1 litre oil,whereas, 0.47 litre in 1 litre
N.B The word tension =partial pressure for gases in SOLUTION.

3. 4-SOLUBILITY AND UPTAKE OF ANAESTHETICS:
A-Oswald’s sol. Coeffi. of ether (12), higher than halothane (2.3) and nitrous oxide
(0.47)>
>>Ether is carried away from the lungs more rapidly>>alveolar air builds up more
slowly.
1-Transport of the
anaesthetic to the lungs (ventilation) by the pipe
joining it to the second reservoir ( ).) 2-removal
of the anaesth.by
pulmonary blood flow >>next pipe as ( ).) 3-N2O
carried away only
relatively slowly by the cardiac output
bl./gas sol. coeff. =0.47.(The upper diagram).
4-The alveolar concentration of
ether
is only slightly above the tissue
level.(lower diagram ).The wide
pipe in the figure.
B-Build-up exponential
curves
(Percentage of anaesth.final values and plotted against
time).
Equilibrium is approached in 10 to 15 minutes with (
N2O).
(concentrations of anaesthetics in the
blood
and brain are close to the alveolar
conc.).
C-_Nitrous oxide is much more soluble than nitrogen.>>slight error (Benedict Roth
spirometer)>>resulting in a rise in the concentrations of O2 &N2>>.
Phenomena called the second-gas effect (anaesth. !!)>>
**A similar phenomenon (diffusion hypoxia.),excretion of nitrous
oxide at
the end of anaesthesia>>fall of oxygen concentration.
5-OIL SOLUBILITY AND ANAESTHETIC POTENCY: Fat and oil are very similar -
It is easier to measure solubilities in oil (used for
measurement).
A-The solubility in oil are in a different order from those in
blood
( nitrous oxide is still the least soluble, the agent with the
greatest
solubility is now halothane , ether being midway
between nitrous oxide and halothane.>>
basis of the Meyer-Overton theory.The potency of the anaesthetic cocept.
B-The vertical axis of the graph is the MAC (minimum alveolar concentration)>>50% of patients cease to move in
response to a stimulus.the horizontal
axis are the oil solubility coefficients of the anaesthetics>> cover the range of anaesthetics in ( logarithmic
scales) >>
anaesthetics are found to lie on a straight line and
from the slope of the line it is seen that an anaesthetic with a high
solubility
in oil has a low MAC value.
C-Anaesthetics >>
interfering with the molecular configuration of the long fatty-acid chains.
**The attachment of the anaesthetic molecules to the chain
molecules is relatively loose and readily reversible with Van der Waals type bond,
**Trace amounts of these anaesthetics may later
be released (generally small)>>malignant hyperpyrexia.
Applications
of
solubility
wide
pipe
net surplus of oxygen.
N20