This document discusses the laws governing the behavior of gases and their applications in anesthesia. It introduces Boyle's law, Charles' law, Gay-Lussac's law, Avogadro's law, Henry's law, and Dalton's law. It then discusses applications of these laws to oxygen, nitrous oxide, Entonox, Heliox, and volatile anesthetic agents. Specifically, it explains how the gas laws can be used to calculate oxygen volumes and nitrous oxide amounts in cylinders. It also discusses how Henry's law relates anesthetic vapor pressures to their blood concentrations.

1. DR UFOEGBUNAM M.P.
DEPARTMENT OF ANAESTHESIA UNTH
ENUGU.

2. INTRODUCTION
 All matter exists in one of three states or phases-solid,
liquid or gas.
 When a gas co-exists in equilibrium with its
corresponding liquid, the gas is termed a vapour
 The critical temperature is defined as the temperature
above which a substance cannot exist in a liquid state

3. Laws governing the flow of gases
 Pressure, Volume and Temperature : when describing the
behavior of gases the temperature, pressure and volume are
related in a consistent manner, which makes it possible to
formulate three gas laws
 Boyle’s law
 Charles’s law
 Gay-lussac’s law
 Others are
 Avogadro’s law
 Henry’s law
 Dalton’s law

4. Boyle’s law
 This is the first gas law and states that at constant
temperature, the volume of a fixed mass of a perfect
gas varies inversely with pressure.
 PV=K (at constant temperature)

5. Charles’s law
 This is the second gas law and states that at constant
pressure, the volume of a fixed mass of a gas is
proportional to its temperature.
 V/T =K (at constant pressure)

6. Gay-Lussac’s law
 This is the third gas law and states that at constant
volume, pressure of a fixed mass of gas is directly
proportional to its temperature
 P/T =K (at a constant volume)

7. The combined gas law
 Is derived from all three gas laws and is as follows
 PT= T OR alternatively expressed P1V1=P2V2
T T1T2
Provided two of the variables are known, the third can
be assumed to remain constant, the gas laws can be
used to calculate changes in one of the known
variables when the other alters. For example, if the
initial pressure and volume of an oxygen cylinder is
137bar and 1.2 litres respectively(and temperature is
assumed to remain constant), then the volume of the
oxygen at a pressure of 1 bar can be calculated.

8. Cont…
 Assuming room pressure is also roughly 1 bar then this
approximates to the volume of oxygen available to the
patient, before the cylinder empties.
 Furthermore, if the rate of oxygen use is known, then
the time remaining for use of that oxygen cylinder can
be calculated
 When making these calculations it does not matter
which units are used for pressure or volume provided
consistency is maintained

9. Units of pressure
Units Equivalent value
Atmospheres (atm)
Bar (bar)
Kilopascal (KPa)
Millimeters of mercury(mmHg)
Torr (mmHg O0C)
Centimeters of water (cmH20)
Pounds per square inch(PSI)
1
1
1O1.3
760
760
1033.2
14.7

10. Avogadro’s law
 Avogadro’s law states that at a constant temperature and
pressure a given volume of any gas will contain the same
number of molecules. The converse of this is that at a
given temperature and pressure, 1 mole of any gas will
occupy the same volume( 22.4 litres at 1 atmosphere and
0oC
 Both Avogadro’s law and combined gas law are used to
derive the ideal gas law
PV=nRT
n=number of moles of gas, R=universal gas
constant(8.314). The ideal gas law is of relevance when
considering the behavior of nitrous oxide in cylinder

11. Henry’s law
 States that at a constant temperature, the amount of a
given gas dissolved in a given liquid is directly
proportional to the partial pressure of the gas in
contact with the liquid.
 Temperature affects the solubility of gases such that at
a higher temperatures gas will be less soluble in a
liquid
 Henry’s law is of importance when considering the
way inhaled anaesthetic vapours and gases behave
physiologically

12. Dalton’s law
 States that the pressure of a gas in a mixture of gases is
independent of the pressure of the other gases in the
mixture.
 Dalton’s law can be explained by the fact that in a
mixture of gases the molecules are so far apart from
one another, that each gas behaves as though the
others were not present
 Dalton’s law explains why vapour pressure is not
affected by ambient pressure.

13. Imperfections of the gas laws
 In reality, the gas laws described are not always true
and they refer to a theoretical “ideal” gas
 However, these concepts generally hold true and as
long as correction factors are used, the gas laws have
practical applications
 Note that the gas laws are only accurate when applied
to gases above their critical temperature

14. APPLICATIONS: OXYGEN
 Boiling point -1830C, critical temperature of -1190C , it
exists as gas at room temperature and therefore obeys
gas laws
 Boyle’s law can be applied to oxygen, which means that
the reading on the pressure gauge of an oxygen
cylinder gives a true indication of the volume
remaining.
 However, inaccuracies may arise in this respect if large
alterations in ambient temperature occur.

15. Cont..
 Oxygen can be stored under pressure in cylinders, it can
also be combined to form cylinder banks attached to
manifold to reduce cost, transportation and constant
change of exhausted cylinders.
 Oxygen has to be cooled to below -1180C to change to a
liquid and as the change occurs , it occupies a much
smaller volume.
 When a small volume of liquid oxygen is warmed it will
make a very large volume of oxygen gas
 In liquid form, a very large quantity of oxygen can be
transported or stored in a low volume

16. Cont..
 Vacuum insulated evaporator (VIE): Is a container
designed to store liquid oxygen

17. Nitrous oxide
 Boiling point of -88.60C and critical temperature is
+360C. Since in most countries it exists as a vapour in
equilibrium with its liquid phase, the gas laws do not
apply to nitrous oxide.
 Unlike oxygen, the pressure gauge tells you nothing
about the amount of nitrous oxide remaining in the
cylinder- it always reads around 52 bar at room
temperature
 To determine the quantity left in a cylinder it must be
weighed, the weight of the empty cylinder subtracted,
and then the number of moles of nitrous oxide
calculated using the Avogadro’s number

18. Cont…
 The ideal gas law can then be used to calculate the
approximate volume of gas remaining
 It is now easy to understand why nitrous oxide cylinders
are not filled to a given pressure.
 A value called the filling ratio is used
 Is the ratio of the weight of the cylinder filled with nitrous
to the weight of the cylinder when filled with water
 Filling ratio in U.K is 0.75, however this is reduced to 0.67
in hotter climates

19. Entonox
 BOC Medical trade name for 50% oxygen and 50% nitrous
oxide.
 For Entonox the critical temp. is termed the pseudo
critical temperature. The term is used to describe the
temperature at which Entonox starts to separate into its
constituent parts.
 The pseudo critical temperature of Entonox is -60C
(occurs mainly in temperate climate)
 If there is a possibility of separation occurring, the
manufacturers recommended that prior to use, cylinders
should be stored horizontally for 24hours at temp. above
100C or warmed to 100C for two hrs (or body temp. for 5
mins and then completely inverted 3 times

20. Heliox
 A mixture of oxygen and helium. Percentage of O2
may be as low as 21% but not higher than 50%
 Heliox is useful in patients with upper airway
obstruction.
 Theoretically patients with airway obstruction have
greater amount of turbulent compared to laminar flow
within their airways
 Helium has a lower density than oxygen ( and
nitrogen) and this may increase airway gas flow when
flow is turbulent. The density of a gas has no effect on
flow when the flow is laminar.

21. Application to volatile anaesthetic
agents
 Volatile anaesthetic agents exert their effects according
to the partial pressure of the agent in the blood
 According to Henry’s law, the partial pressure of
anaesthetic agent dissolved in the blood is
proportional to the partial pressure exerted by the
anaesthetic vapour in the alveli
 Dalton’s law tells us that the partial pressure exerted by
the anaesthetic vapour in the alveoli is independent of
the other gases present in the mixture.

22. Application to vaporizers
 A vaporizer does not generally alter the vapour
pressure of an anaesthetic agent, and this remains
relatively constant.
 One variable that can alter the anaesthetic vapour
pressure in a vaporizer is temperature
 During vaporization latent heat of vaporization cools
the liquid agent and cause a fall in the vapour pressure
and agent concentration
 Modern vaporizers incorporates various mechanisms
to compensate for any drop in temp. to overcome this
problem

23. Do you need to adjust your
vaporizer at high altitude
 Two concepts answer this question
 First, anaesthetic dept is controlled by altering the
partial pressure of anaesthetic agent in the alveolus
(Henry’s law)
 Second, only two factors affect the partial pressure
namely the vapour pressure of agent in the vaporizer
and the conc. of vapour in the inhaled gas mixture.
 The answer is that no change needs to be made to
vaporizer setting at altitude

24. THANK YOU