This topic deals with the important gas laws which govern the behaviour of anaesthetic gases.

1. GAS LAWS FORGAS LAWS FOR
ANAESTHESIOLOGISTSANAESTHESIOLOGISTS

2. Is it necessary for an Anesthetist to studyIs it necessary for an Anesthetist to study
about theabout the BORINGBORING gas laws..?gas laws..?

3. Yes.,of course…..Yes.,of course…..
•Better understanding of the behavior of anesthetic gasesBetter understanding of the behavior of anesthetic gases
•Smooth conduct of anesthesiaSmooth conduct of anesthesia
•For the proper use of Boyles machineFor the proper use of Boyles machine
•To escape from an explosion…!To escape from an explosion…!

4. An new oxygen cylinder is being opened . How do you knowAn new oxygen cylinder is being opened . How do you know
whether it will last for the entire surgical procedure or not..?whether it will last for the entire surgical procedure or not..?

5. Pressure gauges show the pressure of thePressure gauges show the pressure of the
contents…not the volumecontents…not the volume
We assume…if pressure of cylinder getsWe assume…if pressure of cylinder gets
halved, the contents have also become halved…halved, the contents have also become halved…
Is it true….?Is it true….?

6. Knowing the contents of the cylinder…Knowing the contents of the cylinder…
Relationship between the volume,pressure and temperatureRelationship between the volume,pressure and temperature
O2O2
N2ON2O
Boyle’s lawBoyle’s law
Charles lawCharles law
Gay-Lussac’s lawGay-Lussac’s law

7. Boyle’s law: At constant temp, volume is inversely proportionalBoyle’s law: At constant temp, volume is inversely proportional
to its absolute pressure.to its absolute pressure.
Charles law: At constant press, volume is directly proportional toCharles law: At constant press, volume is directly proportional to
Its absolute temp.Its absolute temp.
Gay-Lussacs law: At constant volume, pressure is directlyGay-Lussacs law: At constant volume, pressure is directly
proportional to its absolute temp.proportional to its absolute temp.
PiVi/Ti = PfVf/TfPiVi/Ti = PfVf/Tf

8. Try this calculation…!Try this calculation…!
An O2 cylinder indicates a pressure of 132 atm while
being transported in hot weather when the temp is 440C.
What pressure will be indicated when it is placed in a
cool operation theatre when the temp is 200
C?

9. PiVi/Ti = PfVf/TfPiVi/Ti = PfVf/Tf
132 x V1 / 44132 x V1 / 44 == P2 x V2 / 20P2 x V2 / 20
132 / 44132 / 44 == P2 /20P2 /20
P2P2 = 132 x20 / 44= 132 x20 / 44 == 60 atm60 atm
ANSWER:

10. NITROUS OXIDENITROUS OXIDE
• When compressed - NWhen compressed - N22O gets converted to liquidO gets converted to liquid
• So, NSo, N22O cylinders are filled with liquid.O cylinders are filled with liquid.
THIS PROCESS IS TEMPERATURE DEPENDENTTHIS PROCESS IS TEMPERATURE DEPENDENT
Above certain temperature NAbove certain temperature N22O cant be liquefied,O cant be liquefied,
however much pressure is applied…That temperaturehowever much pressure is applied…That temperature
is called “critical temperature”is called “critical temperature”

11. Can we fill the NCan we fill the N22O cylinders completelyO cylinders completely
with liquid Nwith liquid N22O….?O….?

12. NN22O CYLINDERS:O CYLINDERS:
•Filling ratio
•Critical temperature
•Why older machine doesn’t have
a pressure gauge..?
0.67
“Wt of N2O in the cylinder/ Wt of H2O,the cylinder can hold”
In India, N2O cylinders are filled up to 67% of its total capacity

13. 51 atm 25 atm 18 atm
75atm 94atm
20C -10C -20C
37C 45C
PRESSURE IN NPRESSURE IN N22O CYLINDERSO CYLINDERS
As the N2O cylinders are exposed to high temperature,
the liquid N2O gets converted to gaseous form, thus
increasing the pressure inside the cylinder

14. Oil or grease application in any part of theOil or grease application in any part of the
Boyle’s apparatus is not advised…Boyle’s apparatus is not advised…
Why…?Why…?

15. ADIABATIC COMPRESSION:ADIABATIC COMPRESSION:
When a gas is subjected to sudden compression, heat energyWhen a gas is subjected to sudden compression, heat energy
is produced rapidly. If there is no time for dissipation of thisis produced rapidly. If there is no time for dissipation of this
heat, the temperature of the system rises enormouslyheat, the temperature of the system rises enormously.
First law of thermodynamicsFirst law of thermodynamics

16. When an Oxygen cylinder is opened , the compressed
gas expands and surges inside the Boyles machine.
Once it enters into the system , again it gets compressed
rapidly inside the pressure gauge and pressure regulator.
There adiabatic compression occurs and temperature rises
enormously which may lead on to an explosion , if oil is
used to lubricate the tight cylinder valve…

17. Have you seen ice crystal formation over a cylinder valveHave you seen ice crystal formation over a cylinder valve
if Nif N22O leaks through it?O leaks through it?
Joule-Thomson effectJoule-Thomson effect

18. Joule-Thomson effect:Joule-Thomson effect:
When a compressed gas is allowed a free expansion
through a narrow opening, the temperature of the
system falls rapidly.

19. Joule – Thomson effectJoule – Thomson effect
Anaesthetic significancesAnaesthetic significances
1.1. NN22O cools rapidly. vaporisation decreases..O cools rapidly. vaporisation decreases..
2.2. If the NIf the N22O cylinder contains water vapour, iceO cylinder contains water vapour, ice
crystals form inside the cylinder valve whichcrystals form inside the cylinder valve which
blocks the flow.blocks the flow.
3. Application in N3. Application in N22O cryoprobeO cryoprobe

20. We often talk about partial pressure when we refer gaseousWe often talk about partial pressure when we refer gaseous
pressures. What do you mean by that..?pressures. What do you mean by that..?
Daltons law of partial pressureDaltons law of partial pressure

21. 0
0 0
x
X/4
Total pressure=X + x/4
Daltons law of partial pressure

22. Daltons law:Daltons law:
In a mixture of gases having no chemical interactionIn a mixture of gases having no chemical interaction
between them,each constituent gas exerts the samebetween them,each constituent gas exerts the same
pressure which it would have exerted if it alone hadpressure which it would have exerted if it alone had
occupied the volume of the mixture,provided theoccupied the volume of the mixture,provided the
temperature be the same as that of the mixture.temperature be the same as that of the mixture.

23. It follows that in a mixture of gases, the pressure exertedIt follows that in a mixture of gases, the pressure exerted
by each gas is the partial pressure and the pressure of aby each gas is the partial pressure and the pressure of a
mixture of gases is the sum of the partial pressures of itsmixture of gases is the sum of the partial pressures of its
constituents.constituents.
Daltons law:Daltons law:

24. Daltons law:Daltons law:
In a mixture of gases, each constituent exerts the sameIn a mixture of gases, each constituent exerts the same
proportion of total pressure as its volume is to theproportion of total pressure as its volume is to the
total volumetotal volume
What happens in an alveoli?What happens in an alveoli?

25. O2 – 100
CO2 – 40
H2O – 47
N2 - 573
Pulmonary capillary
vein arteryO2- 40
CO2- 46
H2O- 47
N2- 627
O2- 100
CO2- 40
H2O- 47
N2- 573
alveolus
“Gases move across a partial pressure gradient”

26. On the Mount Everest…On the Mount Everest…
Altitude – 8800 m ( 29,000 ft )Altitude – 8800 m ( 29,000 ft )
Atm. Pressure – 240 torrAtm. Pressure – 240 torr
Alveolar OAlveolar O22 – 23 torr– 23 torr
PVOPVO22– 40 torr– 40 torr
Life is possible only when 100% OLife is possible only when 100% O22 is supplemented, whenis supplemented, when
PAOPAO22 becomes 153 torr.becomes 153 torr.

27. Recently filled D-type NRecently filled D-type N22O cylinder weighs 53.45 kg withO cylinder weighs 53.45 kg with
its contents. The tare weight of the cylinder is 24.48 kg.its contents. The tare weight of the cylinder is 24.48 kg.
How long will it last if the anesthetist draws 2 lpm of NHow long will it last if the anesthetist draws 2 lpm of N22O?O?
Try this calculation…!Try this calculation…!
Avagadros hypothesisAvagadros hypothesis

28. Avagadros hypothesis:Avagadros hypothesis:
One gram molecular weight of any gaseous substanceOne gram molecular weight of any gaseous substance
will occupy a volume of 22.4L at S.T.P and will containwill occupy a volume of 22.4L at S.T.P and will contain
6.023 x 106.023 x 102323
number of moleculesnumber of molecules

29. ANSWER:ANSWER:
•Gram molecular wt of NGram molecular wt of N22O = 44O = 44
•If 44 grams of NIf 44 grams of N22O gives rise to 22.4 litres,O gives rise to 22.4 litres,
then 28.97 k.g ( 53.45 – 24.48 ) will getthen 28.97 k.g ( 53.45 – 24.48 ) will get
converted to 14,748 litres.converted to 14,748 litres.
•If anaesthetist draws 2 LPM , it willIf anaesthetist draws 2 LPM , it will
last for 122.9 hourslast for 122.9 hours

30. If you want to transfuse blood rapidly…If you want to transfuse blood rapidly…
What will you do?What will you do?
1.Put a wider gauge canula
2.Increase the drip stand height
3.Use a rapid infusion bag
Hagen-Poiseuille formulaHagen-Poiseuille formula

31. Hagen-Poiseuille Law:Hagen-Poiseuille Law:
When a fluid flows through a tube, then the quantity of the flow(Q) is
1. Proportional to the pressure difference between the two ends of the tube
2. Proportional to the fourth power of the radius of the tube
3. Inversely proportional to the viscosity of the fluid
4. Inversely proportional to the length of the tube
Q = π r4
(P1- P2)/ 8ηl
As the flow of liquid is proportional to the 4th
power of the radius,
increasing the size of the venous canula will help us to rapidly transfuse
the blood

32. Try to answer these questions..!
1.Why acute bends in the breathing circuits are avoided?
2.What is the rationale behind in giving Helium-O2 mixture
for the patients with partial upper respiratory tract
obstruction?

33. Flow through tubes…Flow through tubes…
•LAMINAR FLOW
•TURBULENT FLOW
Reynold’s number
“Viscosity affects laminar flow,whereas density affects the
Turbulent flow.”

34. • Bends in the breathing circuits are avoided
To favour laminar flow of the gases
• Laminar flow offers less resistance.
• Heliox has got low density and can
easily flow through the constricted airway where
there will be a turbulent flow.

35. Improvement of Peak Expiratory Flow
Following Heliox inhalation in airway obstruction

36. What is the common factor among the following..?
1.Gas stove
2. Insect sprayer
3. Venti oxygen mask
4. Nebuliser
5. Ventilator
6. Bunsen burner
All these instruments are based on “venturi principle”.

37. Have you used a Bunsen burner?Have you used a Bunsen burner?
Bernoulli’s theoremBernoulli’s theorem

38. 0
0
0+-
Clinical application of Bernoulli’s theorem:Clinical application of Bernoulli’s theorem:
The speeding fluid looses its kinetic energy on its
sidewall, represented by the falling pressure gauge.

39. DIFFUSER ( VENTURI TUBE )DIFFUSER ( VENTURI TUBE )
When a fluid jets through a narrow opening ,it tries to
create a negative pressure along the side wall of the diffuser
Because of the loss of kinetic energy..
This negative pressure can be used for various purposes,
like mixing gases , suctioning etc
Negative pressure

41. CLINICAL APPLICATION
OF VENTURI PRINCIPLE
VENTI MASKS
FOR OXYGEN THERAPY

42. Try to reason out…!Try to reason out…!
When the lung decreases in size during expiration
how the alveoli remain patent without getting emptied
into nearby alveoli?
Laplace lawLaplace law

43. Laplace law:Laplace law:
The excess pressure inside a spherical gas-liquid
interface is equal to twice the co-efficient of surface
tension divided by the radius of the interface
ΔP= 2γ/ r

44. As the alveoli decreases in size during expiration,
the tension inside the alveoli tends to increase and
radius decreases which will try to collapse the alveoli.
But this doesn’t happen…
Because of the decrease of the surface tension
by the surfactant.
As the tension decreases , the force to collapse
the alveoli decreases , thereby avoiding collapse.

45. THANK YOU
dr.r.selvakumar.M.D.D.A.DNB
professor of anaesthesiology
k.a.p.viswanatham govt medical college,
trichirapalli
tamilnadu-india