basic physics for anaesthesiologists
TRANSCRIPT
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BASIC PHYSICS FOR THE
ANAESTHESIOLOGISTS
RATHER APPLIED PHYSICS
dr.r.selvakumar.professor of anaesthesiologyk.a.p.viswanatham govt medical collegetrichy
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WE HATE PHYSICS…
THAT’S WHY WE SELECTED MEDICINE
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BUT… THERE IS A LITTLE BIT OF PHYSICS IN ALL ACTIVITIES OFOUR DAY TO DAY LIFE….
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JUST NOW I’VE FITTED THE MACHINEWITH A NEW N2O CYLINDER….
HOW LONG WILL IT LAST…?
CAN I TAKE A CASE WHICH MAY LAST FOR 6 HOURS….?
How the physics knowledge helps the Anaesthesiologists?
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1.HOW TO CALCULATE THE AMOUNT OFN2O GAS IN A CYLINDER..?
• STORAGE OF N2O IN A CYLINDER
• PARTLY AS GAS ,PARTLY AS LIQUID
STORY OF O2 – IDEAL GAS
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MOLECULAR WEIGHT
ADDITION OF ATOMIC WEIGHT
ATOMIC WEIGHT OF NITROGEN – 14, OXYGEN – 16
MOLECULAR WEIGHTOF N2O – 14+14+16 = 44
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GRAM MOLECULAR WEIGHT
MOLECULAR WEIGHT EXPRESSED IN GRAMS.
AVAGADRO HYPOTHESIS:
ONE GRAM MOLECUALR WEIGHT OF ALL GASES WILL CONTAIN THE SAME NUMBER OF MOLECULES AND OCCUPY THE SAME VOLUME ( 22.4 L)AT S.T.P
32 g OF O2 = 44 g OF N2O = 28 g OF N2 =22.4 LITERS
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AMOUNT OF N2O GAS IN A CYLINDER
TARE WEIGHT OF THE CYLINDER = 12.5 K.G
CYLINDER WEIGHT WITH N2O = 15 K.G
SO.. WEIGHT OF THE N2O = 2.5 K.G = 2500G
44G OF N2O = 22.4 L
THEREFORE 2500 G = 22.4/44 x 2500 = 1272 L
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2. HOW DOES THE ALVEOLAR GASEXCHANGE OCCURS….?
O2 AND CO2 MOVE IN OPPOSITEDIRECTION BECAUSE OF PARTIAL PRESSURE GRADIENT.
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WHAT DO YOU MEAN BY PARTIAL PRESSURE?
@ @
@@
@
$ $$
@ @@@
@ $$
IN A MIXTURE OF GASES,THE PRESSURE EXERTED BY EACH GASIS THE SAME AS THAT WHICH IT WOULD EXERT IF IT ALONEOCCUPIED THE CONTAINER
$
5 PSI 3 PSI 8 PSI
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DALTON’S LAW OF PARTIAL PRESSURE
THE PROPORTION OF THE PRESSURE EXERTED BY A GASIN THE TOTAL PRESSURE IS EQUATED WITH THE VOLUMEIT OCCUPIES
@ @@@
@
$$
$
PRESSURE OF @ GAS = 5 PSIPRESSURE OF $ GAS = 3 PSITOTAL PRESSURE = 8 PSI
What is the % contribution of pressure of @ gasIn the total pressure?5/8 x 100 = 62.5%
So, gas @ will occupy62.5% of the total volume
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THE % OF O2 IN THE ROOM AIR = 21% BY VOLUME
THAT MEANS…IT CONTRIBUTES 21% OF THETOTAL ATMOSPHERIC PRESSURE
21/100 X 760 = 159.6 mm of Hg. pO2 in room air = 159.6
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Gaseous Component
Percentage in Air
% x 760 (mm Hg)
Partial Pressure exerted (mm Hg)
Symbol
Nitrogen 79.04 0.7904 x 760 600.7 PN
Oxygen 20.93 0.2093 x 760 159.1 PO2
Carbon Dioxide
0.03 0.0003 x 760 0.228 PCO2
Total 100 760
Partial Pressure of Atmospheric Gases Calculation
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WHAT WILL HAPPEN IN Mt EVEREST…?
ATMOSPHERIC PRESSURE = 253 mm of Hg.
PARTIAL PRESSURE OF O2 = 21/100 X 253 = 53 mm of Hg
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3. DIFFUSION OF GAS IN A LIQUID
GAS GOES INTO SOLUTION
SATURATION POINT
THE PRESSURE EXERTED BY THE GAS IN THE LIQUID IS CALLED AS “TENSION”
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DIFFFUSION OF GAS IN A LIQUID
100 PSI 200 PSI
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THE AMOUNT OF GAS DISSOLVED IN A GIVEN LIQUIDIS DIRECTLY PROPORTIONAL TO THE PARTIAL PRESSURE OFTHE GAS IN EQUILIBRIUM WITH THE LIQUID
HENRY’S LAW
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DIFFUSION OF GAS IN A LIQUID
PARTIAL PRESSURE OF THE GAS
TEMPERATURE
NATURE OF THE GAS
TYPE OF LIQUID
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PARTITION COEFFICIENT
1 litre of BLOOD
1 litreN2O
0.47 L N2O
RATIO OF THE AMOUNT OF SUBSTANCE PRESENT IN ONE PHASECOMPARED WITH ANOTHER,THE TWO PHASES BEING OF EQUAL VOLUMEAND IN EQUILIBRIUM
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PARTITION CO-EFFICIENT
THE AMOUNT OF DISSOLVED GAS TO RAISE THE SAMEPARTIAL PRESSURE AS IN THE CONTAINER MAY NOT BETHE SAME……
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#N2O0.47
HALOTHANE 2.3
ETHER 12
SOLUBILITY OF ANAESTHETICS IN BLOOD
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THATS WHY, INDUCTION WITH ETHER IS SLOW……
Partial pressure in the alveoli builds upvery slowly as most of the ether goes into solution in the blood.
Only when blood gets fully saturated, thePartial pressure in the alveoli and then brain rises…..
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BOURDON PRESSUREGAUGE
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4. WHICH PART OF THE BOYLE’S MACHINEUTILISES THE SAME PRINCIPLE AS THIS TOY…?
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BOURDON PRESSURE GAUGECBE-SZ
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IN A CONTAINER,PRESSURE EXERTED BY A COMPRESSED GAS
EXERTS ITS EFFECT EQUALLY ON ALL SIDES OF THE CONTAINER.
IF THE CONTAINER IS MADE SUCH A WAY THAT IT CAN EXPAND OR
STRAIGHTEN OUT IN A DIRECTION, IT WILL DO SO AS LONG AS THE
PRESSURE IS MAINTAINED INSIDE.
ONCE THE PRESSURE IS RELEASED, THE CONTAINER WILL RETAIN ITS ORIGINAL SHAPE
PRINCIPLE OF BOURDON PRESSURE GAUGECBE-SZ
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BASIC PRINCIPLE OF PRESSURE REGULATORS
TALL CONTAINER
PRESSURE = P
AREA OF ACTINGFORCE = a
SHORT,WIDECONTAINER
Pressure = pArea = A
P
ap
A
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5. HOW THE CYLINDER PRESSURE IS REDUCED TO LOWER LEVEL IN THE BOYLE’S MACHINE…?
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BASIC PRINCIPLE OF PRESSURE REGULATOR
AT EQUILIBRIUM
FORCE ACTING IN THE TALLCONTAINER
=FORCE ACTING IN THESHORT,WIDE CONTAINER
P x a = p x A
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A LARGE FORCE ACTING ON A SMALL SURFACE AREA
CAN BE BALANCED BY
A SMALL FORCE ACTING ON A LARGE SURFACE AREA
BASIC PRINCIPLE OF PRESSURE REGULATORS
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CYLINDER
PRESSURE = P
SMALL DIAPHRAGM
AREA = a
LARGE DIAPHRAGMAREA = A
REDUCED PRESSURE = p
FLOWMETER
SCHEME OF A SIMPLE PRESSURE REGULATOR
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SIMPLE PRESSURE REGULATOR
FORCE ACTING TO OPEN THE SMALL DIAPHRAGM = P x a
AT EQUILIBRIUM….
FORCE ACTING TO CLOSETHE SMALL DIAPHRAGM = p x A
BALANCED BY
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EXAMPLE:
CYLINDER PRESSURE = 2000 psi
SURFACE AREA OFSMALL DIAPHRAGM = 10 mm
PRESSURE OF THE GASIN THE REGULATOR = p
SURFACE AREA OFLARGE DIAPHRAGM = 100 mm
2000 X 10 = p X 100p = 2000 X 10 /100p = 200 psi
SO, IF THE CYLINDER PRESSURE IS 2000, THE REGULATOR WILL REDUCEIT TO 200 psi.
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2000 X 10 = p X 100p = 2000 X 10 /100p = 200 psi
IN OTHER WORDS, THE RATIO OF THE SURFACE AREA
OF THE TWO DIAPHRAGMS DECIDE THE RATIO OF
PRESSURE REDUCTION
HERE IT IS 10 : 100 – SO, PRESSURE REDUCTION IS 1/10
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CYLINDER
PRESSURE = P
SMALL DIAPHRAGM
AREA = a
LARGE DIAPHRAGMAREA = A
REDUCED PRESSURE = p
FLOWMETER
SCHEME OF A MODIFIED PRESSURE REGULATOR
S1
S2
P X a X S1 = p X A X S2
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MODERN REGULATOR:
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PRESSURE OF THE SPEEDING FLUID
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6.WHAT DO YOU MEAN BY VENTURI PRINCIPLE…?
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CBE-SZ
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BASIC PRICIPLE OF VENTURI INJECTOR
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BASIC PRINCIPLE OF A NEBULISER
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VENTI MASKS FOR OXYGEN THERAPY
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Some more applicationsOf Venturi principle….
Bunsen burner
Insect sprayer
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THE BRILLIANCE OF ANCIENT CHOLA KING, RAJA RAJA CHOLAN
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IRRIGATION SYSTEM OF THE CAUVERY BASINWATER LET-OUT SYSTEM FROM THE LAKES
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DANIEL BERNOULLI - 1738
GIOVANNI BATTISTA VENTURI - 1746 - 1822
RAJA RAJA CHOLAN - 985 - 1014
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THANK YOU
dr.r.selvakumar. m.d.d.a.dnbprofessor of anaesthesiology,k.a.p.viswanatham govt. medical college,trichy