BASIC PHYSICS FOR THE

ANAESTHESIOLOGISTS

RATHER APPLIED PHYSICS

dr.r.selvakumar.professor of anaesthesiologyk.a.p.viswanatham govt medical collegetrichy

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….

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?

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

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

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…?

BOURDON PRESSURE GAUGECBE-SZ

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…?

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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BASIC PRICIPLE OF VENTURI INJECTOR

BASIC PRINCIPLE OF A NEBULISER

VENTI MASKS FOR OXYGEN THERAPY

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Some more applicationsOf Venturi principle….

Bunsen burner

Insect sprayer

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

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