Physics – insights

Dr. S. Parthasarathy MD., DA., DNB, MD (Acu),

Dip. Diab.DCA, Dip. Software statistics PhD (physio)

Mahatma gandhi medical college and research institute, puducherry, India

Definitions

• What is a gas?

A gas is a substance that is in its gaseous phase, but is above its critical temperature.

Critical temperature is the temperature above which a gas cannot be liquefied no matter how high the pressure.

A vapour is a substance in the gaseous phase but is below its critical temperature.

For example

• 20 degrees – gas – apply 50 PSI – liquifies

• 30 degrees gas – apply 500 PSI – liquifies

• 36.1 degrees – apply any PSI does not

• 36 – critical temperature

Pressure, volume and temperature

• Pressure = f/a • Units = 1 bar = 1 atm = 100 kpa = 760 mmHg=

14.7 PSI • 138 bar = 2000 psi

• Temperature – units – 00 C = 320F = 2730k

• Volume == litres or Cubic cm ( cc = ml)

There are some gas laws which inter relate the three variables

• Boyle’s law • “For a fixed mass of gas at constant

temperature, the pressure is inversely proportional to the volume”

P is proportional to 1 / V or PV = K

Volume decrease – pressure increase

Application in anaesthetic practice

Oxygen cylinder of volume 10 L, molybdenum steel – 138 bars.

So how much oxygen is stored ?P1V1 = P2V2

138*10 = 1*V2

So, V2 = 1380 L

Transfer

• 100 bar – 15 litre cylinder • P1 V1 = P2V2 one

• 1500 litres of oxygen

• 5 litres / minute --- 300 minutes approx

Charles law

• “For a fixed mass of gas at a constant pressure, the volume is directly proportional to the temperature”

V is proportional to T or V / T = K

Charles’ Law (Temperature-Volume Law)

Gas volume varies directly with temperature at a constant pressure

V1/T1=V2/T2

Application of charles law

• Respiratory gas measurements of tidal volume & vital capacity etc are done at ambient

temperature while these exchanges actually take place in the body at 37 OC.

Pressure law- Gay lussac law

• “For a fixed mass of gas at a constant volume, the pressure is directly proportional to the temperature”

P is proportional to T or P / T = K

Gay lussac law

• Cylinders are kept in high temperature??

• Volume same

• Pressure raises to explode • Molybdenum steel can withstand 210 bars , if

there is any damage , temperature rise ??

Universal gas law • PV = nRT

• Look at the gauge of the oxygen cylinder • Pressure • Volume, R, temperature constant

• P ∝ n • Look at the gauge and tell

• But for nitrous ??

Is there a pneumonic ??

• Pakistan Tele Vision -- Can Be Good• P constant – C • T constant – B • V constant – G

Avogadro’s hypothesis

• Equal volumes of gases, under the same conditions of temperature and pressure, contain equal numbers of molecules.

• One mole of a gas • Oxygen (O2) = 32 • 32 grams of oxygen will have 6.022 * 1023

atoms

If we describe in volume

• One mole of a gas will occupy 22.4 litres• Gram molecular weight –( Oxygen – 16 ) =O2- 32

• i.e 44 gm of CO2, 2 gram of hydrogen, 32 gm. O2, • Will occupy ??

• 22.4 litres

Take sevoflurane as example

• MW = 200 • i.e. 200 gm of sevo will occupy 22.4 litres • 20 gm = 2.24 litres

• flow of oxygen through vaporizer of 224 litres this 2.24 will be 1 %

Nitrous oxide ??

N2O is stored in cylinder as liquid.

Exists partly as liquid and partly as gas.So customary to weigh the cylinder along

with its contents.From known cylinder wt. and measured

wt. amount of N2O and usage is found out using Avogadro’s hypothesis

E.g. Wt of cylinder with N2O - 5.6kg

Tare wt. - 4.5kgSo, wt of N2O - 1.1kg

44 g of N2O = 22.4 l

Therefore 1.1 kg of N2O = = 560 l

So, if we give 2 liters of N2O / min, this cylinder will come for 280 min or 4.6 hrs

Dalton’s law

• total pressure of a gas mixture was the sum of the pressures of each of the gases if they were to exist on their own.

• P = p1+p2+p3…..• If a cylinder of air = 100 kPa• Nitrogen is 79 kPa and oxygen 21 kPaEntonox 100 kPa == ??

Dalton’s law • What is the partial pressures of O2 and N2O if you are administering a ratio of 70/30?

N20 70% X 760 mmHg = 532 mmHgO2 30% X 760 mmHg = 228 mmHg

760 mmHg

• Would this differ if you were administering anesthesia at Ooty General Hospital?

N20 70% X 630 mmHg = 441 mmHgO2 30% X 630 mmHg = 189 mmHg

630 mmHg

Dalton’s law

Graham’s law of diffusion • rate of diffusion was inversely proportional to the square root of the

molecular mass of the gasApplications:

1. Flow meters: each gas with its own phy property must pass through its own calibrated flow meter.

2. Rate of diffusion is slower in liquids and thus local anaesthetics, if not injected in close proximity to the nerve fibre will not be effective.

3. Helium, a lighter gas is used in airway obstruction to improve diffusion and gas exchange

Gas B more mass than A

• A will effuse out of the alveolus quicker than B, leaving behind more of B and so raising its concentration.

• for example, halothane is more massive than nitrous oxide,

• Graham’s law will indicate that the nitrous will diffuse quicker and so raise the concentration of the halothane in the alveolus.

Fick law

• Diffusion – concentration gradient pressure gradient 1. DLCO 2. vapour into circuits 3. Nitrous into cuffs and air filled cavities

Carbondioxide is 20 times more soluble than oxygen – membrane defects affect oxygenation

Henry’s Law

• Henry’s law states that for a gas-liquid interface the

amount of the gas that dissolves in the liquid is

proportional to its partial pressure.

• So Henry’s law helps to predict how much gas will be

dissolved in the liquid.

• The actual amount also depends on the solubility of

the gas.

Adiabatic expansion

• Adiabatic, when applied to expansion or compression of a gas, means that energy is not added or removed when the changes occur.

– Compression of gas – temperature rises– Expansion of gas – temperature falls– Think there are no pressure regulators in Boyle s

machine

Reynolds number

• whether flow is laminar or turbulent

Where – Reynold’s numberdensity of fluidv – velocity of fluidd – diameter of tube and – viscosity of fluid

Reynold s number

• Reynold s number of 2000 – borderline • When– Re < 2000 – laminar– Re > 2000 – turbulent

Viscosity is the important property of laminar flow Density is the important property of turbulent flow Reynold’s number of 2000 delineates laminar from

turbulent flow

Applications of reynold number • 1- undersized ETT may cause a tremendous decrease

in the flow of gases• 2- Every piece of anaesthetic equipment; because of

diameters & shape of connectors,FGF affected • 3- In respiratory tract obstruction, oxygen – helium

mixtures are given to reduce density and improve the flow.

• 4- Laminar flow during quiet breathing is changed to turbulent during speaking & coughing- dyspnea.

• Flow meter – low flows – laminar flow – density • High flows – turbulent flow – viscosity

Roughly speaking

• Numerical value for critical value in l/min for O2 + N2O is same as ID of ETT in mm.

• Flow changes to turbulent from laminar.

• Flow – 5 litres , ok for 5 mm ID ET tube

Hagen poiseille equation

• Flow – Q is determined by a few factors

Pressure bags and height

• Viscosity of common infusions:• 1.0 centipoise - Lactated Ringers• 4.0 cP- Hetastarch• 40.0 cP- 5% albumin

• Warming decrease viscosity

Joule-Thompson Effect

• When a compressed gas is allowed to escape

freely into an open space, cooling occurs.

• Condensation of water or frost may

accumulate on the cylinder valve.

• A cryoprobe operates on the Joule-Thompson

effect.

(Joule-Thompson Effect)

• As gas escapes from a N2O cylinder, the liquid N2O in the cylinder vaporizes.

• Heat is lost as the liquid vaporizes (latent heat of vaporization)

• the temperature in cylinder falls Hot summer – what is necessary ??

Bernoulli principle

• An increase in the flow velocity of an ideal fluid will be accompanied by a

simultaneous reduction in its pressure.

Kinetic + potential = same

Venturi

• The effect by which the introduction of a constriction to fluid flow within a tube causes the velocity of the fluid to increase, therefore, the pressure of the fluid to fall.

Venturi -- clinical uses

• Suction • Scavenging • Venturi mask • Nebulizers • Checking Bains circuit

Coanda effect

• This effect was named after a Romanian aircraft designer Henri Coanda, after an aircraft he designed went up in flames as a consequence of this effect.

Coanda Effect

If a constriction occurs at bifurcation because of increase in velocity and reduction in the pressure, fluid (air, blood) tends to stick to one side of the branch causing maldistribution.

Coanda effect

1. Mucus plug at the branching of tracheo-bronchial tree may cause maldistribution of respiratory gases.

2. Unequal flow may result because of atherosclerotic plaques in the vascular tree

3. Fluid logic used in ventilators employs this principle to replace valves or mobile parts.

• Boyle law, charles law , Gay lussac, universal • Graham s law • Fick law • Bernoulli and venturi • Reynold number • Poiseulli • Joule thomson effect • Coanda effect

Thank you all