Physics and Anesthesia Final Report

Physics in AnesthesiaGerald Caesar O. Libranda, MD

Common gas laws applicable to anaesthesia delivery systemBoyles lawCharles lawGay lussac s lawAvagadros hypothesisDaltons law of partial pressuresBernoulis principleVenturi effectHagen poisulles lawGrahams law

Reynolds's numberCoanda effectJoule Thomson effectAdiabatic compressionRaoults lawAzeotropic mixturesCritical temperaturePoynting effectHenrys law

Simple mechanics

Simple mechanics

Pressure Conversions

1 atm = 101.3kPa = 760 mmHg = 1033 cmH2O = 14.6 lb/in2 100 kPa = 1 bar1000 kPa = 1 mPa = 10 bar1 kPa = 10 cmH2O = 7.5 mmHg

Simple mechanicsEnergythe capacity to do work (joules, J)

WorkForce acting upon an object to cause its displacement in the direction of the force applied (joules,J).J = FD = Fl = pA = pV (pressure and volume)

J is work, F is force and D is distance (displacement) travelled in the direction of the force.

Simple mechanicsJoule The work done when a force of one newton moves one metre in the direction of the force is one joule.

It can be shown that work is given by pressure x volume. This enables indices such as work of breathing to be calculated simply by studying the pressure-volume curve

Simple mechanicsWattthe power expended when one joule of energy is consumed in one second is one watt.

P = W/T P = FV (force and velocity) P = p Q (pressure and flow)

Importance: if a pressure volume loop for a resp cycle is plotted, the work of breathing may be found. If the resp rate is now measured then the power may be calculated.

Gases, liquids and solidsFreezing point: at any given pressure, the transition between solid and liquids occurs at a fixed temperature.

Boiling point: transition between liquid and gas

Changes in ambient pressure causes boiling and freezing temperature to vary.

Vapours and gasesGas: Substance which is normally in gaseous state at room temperature and atmospheric pressure.

Vapour: gaseous substance which is normally in liquid form at room temperature and atmospheric pressure, since its critical temperature is above the room temperature

Vapours and gasesVapour formed from liquid by evaporation.

Occurs at surface of liquid and the concentration of vapour increases.

Continues till there is a equilibrium when no further increase in vapour concentration is possible.

This is called saturated vapour pressure

Saturated Vapour PressureSVP increases with temperature

The temp at which SVP is equal to atm pressure is called the boiling point

Important: Vapour pressure depends only on the liquid (physical properties) and temperature. Not affected by ambient pressure.

VaporizationVapor Pressures at 200C

Amount of vapour depends on temperature As anesthetic vapour is removed, latent heat is removed from the remaining liquid and the vaporizer walls Vaporizer has to be heated to be able to produce sufficient amounts of vapour

Isoflurane239mmHgEnflurane175mmHgHalothane243mmHgDesflurane669mmHgSevofurane157mmHg

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Gas laws applicable to Cylinders

What is the volume of oxygen in a full E type ( 2000 psig) of cylinder that is available for use at 15 psig (pressure at common gas outlet)?How long it will last if you use 3 liters/min

Boyles law

states that at constant temperature(T)the volume(V) of a given mass of a gas is inversely proportional to the absolute pressure(P).V 1/pPV = constant (if T is kept constant)Same number of molecules at the same temperature occupying different volumes will exert different pressures. However, their product will be the same.

pressure in cylinder = P1( 2000 psig) volume of the cylinder = V1 ( 5L) pressure at common gas outlet = P2 (15 psig), volume of oxygen available( V2) = ?P1V1= constantP2V2= constantHence P1V1= P2V22000 X 5= 15 X V2V2= 2000 x 5/15 = 665 litres

Boyles Law

So if we use 3 litres of oxygen, the E type full cylinder will last for about 220 mins (5 liters of oxygen will remain in the cylinder and cannot be used).

Can we use Boyles Law in calculating the amount of N2O available in N2O cylinder?At room temperature N2O exists as liquid and O2 exists as gas

Why N2O exists as liquid at room temperature?Nitrous oxide has a critical temperature of 36.5o C.This is the temperature above which the gas cannot be compressed to its liquid state with any amount of pressure. So a gas can remain in liquid state below its critical temperature.Critical pressure is the pressure of the gas at its critical temperature.

Critical temperature:Gases can be liquefied by increasing the pressure or cooling.

However , there is a temperature above which any gas cannot be liquefied by increasing pressure. This is criticaltemperature.

Temperature above which a gas cannot be liquified by pressure alone

> N2O = 36.5 C> O2 = -116 C

Critical pressureMinimum pressure at critical temperature required to liquefy a gas

The pressure at which a gas liquefies at its critical Temperature

> N2O ~ 73 bar @ 36.5 C> N2O ~ 52 bar @ 20.0 C

Critical volumeVolume occupied by 1 mole a gas at critical pressure and critical temperature

What is the law that is used to calculate amount of N2O in a full E type of cylinder?Avagadros hypothesis

Avagadros hypothesis

States that equal volume of gases contain equal number of molecules at Standard Temperature and Pressure (STP).Pressure exerted by a given number of molecules of ideal gas given in a given temperature is constant and is independent of its molecular weightOne mole of a gas contains 6.023 x10 23 moleculesThis law can also be defined as -

One mole (molecular weight) of any gas at STP occupies 22.4 litres of volume.

When mole is expressed in grams its called as gram molecular weight.

So 1 gram molecular weight of any gas at STP will contain 6.023x10 23 molecules and occupies 22.4 litres of volume.

Standard temperature is 273 K and Standard Pressure is 760 mm of Hg.

How do you use this law to know how much of nitrous oxide is present in a E type of cylinder for use?

Weight of liquid N20 = weight of full cylinder tare weight

= 3.3 kgs

Using Avagadros Hypothesis 1 gram molecular weight of any substance will occupy 22.4 litres; The molecular weight of N2O = 44 gSo 44g N2O will give 22.4 litres 3300 g will give 3300 X 22.4= 1680 litres.

441680 LITRES AT 273 KELVNS

Osmosis and colligative propertiesOsmole:one osmole is an amount of particles equal to Avogadro's number

Osmolarity: The amount of osmotically active particles present per litre of solution(mmol/l)

Osmolality: The amount of osmotically active particles present per kilogram of solvent (mmol/kg)

Osmosis and colligative properties

Osmosis and colligative properties

Raoults lawThe depression of freezing point or reduction of the vapour pressure of a solvent is proportional to the molar concentration of the solute.

Surface tensionThe force per unit length acting across any line in the surface and tending to pull the surface apart across the lines.In the surface layer, some of the forces of attraction between the molecules act in a direction parallel to the surface of the liquid and result in the liquid surface behaving as though a shin were present

What is the law that is used to calculate amount of N2O available at room temperature?Charles Law

States that at constant pressure, volume of a gas is directly proportional to the temperature.V T or V / T = constant.

A full E type nitrous oxide cylinder will give 1680 liters of gas at STP( 273 K and 760 mm of Hg)How much of N2O is available for use at room temperature ---- 200 C?

full E type nitrous oxide cylinder = V1 = 1680 LStandard temperature = T1 = 273 Kroom temperature = T2 =( 293 K)Volume of N2O at room temperature= V2 = ?V1/ T1= V2/ T2V2 = V1 X T2

T11680 X 293 = 1803 litres.

273A full E type nitrous oxide cylinder will give 1803 liters of gas at room temperature.

Why oxygen cylinders should not be kept under the sun?

Heated upIn the sun

Gay Lussacs lawAt constant volume, the pressure of a fixed amount of a perfect gas varies in proportion to its absolute temperature.P T or P/T = constant

Tip : Water Boyles at a constant temperature and that Prince Charles is under constant pressure to be King.

Why will the cylinder explode?If oxygen cylinder is kept under the sun its temperature increases and according to GAY LUSSACs law pressure is directly proportional to temperature, volume being constant.Pressure increases inside the cylinder so much that the cylinder may even explode. Hence the oxygen cylinders should be stored in a cooler place.

What are the precautions taken to prevent explosion in cylinders?To prevent explosion there are 3 types of safety valves in the cylindera)Fusible plug. b) Frangible disc. And c) Safety relief valve.

FRANGIBLE DISC has a diaphragm that breaks at a particular pressure. As the temperature inside the cylinder increases, the pressure also increases (GAY LUSSACS LAW), the diaphragm in the frangible disc breaks and gives vent to the gas and prevents explosion.

SAFETY RELIEF VALVE opens at a particular pressure and closes once the pressure inside the cylinder decreases. Pressure inside the cylinder can increase as a result of increase in temperature (GAY LUSSACS LAW) if accidentally the cylinder is heated up.

Pressure Reducing ValvesHigh and low pressure valves push against an area of diaphragm connected to a rod. When low gas pressure decreases, the tension of the spring overcomes the forces of the gas and the rod is pushed out, opening the high pressure inlet. Low and high pressure would then overcome the rod, closing the high pressure inlet.

What is Universal gas constant?

Boyles law, Charles law and Gay Lussacs law when combined with that of Avagadros hypothesis i.e.PV = K1 (Boyles law)V/T = K2 (Charles law)P/T = K3 (Gay Lussacs law)PV/T = universal gas constant= R

Using Avogadros Hypothesis at constant pressure and temperature V= number of molecules (n). Combining all the above laws PV=nRT. Where n is the number of molecules of the gas.Universal gas constant is 1.987 joules/degree/mole in SI units.

How Bourdons pressure gauge will indicate the content of the oxygen cylinder?

Why the Bourdons pressure gauge of N2O does not show the contents of the cylinder?

PV = nRT

Using UNIVERSAL GAS LAW PV= nRT.Where P=pressuren=number of moleculesR=universal gas constant and T= temperatureGauge pressure = n ( content of the cylinder)Hence the pressure gauge acts as a content gauge

N2O is a liquid at room temperature and hence it will not follow the universal gas constant equation. The pressure in the N2O Bourdon pressure gauge always shows 750 psig till all the liquid N2O becomes vapour. 750 psig is the saturated vapour pressure of N2O at 20oC. When all the liquid nitrous oxide converts into the vapor state, the Bourdons pressure gauge will act as the content gauge and BOYLES LAW will be applicable.

Which law is used in filling a mixture of CO2 and O2 in a same cylinder?Daltons law of partial pressure

Daltons law of partial pressure

States that in a mixture of gases the pressure exerted by each gas is same as the pressure exerted as if it alone occupied the container.It can also be defined as in a mixture of gases the total pressure exerted by the mixture is equal to the sum of pressures exerted by the individual gases. If a pressure exerted by a gas is 50 % of the total pressure exerted by all gases in that container, then it will occupy exactly 50 % of its volume.

The pressure exerted by each of the gases is called as the PARTIAL PRESSURE OF THAT GAS. The partial pressure of the component gas must be proportionate to its percentage in the gas mixture.Partial pressure= fractional concentration X total pressure.

How do you fill CO2 and O2 in the same cylinder?

In order to have CO2 10% in a cylinder, first the cylinder can be filled with CO2 upto a pressure f 1380 kPa and then O2 is filled to a total pressure of 13800 kPa ( total pressure in an E type of O2 cylinder)

Why should you open the cylinder slowly?

Rapid opening of the valve -> rapid compression of oxygen in the narrow tube -> very high temperature -> explosion. ADIABATIC PROCESS.Hence oxygen cylinder should be opened slowly to prevent adiabatic process.

Adiabatic compression or expansion of gasesAdiabatic, when applied to expansion or compression of a gas, means that energy is not added or removed when the changes occur.The term adiabatic implies a change in the state of a gas without exchange of heat energy with its surroundings

Compression of gas temperature risesExpansion of gas temperature falls

Adiabatic compression or expansion of gases

Practical application:

Compression of gases will require added cooling

In cyroprecipitate expansion of gas in the probe low temp in probe tip

Flow Meters

What are the laws governing the flows in a flow meter?

FluidsFluids are gases or liquids. Flow is the quantity passing a point/cross sectional area per unit time; represented by Q.Flow can be Laminar movement in a steady manner without eddiesTurbulentFlow changes from laminar to turbulent and is halved when the Reynolds number which is a product of certain factors crosses the value of 2000.Flow is related to the distance from the side

Reynolds number

Reynolds numberIt is dimensionless and has no units.When Re < 2000 laminar Re > 2000 turbulent

Points to remember:Viscosity is the important property of laminar flowDensity is the important property of turbulent flowReynolds number of 2000 delineates laminar from turbulent flow

Here low flow of O2 is used thus the flow is LAMINAR FLOW- Hegan- Poissuilles Law is applicable.

Laminar FlowA steady flow greatest at the centre and slowest at the periphery of tube. Physical property effecting laminar flow is viscosity to which it is inversely proportional.

Law applicable Hagen Poiseuilles law applies only to Newtonian fluids

ViscosityViscosity may be thought of as stickiness of the fluidViscosity will affect the flow of fluids through a tube: the more viscous the fluid, the slower the flow.Viscosity is defined as that property of a fluid that causes it to resist flow. The coefficient of viscosity (h) is defined as : (h) = force x velocity gradient area

Coefficient of viscosity

Viscosity(h) =force/area x velocity gradient

Fluids that obey this formula are referred to as Newtonian fluids Some biological fluids are non Newtonian A prime example is blood . Viscosity changes with the rate of flow of blood ,in stored blood, with time (blood thickens on storage) Viscosity of liquids diminishes with increase in temperature Viscosity of a gas increases with increase in temperature

Hegan- Poissuilles lawIs applied for laminar flow. It states that the flow through the tube is directly proportional to pressure gradient and 4th power radius and inversely proportional to length of the tube and viscosity of the gas.

Q= r 4(P1 P2)

8LWhere Q= flow of liquid

r= radius of the tubingP1 P2 = pressure gradient across the tubing (eta) = viscosityL= length of the tubing.

Here the flow is orificial. Grahams Law of turbulent flow is applicable

Turbulent FlowTurbulent flow describes the situation in which fluid flows unpredictably with multiple eddy currents and is not parallel to the sides of the tube through which it is flowing.

Facilitated by corners, irregularities and sharp angles etc.

Laminar flow passing through a constrictionVelocity in constriction is increased overall, greater friction, and the resistance to flow is no longer constantAffected by density of gas.

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Grahams law for turbulent flow

Why should you know that Graham's law is applicable for high flows? The flow meters are always calibrated at 760 mm of Hg. If the anaesthesia machine is used in a high altitude area, where the atmospheric pressure is very low, the density of the gas decreases, but viscosity will not change

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As higher flow depends on density and as per GRAHAMS LAW FOR TURBULENT FLOW, flow is inversely proportional to square root of density i.e. FLOW 1/ density Flow will be higher than the actual flows that are set in the flow meters. The opposite will occur under hyperbaric conditions.

Turbulent Flow law applicable

Flow Of Fluids Through OrificesIn an orifice the diameter of fluid pathway exceeds the length. Flow rate of a fluid through an orifice is dependant upon:

The square root of the pressure difference across the orifice

The square root of the diameter of the orifice

The density of the fluid, as flow through an orifice inevitably involves some degree of turbulence

Resistance to gas flow through tracheal tubes of different internal diameter (ID)

Anesthetic breathing systems A sudden change in diameter , irregularity of the wall may be responsible for a change from laminar to turbulent flow.Thus a tracheal and other breathing tubes must possess:

Smooth internal surfacesGradual bends No constrictionsLarge diameterShort length Resistance to breathing is much greater when a tracheal tube of small diameter is used

ApplicationUndersized ETT tremendous decrease in flow of gasesWide bore and curved rather than sharp angles should be preferred.In resp tract obst, HeliOx mix given to reduce density and improve the flowLaminar flow during quiet breathing changed to turbulent during speaking and coughing leading to dsypnea

ApplicationIn flow meter at low flows, Hagen Poiseuilles Law applies laminar, while at higher flows, law applicable to turbulent flow.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.

Why flow meters of each gas should be calibrated for that gas specifically?

Each of the flow meters are calibrated for that particular gas.Different gases have different viscosity and density.Thus if gas other than the particular gas for which the flow meter has been calibrated is used, the flows shown may not be the actual flow that is being delivered to the patient.

Gas laws applicable to vaporizers

How to calculate 1ml of sevoflurane gives how much of vapor?Molecular weight of sevoflurane is 200. Density of sevoflurane is 1.5According to AVAGADROS HYPOTHESIS 200g of sevoflurane gives 22400 ml of vapors. So 1g of sevoflurane will give 22400/200= 112 ml of vapors.

Since the density is 1.5, 1.5 g is equal to 1 ml.So 1ml of sevoflurane liquid =112 x 1.5 = 168 ml.Since this 168 ml of sevoflurane vapour is at standard temperature is 273 KUsing CHARLES LAW one can calculate the vapors available at room temperature i.e. 293 K

According to CHARLES LAW: V/T = constantSo V1/T1 = V2/ T2V1= 168 ml ( vapors at 273 K) T1 = 273 K

V2 = ? T2 = 293 KV2 = 168 X 293 = 180 ml 273Thus 1 ml of sevoflurane will give 180 ml of sevoflurane vapors at room temperature ( 200 C)

How do you estimate the cost of volatile anaesthetics?

If 2% of sevoflurane is used with a fresh gas flow of 6 litres. Then per min 120 ml of sevoflurane vapors will be used upSo 120 X 60 ml = 7200 ml of sevoflurane vapors will be used for 1 hr. duration of sevoflurane anesthesia. 1 ml of sevoflurane gives 180 ml of vapors at 20o C. So 7200 ml of sevoflurane vapors is produced by

7200 = 40 ml of sevoflurane. 180

Cost of 250 ml of sevoflurane is Rs.7500. So 1 ml sevoflurane costs Rs. 30.So the cost of sevoflurane anaesthesia if it is used at 2% per hour 30 X 40 = Rs.1200A simple formula to calculate cost is

3 X fresh gas flow X % kept on dial setting

Breathing circuits

What is the principle used in checking the integrity of the inner tube of the Bains Circuit in PETHIKS TEST?

Bernoullis principle

Bernoullis principle.

when a gas flowing through a tube encounters a constriction, at that point the pressure drops and the velocity increases An increase in the flow velocity of an ideal fluid will be accompanied by a simultaneous reduction in its pressurei.e. kinetic energy increases and the potential energy decreases. This is called as Bernoullis principle.

The law of conservation of energy The fluid has potential energy due to the pressure driving it in the direction of flow and kinetic energy because it is movingGain in kinetic energy (1/2 m v2), potential energy (m g h) decreases so that total remains sameResults in a increase in velocity and reduction in pressure.

Importance : Resultant drop in pressure applications such as nebulizers and Venturi masks.

The Venturi effect 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 is a tube with a cross section gradually decreases and then increases.Entrainment of air from the surrounding due to fall in pressure at the point of constriction is called as VENTURIS EFFECT.

Venturi Masks

Color Delivered FiO2 Fresh gas flow(l/min) Blue 24% 2White 28% 4Orange 31% 6Yellow 35% 8Red 40% 10Green 60% 15Entrainment ratio 100ax21b=30x required FiO2 a+b=30

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Working of a nebulizer

In this case, gas as the driving fluid enters by the central tube, entrains liquid from a side tube breaks it up into droplets suitable for inhalation.

Coanda EffectIf 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 - gas flow through a tube, with two Venturis tends to cling either to one side of the tube or to the other

The application of a small pressure distal to the restriction may enable gas flow to be switched from one side to another

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

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

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

Circle SystemEither a semi-closed or closed system; with the absorber out of circuit it becomes as semiclosed rebreathing systemLow flow has been defined as a total fresh gas flow of 1 l/minute, and minimum flow as 0.5 l/minOxygen must not be reduced below the minimum 250ml/min, the basic metabolic requirement being 200 ml/minAt induction, anesthetic uptake is highConcentration effect: If very low gas flow is used, alveolar oxygen concentration increases while N2O concentration decreases. It is necessary to maintain a high fresh gas flow during induction and until expired concentration of anesthetics is near inspired concentration (10 minutes)

Circle System

Mapleson A (Magill) breathing systemFresh gas inlet remote from the subject while the expiratory valve is near the subject Economic to use during spontaneous respirationDuring spontaneous ventilation, provided that fresh gas flow is at least equal to alveolar ventilation there will be no rebreathing of CO2During controlled breathing the economy of gas flow is lostPositive pressure provided during inspiration opens the pressure relief valve, venting fresh gas, needs to be set at a higher pressure leading to retention of CO2

Mapleson A

T - PieceGeometric opposite of Mapleson A circuitFresh gas flow inlet is near the subject while expiratory valve or port is furthestNo CO2 will be rebreathed if fresh gas flow is approximately twice the minute volume during spontaneous respirationIf minute ventilation is increased, fresh gas flow can be reducedIncrease in either one decreases CO2 tension

T - PieceE Valveless system suitable for spontaneously breathing subjectsF Pediatric valveless system

Gas laws applicable at alveoli

Which law is used to know the diffusion of gases across the alveoli in to the blood Ficks law

Solubility and diffusion

Ficks lawDiffusion across a membrane

=k x pressure gradient x surface area thickness of membrane K = diffusion coefficient molecular weight

The diffusion coefficient for oxygen is 1.0, for carbon dioxide it is 20.3, and for nitrogen it is 0.53.

Application:Alveolar capillary membrane Co transfer testAnaesthetic vapour diffusing into breathing circuits and later acting as Vaporizers at the time of discontiuation of anaesthetic.N2O diffusion into cuff of ETTDiffusion of N2O into air filled cavitiesSolubility and diffusion

Which law governs the amount of O2 dissolved in the blood? Henrys law

Henrys law states that the amount of a gas dissolved in a unit volume of a solvent is directly proportional to its partial pressure at STP.

The law also predicts how much of a gas dissolves in a liquid.According to this law, the volume of gas that dissolves in a liquid is equal to its solubility coefficient times its partial pressure.V = x PGASWhere V = volume of the gas dissolved, is the solubility coefficient of the gas in the liquid and Pgas is the partial pressure of the liquid

The solubility coefficient of oxygen is 0.003 ml/dl. Thus at 100 mmHg of oxygen tension, the amount of oxygen in the dissolved form will be 0.3 ml.

Solubility and diffusion

Solubility and diffusionApplications:

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

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.

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

Solubility and diffusionBlood: gas solubility coefficient The ratio of the amount of substance in equal volume phases of blood and gas in a closed system at equilibrium and at standard temperature and pressure

Solubility and diffusionOil: gas solubility coefficientThe ratio of the amount of substance present in equal volume phases of oil and gas in a closed system at equilibrium and at standard temperature and pressure

Highest lipid solubility greatest potency as anaesthetics.

Basis for Meyer Overton theory of anaesthesia.N2O -1.4Ether-65Halothane-224,

Application : Halothane very potent and needs lesser concentration in alveoli and brain

Solubility and diffusionBunsen solubility coefficientThe volume of gas, corrected to standard temperature and pressure, that dissolves in one unit volume of liquid at the temperature concerned where the partial pressure of the gas above the liquid is one atmosphere.

Solubility and diffusionOstwald solubility coefficientThe volume of gas that dissolves in one unit volume of liquid at the temperature concerned.

The Ostwald solubility coefficient is, therefore, independent of the partial pressure.

Solubility and diffusionSolubility co-efficient

Higher the solubility, easier it is for the gas to diffuse, e.g. Carbon dioxide is 20 times more diffusible than oxygen and thus diseases affecting gas exchange in alveoli affect oxygenation rather than CO2

Solubility and diffusionMembrane area & thicknessDiffusion is inversely proportional to the thickness of membrane and directly proportional to the membrane area across which diffusion has to take place.

Cardiac Output MeasurementThe Fick principleThe total uptake or release of a substance by an organ is equal to the product of the blood flow to the organ and the arterio-venous concentration difference of the substance.

Miscellaneous Knowledge

Heat transferForm of energy while temperature is a measure of the random thermal movements of molecules or atomsProgressive addition of heat results to change in phase Latent heat heat necessary to overcome the cohesive forcesHeat energy can be transferred by

Conduction metal , fixed mean position, by vibrationConvention liquids and gases, convection current or bulk movementRadiation infrared radiation, can in vacuum in absence of any medium or continuity. E.g. sun to earth. Importance heat loss suffered by patient during prolonged periods of anaesthesia and sedation.

HumidityAmount of water present in atmosphereAbsolute humidityRelative humidityHumidification requires addition of latent heatHygroscopic materialOne that attracts moisture from the atmosphere

Importance : the main location of hygroscopic medium is inside heat and moisture exchange (HME) filters.

This equation may be used in anaesthetics when calculating the contents of an oxygen cylinder.- constant room temp- fixed internal volume,- R is a constantOnly variables now are P and n so that P nTherefore pressure gauge acts as a content gauge for gases measure of amount of O2 left in a cylinder.

Remaining time = O2 cylinder pressure / (200 x Oxygen flow rate)

We cannot use a nitrous oxide cylinder pressure gauge in the same way is that these cylinders contain both vapour and liquid and so the gas laws do not apply.

Capacitors and capacitanceCapacitor A device that stores electrical charge.Consists of two conducting plates separated by non conducting material dielectric.CapacitanceThe ability of a capacitor to store electric charge (farads, F).

Inductors and inductanceInductor

An inductor is an electric component that opposes changes in current flow by the generation of an electromotive force.

Inductors and inductance Current does not flow immediately, but increases slowly in step with the built up of magnetic lines of force.Inductors tends to block AC but pass DC, because reactance of inductors increases with frequency

InductanceInductance is the measure of the ability to generate a resistive electromotive force under the influence of changing current

DefibrillatorsCharging When charging the defibrillator, the switch is positioned so that the 5000 V DC current flows only around the upper half of the circuit. It, therefore, causes a charge to build up on the capacitor plates

DefibrillatorsDischargingWhen discharging, the upper and lower switches are both closed so that the stored charge from the capacitor is now delivered to the patient. The inductor acts to modify the current waveform delivered .

Defibrillators

Spectrophotometry basic conceptsDefinition:Radiation is of different wavelengths. If radiation is passed through a solution, different wavelengths are absorbed by different substances.

Beers law Lamberts law

Beers lawAbsorption of radiation by a given thickness and concentration of a solution is the same as twice the thickness with half the concentration. Lamberts lawEqual thickness absorb equal amounts of radiation.

Both laws say that the absorption of radiation depends on the amount of a particular substance. This has been utilised in pulse oximetry.Spectrophotometry basic concepts

Pulse oximetryThe concentration and molar extinction coefficient are constant. The only variable becomes path length, which alters as arterial blood expands the vessels in a pulsatile fashion.

References:Fundamentals of Anaesthesia

Understanding Anesthesia equipment by Dorsch and Dorsch

Physics, Pharmacology and Physiology for Anaesthetists

Basic physics and measurement in ANAESTHESIA

Basic Physics applied to anaesthesiology by Selvakumar. CASCO 2012 august, Coimbatore.

Physics and Anaesthesia by Saeeda Haideer

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