Introduction Physics is a natural sciences attemptto describe the fundamental laws ofworld around us.

Introduction As anaesthesiologists we deal withliquids and gases under pressure atvarying temperatures and volumes.These inter relationships are simple,measurable and their understandingensures a safe outcome for thepatient.

Simple mechanics

Simple mechanics

Simple mechanicsEnergythe capacity to do work (joules, J)Workis the result of a force acting upon anobject to cause its displacement in thedirection of the force applied (joules,J).orJ = FDJ is work, F is force and D is distancetravelled in the direction of the force.

Simple mechanicsJouleThe work done when a force of onenewton moves one metre in thedirection of the force is one joule.It can be shown that work is givenby pressure x volume. This enablesindices such as work of breathing to becalculated simply by studying thepressure-volume curve

Simple mechanicsP = F/A or F = PAandV = DA or D = V/AsoJ = FDbecomesJ = (PA).(V/A)orJ = PVWhere P is pressure, F is force, A is area, V is volume,D is distance and J is work

Simple mechanicsWattthe power expended when one jouleof energy is consumed in one second isone watt.Importance: if a pressure volume loop for aresp cycle is plotted, the work of breathing maybe found. If the resp rate is now measured thenthe power may be calculated.Power for resp. = 700 1000 mW

Hydrostatic pressure

This can be used to convert Kpa andmmhgThus,760 mmHg = 101.4 Kpa7.49 mmhg = 1 Kpa

Gauge pressureWhen unknown pressure ismeasured relative to atmosphericpressure the value obtained is referredto as gauge pressure.- blood pressure- airway measurements

Absolute pressureAn absolute pressuremeasurement includes the effect ofatmosphere, and is therefore equal tosum of atmospheric pressure plus thegauge pressure.Barometric pressure is an absolutepressure measurement

Force required when injecting asyringe In order for fluid to pass out of thebarrel of the syringe the samepressure must be developed in thesyringe. Then the force required to depress theplunger will be dependent on thecross-sectional area of the syringebarrel,Force = pressure x cross sectional area

Force required when injecting asyringe

Force required when injecting asyringe

Force required when injecting asyringeApplications:Thumb pressure produce a force of 25 N atthe end of syringe.For a 2 ml sy. pressure generated is500kPaFor a 20 ml sy pressure generated is100kPaEven this is 6 times more than SBP of16kPa (120 mmHg).So during Biers block, pressure in the veinduring rapid injection can exceed systolicpressure, particularly if a vein adjacent tothe cuff is present.

Another typical e.g. is formation of bedsores.Suppose 20 kg of patient wt comesin contact with 10 * 10 cm and thepressure developed will be 19.6 kPa.this is more than systolic pressureand there is a risk of ischemia and bedsores at this pressure point.

Similar concepts are applied forpressure relief valves and exp. valvesof anaesthetic breathing systems.

Law of La PlaceTension may be defined as the internal force generated by a structureLa Place Law states that for cylinders, T = Pr (where T = walltension, P = pressure of fluid within the cylinder, r = radiusTension Pressure Radius

Gases, liquids and solids Freezing point: at any given pressure,the transition between solid andliquids occurs at a fixed temperature. Boiling point: transition between liquidand gas. Changes in ambient pressure causesboiling and freezing temperature tovary.

Critical temperature:gases can be liquefied byincreasing the pressure or cooling.however , there is a temperatureabove which any gas cannot beliquefied by increasing pressure. This iscritical temperature.

Critical pressureminimum pressure at criticaltemperature required to liquefy a gasCritical volumevolume occupied by 1 mole a gasat critical pressure and criticaltemperature

Vapours and gasesGas: Substance which is normally ingaseous state at room temperature andatmospheric pressure.Vapour: gaseous substance which isnormally in liquid form at roomtemperature and atmospheric pressure,since its critical temperature is abovethe room temperature

Vapour formed from liquid by evaporation. Occurs at surface of liquid and theconcentration of vapour increases. Continues till there is a equilibrium when nofurther increase in vapour concentration ispossible. This is called saturated vapour pressure.

SVP increases with temperature. The temp at which SVP is equal toatm pressure it is called boiling point.Important: Vapour pressure dependsonly on the liquid and temperature. Notaffected by ambient pressure. (and ispractically independent of total environmental pressure)

VaporizationVapor Pressures at 200CIsoflurane 239mmHgEnflurane 175mmHgHalothane 243mmHgDesflurane 669mmHgSevofurane 157mmHg

The Bernoulli principleAn increase in the flow velocityof an ideal fluid will be accompaniedby a simultaneous reduction in itspressure.

The Venturi effectThe effect by which the introduction of aconstriction to fluid flow within a tubecauses the velocity of the fluid to increase,therefore, the pressure of the fluid to fall.

Working of a nebulizerIn this case, gas as the driving fluidenters by the central tube, entrainsliquid from a side tube breaks it up intodroplets suitable for inhalation.

Friction between the oxygen moving athigh speed and the air pulls more airalong with the flow of oxygen, an effectknown as jet entrainment.

The law of conservation of energy . The fluid has potential energy due tothe pressure driving it in the directionof flow and kinetic energy because it ismoving. Gain in kinetic energy, potentialenergy decreases so that totalremains same Results in a increase in velocity andreduction in pressure.

Coanda EffectIf a constriction occurs atbifurcation because of increase invelocity and reduction in the pressure,fluid (air, blood) tends to stick to oneside of the branch causingmaldistribution.

Coanda EffectApplication:1. Mucus plug at the branching of tracheo-bronchial tree may causemaldistribution of respiratory gases.2. Unequal flow may result because ofatherosclerotic plaques in the vasculartree3. Fluid logic used in ventilators employsthis principle to replace valves or

Heat transfer Heat energy can be transferred byConduction metal , fixed mean position,by vibrationConvention liquids and gases,convention current or bulk movementRadiation infrared radiation, can invacuum in absence of any medium orcontinuity. E.g. sun to earth.Importance heat loss suffered by patientduring prolonged periods of anaesthesiaand sedation.

Humidity Amount of water present inatmosphere Absolute humidity Relative humidity Hygroscopic material One that attracts moisture from theatmosphereImportance : the main location ofhygroscopic medium is inside heat andmoisture exchange (HME) filters.

The gas laws

Boyles law

The gas laws Application in anaesthetic practiceOxygen cylinder of volume 10 L,molybdenum steel 138 bars.So how much oxygen is stored ?P1V1 = P2V2138*10 = 1*V2So,V2 = 1380 L

The gas laws

The gas laws

The gas laws Medical gases are stored in clyindershaving a constant volume and highpressures (138 Barr in a full oxygen/aircylinder). If these are stored at hightemperatures, pressures will rise causingexplosions. Molybdenum steel can withstand pressuretill 210 bars. Weakening of metal indamaged cylinders are at a greater risk ofexplosion due to rise in temperature.

Gas contents in a cylinderFrom ideal gas equation, the pressure exertedby any gas is dependent on the number ofmoles present.Therefore in a fixed volume such as cylinder,the pressure is a measure of amount of gascontained.Applies to gas e.g. O2 or CO2 cylinder but notto vapour like full nitrous, where liquid and gasphases are present.

Perfect gas A gas that completely obeys all threegas laws.or A gas that contains molecules ofinfinitely small size, which, therefore,occupy no volume themselves, andwhich have no force of attractionbetween them.

Avogadros hypothesis Equal volumes of gases, under thesame conditions of temperature andpressure, contain equal numbers ofmolecules.

Avogadros number

The universal gas equation

This equation may be used inanaesthetics when calculating thecontents of an oxygen cylinder.- constant room temp- fixed internal volume,- R is a constantOnly variables now are P and n so thatP nTherefore pressure gauge acts as acontent gauge for gases measure ofamount of O2 left in a cylinder.

We cannot use a nitrous oxidecylinder pressure gauge in the sameway is that these cylinders containboth vapour and liquid and so the gaslaws do not apply.

Then how to find out the quantity ofNitrous oxide.N2O is stored in cylinder as liquid.Exists partly as liquid and partly asgas.So customary to weigh the cylinderalong with its contents.From known cylinder wt. andmeasured wt. amount of N2O andusage is found out using Avogadroshypothesis

Heat and temperatureHeatThe form of energy that passes betweentwo samples owing to the difference in theirtemperatures.TemperatureThe property of matter which determineswhether the heat energy will flow to or fromanother object of a different temperature

Fluids Fluids are gases or liquids. Flow is the quantity passing a point ina unit time represented by Q. Flow can be Laminar TurbulentFlow changes from laminar to turbulent andis halved when the Reynolds numberwhich is a product of certain factorscrosses the value of 2000.

Reynolds number

Reynolds number It is dimensionless and has no units. When Re < 2000 laminar Re > 2000 turbulentPoints to remember: Viscosity is the important property of laminarflow Density is the important property of turbulentflow Reynolds number of 2000 delineates laminarfrom turbulent flow

Laminar FlowA steady flow greatest at the centreand slowest at the periphery of tube.Physical property effecting laminarflow is viscosity to which it is inverselyproportional.Law applicable Hagen Poiseuilles law

ViscosityViscosity may be thought of asstickiness of the fluidViscosity will affect the flow of fluidsthrough a tube: the more viscous thefluid, the slower the flow.

Coefficient of viscosity

Turbulent FlowTurbulent flow describes the situation inwhich fluid flows unpredictably with multipleeddy currents and is not parallel to the sidesof the tube through which it is flowing.Facilitated by corners, irregularities andsharp angles etc.Affected by density of gas.

Turbulent Flow lawapplicable

Daltons law of partial pressures- states that if a mixture of gases is placed ina container then the pressure exerted byeach gas (partial pressure) is equal to thatwhich it would exert if it alone occupied thecontainer.- Thus in any mixture of gases (alveolar,fresh inspired gases, air ) the partialpressure exerted by each gas isproportional to its fractional concentration.P =P + P + P

Daltons law of partialpressuresThe proportion of the pressure exertedby a gas in the total pressure isimportant and is equated with thevolume it occupies.If a pressure exerted by a gas is 50 %of the total pressure exerted by allgases in that container, then it willoccupy exactly 50 % of its volume.

Adiabatic compression orexpansion of gases Adiabatic, when applied to expansionor compression of a gas, means thatenergy is not added or removed whenthe changes occur. Compression of gas temperature rises Expansion of gas temperature falls

Adiabatic compression orexpansion of gases Practical application:Compression of gases will require addedcoolingIn cyroprecipitate expansion of gas in theprobe low temp in probe tip

Application1. Undersized ETT tremendousdecrease in flow of gases2. Wide bore and curved rather thansharp angles should be preferred.3. In resp tract obst, Heliox mix given toreduce density and improve the flow4. Laminar flow during quiet breathing changed to turbulent during speakingand coughing leading to dsypnoea

Application5. In flow meter at low flows, Hagen Poiseuilles Law applies laminar,while at higher flows, law applicableto turbulent flow.6. Numerical value for critical value inl/min for O2 + N2O is same as ID ofETT in mm. Flow changes toturbulent from laminar.

Capacitors and capacitanceCapacitorA device that stores electricalcharge.Consists of two conducting platesseparated by non conducting material dielectric.CapacitanceThe ability of a capacitor to storeelectric charge (farads, F).

Inductors and inductanceInductorAn inductor is an electriccomponent that opposes changes incurrent flow by the generation of anelectromotive force.

Current does not flow immediately, but increasesslowly in step with the built up of magnetic lines offorce.Inductors tends to block AC but pass DC, becausereactance of inductors increases with frequencyInductanceInductance is the measure of theability to generate a resistiveelectromotive force under the influenceof changing current

DefibrillatorsChargingWhen charging the defibrillator, theswitch is positioned so that the 5000 V DCcurrent flows only around the upper half ofthe circuit. It, therefore, causes a charge tobuild up on the capacitor plates

DefibrillatorsDischargingWhen discharging, the upper and lowerswitches are both closed so that the storedcharge from the capacitor is now delivered tothe patient. The inductor acts to modify thecurrent waveform delivered .

Osmosis and colligativepropertiesOsmole: one osmole is an amount ofparticles equal to Avogadros numberOsmolarity: The amount of osmoticallyactive particles present per litre ofsolution(mmol/l)Osmolality: The amount of osmoticallyactive particles present per kilogram ofsolvent (mmol/kg)

Raoults lawThe depression of freezing point orreduction of the vapour pressure of asolvent is proportional to the molarconcentration of the solute.

Surface tensionThe force per unit length actingacross any line in the surface andtending to pull the surface apart acrossthe lines.In the surface layer, some of theforces of attraction between themolecules act in a direction parallel tothe surface of the liquid and result in theliquid surface behaving as though a

Surface Tensiont tgravityt tgravityH2O Hg

Solubility and diffusionHenrys lawThe amount of gas dissolved in a liquidis directly proportional to the partial pressureof the gas in equilibrium with the liquid.

Solubility and diffusion

Solubility and diffusionApplications:1. Flow meters: each gas with its own phy propertymust pass through its own calibrated flow meter.2. Rate of diffusion is slower in liquids and thus localanaesthetics, if not injected in close proximity tothe nerve fibre will not be effective.3. Helium, a lighter gas is used in airway obstructionto improve diffusion and gas exchange

Solubility and diffusion

Application:1. Alveolar capillary membrane Co transfer test2. Anaesthetic vapour diffusing into breathingcircuits and later acting as Vaporizers at the timeof discontiuation of anaesthetic.3. N2O diffusion into cuff of ETT4. Diffusion of N2O into air filled cavities

Solubility and diffusionBlood: gas solubility coefficientthe ratio of the amount ofsubstance in equal volume phases ofblood and gas in a closed system atequilibrium and at standard temperatureand pressure

Solubility and diffusionOil: gas solubility coefficientthe ratio of the amount ofsubstance present in equal volumephases of oil and gas in a closedsystem at equilibrium and at standardtemperature and pressure

Highest lipid solubility greatest potency asanaesthetics.Basis for Meyer Overton theory ofanaesthesia.N2O - 1.4Ether - 65Halothane - 224,Application : Halothane very potent andneeds lesser concentration in alveoli andbrain

Solubility and diffusionBunsen solubility coefficientthe volume of gas, corrected to standardtemperature and pressure, that dissolves inone unit volume of liquid at the temperatureconcerned where the partial pressure of thegas above the liquid is one atmosphere.

Solubility and diffusionOstwald solubility coefficientThe volume of gas that dissolves inone unit volume of liquid at thetemperature concerned.The Ostwald solubility coefficient is, therefore,independent of the partial pressure.

Solubility and diffusionSolubility co-efficientHigher the solubility, easier it is forthe gas to diffuse, e.g. Carbon dioxide is20 times more diffusible than oxygenand thus diseases affecting gasexchange in alveoli affect oxygenationrather than CO2

Solubility and diffusionMembrane area & thicknessDiffusion is inversely proportional tothe thickness of membrane and directlyproportional to the membrane areaacross which diffusion has to takeplace.

Cardiac output measurementThe Fick principleThe total uptake or release of asubstance by an organ is equal to theproduct of the blood flow to the organand the arterio-venous concentrationdifference of the substance.

Spectrophotometry basicconceptsDefinition:Radiation is of differentwavelengths. If radiation is passedthrough a solution, differentwavelengths are absorbed by differentsubstances. Beers law and Lamberts law

Beers law Absorption of radiation by a given thickness andconcentration of a solution is the same as twicethe thickness with half the concentration. Lamberts law Equal thickness absorb equal amounts ofradiation.Both laws say that the absorption of radiationdepends on the amount of a particular substance.This has been utilised in pulse oximetry.

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

References:1. Fundamentals of Anaesthesia2. Understanding Anesthesia equipment by Dorschand Dorsch

3. Physics, Pharmacology andPhysiology for Anaesthetists4. Basic physics and measurement inANAESTHESIA

5. Basic Physics applied toanaesthesiology by Selvakumar.CASCO 2012 august, Coimbatore.6. Physics and Anaesthesia by SaeedaHaideer