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