Physics and its laws in anaesthesia

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This is a presentation about the basic laws in physics for Anesthesiologist and its application in day to day practice.

Transcript of Physics and its laws in anaesthesia

  • 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 hightem