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Introductory Physics for Anesthesiologists

Royal College of Physicians and Surgeons of Canada: Objectives of Training and Training requirements for certification. Specific Requirements Demonstrate knowledge of the basic sciences as applicable to anesthesia, including anatomy, physiology, pharmacology, biochemistry and physics. . Royal College of Physicians and Surgeons of Canada: Objectives of Training and Training requirements for certification.

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Introductory Physics for Anesthesiologists

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    1. Introductory Physics for Anesthesiologists T. Turkstra, M. Eng, P. Eng, MD, FRCPC April 15 2009

    3. Royal College of Physicians and Surgeons of Canada: Objectives of Training and Training requirements for certification Specific Requirements Demonstrate knowledge of the basic sciences as applicable to anesthesia, including anatomy, physiology, pharmacology, biochemistry and physics??

    4. Training Objectives (Resident Handbook) The Anaesthetist will possess the scientific knowledge to provide a sound basis for good clinical practice. This will include.. Physics especially the physics of gases and fluids, and the principles of electrical safety.

    5. Patient Scenario 52 yr old male, prev healthy, med/allergy ~2 week Hx of dyspepsia, 10 lb wt loss AXR/CT shows obstructing 5 cm mass near cecum Admitted to floor, no resolution of symptoms, NPO, has NG insitu Exam unremarkable including airway Anesthetic plan?

    6. Patient Anesthetic Concerns Airway? RSI? Cricoid pressure? How much cricoid pressure? Brimacombe JR et al: Cricoid pressure CJA 1997; 44: 414-25 Recommendations: 20-44 N. Cricoid pressure is a force.

    7. Physics? Why Bother? Fewer and fewer physics MCQs since about 2000 Still fair game Fairly important aspect of much of our daily practice

    8. Today's Objectives To outline some of the core principles and definitions as applicable to: Force Pressure Gases Fluids Flow Work and Power Electrical Safety Thermodynamics Light transmission/optics Thermodynamics Patient temperature control Light transmission/optics pulse-oxThermodynamics Patient temperature control Light transmission/optics pulse-ox

    9. Today's Objectives To outline some of the core principles and definitions as applicable to: Force Pressure Gases Fluids Flow Work and Power Electrical and Fire Safety Thermodynamics Light transmission/optics Thermodynamics Patient temperature control Light transmission/optics pulse-oxThermodynamics Patient temperature control Light transmission/optics pulse-ox

    10. Sample Exam Question (2004) According to NIOSH, exposure to N2O should be limited to? a) Time-weighted 8 hours of 10 ppm. b) Time-weighted 8 hours of 25 ppm. c) Time-weighted 8 hours of 100 ppm. d) Max exposure 200 ppm per case. e) Max exposure 50 ppm per day.

    11. Force 1 Newton (N) of force applied to 1 kg of matter, will accelerate it by 1m/s2 How much is that? About 1/4 lb or about 100 gAbout 1/4 lb or about 100 g

    12. Pressure Pressure is defined as force exerted over a given area P = F/A 1 Pascal of Pressure = 1Nm-2 = 1N/m2 1kPa = 1000 Pa 1 PSI = 1 pound per square inch (lb/in2) 1bar = 101.3 kPa = 1 atmosphere = 760 mmHg = 14.7 psi

    13. Force in Context Consider the Pressure Reducing Valve High pipeline pressure has to be reduced to low anaesthesia machine or breathing system pressure, to prevent injury The Pressure Reducing Valve uses a diaphragm attached to a spring to open or close a piston valve in a high pressure chamber.

    14. The Pressure Reducing Valve The low pressure (P2) is applied over a the large area diaphragm, exerting sufficient force against the spring to raise the piston and stop flow from the High Pressure inlet. (F=PA) As the pressure falls in the low pressure system, the spring pushes the diaphragm down, allowing more gas into the system

    15. Question (2004) An anesthesiologist is working in Vancouver (Patm = 760 mmHg) and sets oxygen at 2L/min, nitrous oxide at 1L/min and the Halothane vaporizer at 1 volume %. Which of the following is true? a) The gas mixture at the common gas outlet will be 1 MAC. b) All of the fresh gas flow will pass through the vaporizing chamber of the halothane vaporizer. c) The partial pressure of halothane at the common gas outlet will be 7.6 mmHg. d) 3 mL per min of halothane will enter the gas mixture. e) 300 mL per min of halothane will enter the gas mixture.

    16. What is a Gas? Molecular theory; Substances are composed of a lattice of molecules. Molecules all vibrate, oscillating about a mean position. Molecules exert force (attraction) on surrounding molecules If heat is added, the vibration amplitude is increased, and molecules exert less force on their neighbours With increased kinetic energy, some molecules break free to enter atmosphere as a gas or vapour

    17. Gas vs. Vapour Molecules may transfer from the liquid phase to the vapour phase and back again Once equilibrium of transfer has been reached, the vapour is saturated If the liquid is heated to its boiling point, all the molecules escape to the gaseous phase As gas molecules collide with the wall of the container holding it, they exert a net force, which when exerted over a certain area is defined as pressure

    18. The Ideal Gas Laws BOYLES LAW (First perfect gas law)

    19. The Ideal Gas Laws BOYLES LAW (First perfect gas law) At a constant temperature, the volume of a given mass of gas varies inversely with the absolute pressure V ? 1/P or, PV = Constant (k1)

    20. Practical Application Consider a patient who needs high flow O2 being transferred to a different Hospital. You have a10 L O2 cylinder, with a gauge pressure of 13,700kPa How long do you have on that O2 cylinder?

    21. Boyles Law Quiz If a 10 litre oxygen cylinder has a gauge pressure of 13,700 kPa, how many litres of oxygen does it hold? Hint: from PV = Constant (k1); P1V1 = P2V2

    22. Answer Absolute pressure = gauge pressure plus atmospheric pressure Using P1V1 = P2V2 (13,700 +100) x 10 = 100 x V2 V2 = 13800 ? 10 = 1380 litres (10 litres will stay behind in the cylinder, so 1370 litres are available for delivery at atmospheric pressure At 10 l/min ? 1370/10 = 137 min =just over 2 hours

    23. Question (2004) You are transporting a patient by ambulance. The patient requires 4l/min O2. You are taking along a full E tank of O2. The trip takes 2 hours. At the end of the trip, how much O2 is left in the tank? a) 60 L b) 180 L c) 360 L d) 400 L e) 620 L

    24. The Ideal Gas Laws CHARLES LAW (Second perfect gas law also known as Gay Lussacs law) At a constant Pressure, the volume of a given mass of gas varies directly with the absolute temperature V ? T or V/T = Constant (k2)

    25. The Ideal Gas Laws The Third Perfect Gas Law (The pressure law) At a constant volume, the absolute pressure of a given mass of gas varies directly with the absolute temperature P ? T or P/T = Constant (k3)

    26. Question Consider an Oxygen cylinder filled to absolute pressure of 138 atmospheres (bar) or 13800kPa, at 17C. Cylinders are tested to withstand 210 bar If this cylinder accidentally makes it into a furnace at 100C, what happens to the cylinder?

    27. EXPLOSION? Doubling the temperature will double the pressure. Why does the cylinder not explode at 340C.?

    28. EXPLOSION? Doubling the temperature will double the pressure. Why does the cylinder not explode at 340C.? Because the equation relates to absolute temperature. 170C = 290K, and 1000C = 3900K. At 1000C the pressure is only 185 atmospheres (P1/ T1 = P2 / T2)

    29. Standard Pressure and Temperature: s.t.p. Because gas volumes are so greatly affected by changes of pressure and temperature, it is important to specify the temperature and pressure at which volumes are measured s.t.p. is 273.15 K and 101.325kPa or 760 mmHg

    30. AVAGADRO Avagadros Hypothesis states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules Because the molecular weights of gases differ, there will be a different mass of any gas in a given volume at the same temperature and pressure Therefore it is more convenient to express a quantity of a gas in terms of the number of molecules, rather than in terms of mass.

    31. AVAGADRO and the MOLE A MOLE is the quantity of a substance containing the same number of particles as there are atoms in 0.012kg of carbon12 There are 6.022 x 1023 atoms in 12 g of carbon 12. This is called Avagadros Number One mole of any gas at s.t.p. occupies 22.4litres

    32. The Mole THUS: 2g of Hydrogen 32g of Oxygen 44g of Carbon Dioxide All occupy 22.4 litres at s.t.p

    33. Physics in Context Calibration of vaporizers is done using Avagadros hypothesis. Molecular weight of Sevoflurane is 200, so 200 g Sevo is 1 mole, and would occupy 22.4 l at s.t.p. If we put 20g of Sevo (0.1 mole) into a vaporizer, and allow it all to vaporize, it would occupy 2.24 litres

    34. Physics in context If we ran oxygen through the vaporizer to a volume of 224 litres, the Sevo would make up 22.4l of the 224 litres, so would make up 1% of the 224 litres Similarly 40 g of Sevo would occupy 44.8l or 2 % of the 224l volume.

    35. How does a vaporiser work?How does a vaporiser work?

    37. The Universal Gas Constant PV = Constant (k1) Boyle V/T = Constant (k2) Charles P/T = Constant (k3) (3rd Law) By combining the perfect gas laws with Avagadros hypothesis we arrive at the following equation: PV/T = Constant (k4), for any given quantity of gas

    38. The Universal Gas Equation For any 1 mole of any gas, this constant (k4) is the UNIVERSAL GAS CONSTANT R Rearranging this equation we come to the generally applicable equation (Universal Gas Law) of: PV = nRT Where n is the number of moles of the gas R depends on the units. Metric: Its value is 8.3144J/K/mol

    39. Daltons Law Daltons law of Partial Pressures states that in a mixture of gases, the pressure exerted by each gas is the same as that which it would exert if it alone occupied the cylinder By Applying Boyles Law (PV = Constant ) and Daltons Law we can conclude that the the partial pressure of a gas in a mixture is obtained by multiplying the total pressure by the fractional concentration of the gas

    40. Daltons Law in Practice For example, in a cylinder of entonox pressurised to 100kPa, The Oxygen is exerting 50 kPa, and the Nitrous Oxide is also exerting 50 kPa In a cylinder of air at an ambient pressure of 100kPa, the oxygen exerts a pressure of 20.93kPa, and the Nitrogen a pressure of 79.07kPa

    41. Daltons Law in Practice Consider alveolar gas: If the end tidal CO2 is measured as 5.6% at 101.3 kPa, what is the true pressure of the alveolar CO2 (PACO2)

    42. Answer The presence of water vapour must be taken into account for humidified gas when calculating partial pressures, and Water Vapour pressure in humidified alveolar gas is 6.3kPa (x7.5 for mmHg) CO2 is measured as a dry gas, so: PACO2 =(101.3 6.3) x 5.6 kPa = 5.3 kPa 100

    43. Adiabatic Changes of State The three gas laws describe the behaviour of a gas when one of the three variables (P/V/T) is constant If these conditions are applied, heat energy must be added or taken from a gas if it changes pressure or volume

    44. Adiabatic Changes of State The state of a gas can however be changed without allowing the gas to exchange heat energy with its surroundings the heat is retained within the system Example is the theoretic hazard when high pressure pipelines are opened into a low pressure anaesthetic machine, without regulator valves. The rapid pressurization is associated with a local large temperature rise, and risk of fire and explosion

    45. Question (2004) What is the least likely cause of decreased ETCO2? a) endobronchial intubation b) hypothermia c) hyperventilation d) increased cardiac output e) pulmonary embolism

    46. Question (2004) All of the following are disadvantages of a closed circuit system, EXCEPT? a) Need to vent the circuit intermittently to remove nitrogen build-up. b) Cannot monitor ventilation. c) Cannot easily increase depth of anesthesia. d) There is a 200 mL/min loss through the gas sampler.

    47. Flow Flow can be defined as the amount of a substance (gas or liquid) passing over a given point per unit time F = Q t Flow may be Laminar or Turbulent Many clinical measurements assume laminar flow

    48. Laminar Flow Smooth tubes at low flow rates There is a linear relationship between pressure difference across the tube, and the rate of flow i.e resistance to flow is constant

    49. (2005) According to the Hagen-Poisseuille equation which parameter will be inversely proportional to laminar flow? A. Radius of the tube to the power of 4 B. Pressure gradient across the tube C. Velocity of fluid D. Viscosity of fluid

    50. Laminar Flow Laminar flow is governed by : Pressure gradient across the tube; P Radius of the tube; r Length of the tube; l Viscosity of the fluid; ? The Hagen-Poiseuille equation describes the relationship between these factors

    51. The Hagen-Poiseuille Equation

    52. Turbulent Flow A constriction results in an increase of the velocity of the fluid Flow eddies, with resulting higher resistance Flow is no longer directly proportional to pressure

    53. Turbulent Flow The analysis of turbulent flow is highly complex With Turbulent Flow, in a rough tube, the following approximations apply: Q ? ? P or P ? Q2 Q ? ? l-1 thus P ? l Q ? ? ? -1 thus P ? ? Where Q is Flow, P is pressure across the tube, l is length of the tube and ? is the density of the fluid

    54. Turbulent Flow For turbulent flow in a smooth tube, the resistance shows behaviour intermediate between turbulent flow in rough tubes, and laminar flow. Thus there is some dependence on viscosity as well as density

    55. Onset of Turbulent Flow The following factors influence the type of flow: ? = Linear Velocity of fluid ? = Density of fluid d = Diameter of tube ? = Viscosity of fluid

    56. Reynolds Number If Reynolds number exceeds 2000, in a cylindrical tube, turbulent flow is likely to be present The Reynolds number is calculated as follows Reynolds number = ??d ?

    57. Clinical Applications Turbulent flow often occurs where there is an orifice, a sharp bend or other irregularity causing an increase in velocity Helium reduces the density of inhaled gas, reducing Reynolds number, and converting turbulent flow to laminar flow with resultant reduction in resistance Warming and humidification of inhaled gases reduces their density, and also reduces resistance to flow But there is something useful hidden here.But there is something useful hidden here.

    58. Work One Joule of work is done when one Newton of force moves an object one metre W = F x D Remember that P = F/A, or F = P x A and Volume = D x A, or D = V/A. Substituting; W = PA x V/A = PV or Work = Pressure x Volume Resident fatigue How much work do residents actually do?Resident fatigue How much work do residents actually do?

    59. Power Power is the rate of work, and is expressed in watts 1 watt is 1 joule / second How powerful are residents?????How powerful are residents?????

    60. Question (2004) Regarding the line isolation monitor, all of the following are true, EXCEPT? a) Faulty equipment plugged into the wall converts the system to a standard grounded system. b) It will alarm when a 2-5 mA leak is detected. c) The number displayed on the gauge is the total current running on the system at that time. d) It continuously monitors the integrity of an isolated power system.

    61. Figure 8-24. A faulty piece of equipment plugged into the isolated power system does not present a shock hazard. It merely converts the isolated power system into a grounded power system. The figure insert illustrates that the isolated power system is now identical to the grounded power system. The dashed line indicates current flow in the ground wire.Figure 8-24. A faulty piece of equipment plugged into the isolated power system does not present a shock hazard. It merely converts the isolated power system into a grounded power system. The figure insert illustrates that the isolated power system is now identical to the grounded power system. The dashed line indicates current flow in the ground wire.

    62. Figure 8-27. When a faulty piece of equipment is plugged into the isolated power system, it will markedly decrease the impedance from line 1 or line 2 to ground. This will be detected by the LIM, which will sound an alarm.Figure 8-27. When a faulty piece of equipment is plugged into the isolated power system, it will markedly decrease the impedance from line 1 or line 2 to ground. This will be detected by the LIM, which will sound an alarm.

    63. Question (2004) Line isolation monitor may be triggered when: a) microshock may occur b) if ungrounded material is used in the OR c) If the leakage current exceeds preset value d) if the patient becomes grounded e) if the electrocautery unit is used without a grounding pad

    64. Question (2005) Regarding power isolation: A. prevents macroshock B. prevents explosion of flammable gases C. prevents interruption of power in the case of short circuit D. prevents burns from high frequency electrical cautery

    65. Electrical Safety Electrical safety in the OR is often regarded as being of historical interest only Reality is that the OR environment is becoming more electrically complex by the year More complications arise with the networking of electronic equipment which may not conform to the rigid safety standards of conventional medical equipment 10 000 device related injuries in USA every year Electrocution 5th leading cause of accidental death in US

    66. Historical Perspective As the paranoia of the era of flammable anaesthetics recedes, so does the concern re electrical safety Dr.W Stanley Sykes Essays on the First Hundred Years of Anaesthesia has a chapter entitled Thirty seven little things that have all caused death: One thing is certainall of them have happened. All have killed, and they are waiting to do the same thing again unless you know about them.Therein lies the value of history That chapter effectively opens and closes with events related to electrical risks.

    67. Definitions When electrons move from one atom to another in a consistent direction, current is said to flow The applied force to do this is described as potential difference, and energy is used up by the process (volts) This energy can both fulfill its function or injure our patients if care is not taken Materials that permit easy transfer of their electrons from one atom to another are termed conductors Those that do so reluctantly are termed resistors

    69. Definitions Materials that will not transfer electrons under normal circumstances are termed insulators An excess of charge may be carried by some materials as a result of friction (static electricity) This may later be discharged by contact with a conductor, or if the potential is sufficiently high by jumping a gap as a spark.

    70. The Effects of Current On The Human Body - (Source -Hand)

    71. Protecting the Operator Adequate earthing of casing Dont let operator touch casing Dont assume all equipment is always in good shape - regular checks Extension cables are frowned upon - frayed from over use, on floor, exposed to saline etc.

    72. Protecting the Patient In modern ORs patients are rarely grounded We use Floating Circuitry to ensure this OR table may be source of grounding, so make sure no contact to metal e.g. Ether screen etc Diathermy safety Use bipolar diathermy if pt has cardiac device Remember that leakage can occur and is source for microshock:

    73. Microshock Of the current passing through a human hand, less than 0.1% passes through the heart Therefore any cardiac effects result from tiny currents The implication is that if you passed a current directly through the heart, much smaller currents can cause injury 5 seconds of sustained 50 ?A AC current produces sustained VF This phenomenon is known as microshock.

    74. Microshock Anaesthetist can be earthing point for patient, and source for microshock - IF you touch a faulty apparatus and SG catheter at the same time, small leakage current from poorly grounded device can be sufficient to cause VF, even though you dont feel a thing

    75. Capacitative Linkage If a material carries a negative charge, other nearby electrons will tend to move away If the potential at that point varies from positive to negative, such as happens with all alternating current sources (most obviously with mains electricity) then the surrounding electrons will be attracted and repelled alternately In other words, an alternating current can be induced in a material without an electrical source being directly connected to it. This is termed capacitative linkage.

    76. Inductive Linkage Moving electrons generate a magnetic field A moving magnetic field causes movement of electrons AC current source will produces a moving magnetic field and therefore induces secondary current in any nearby wires without the need for direct contact Inductive linkage is intentionally utilized in some devices e.g. transformers

    77. Inductive Linkage When two transformers are placed in series in a power supply it allows the power source for a medical device to be separated from any other parts of the circuitry Consequent reduction in the risk of direct transmission of mains energy to the patient This is known as a floating circuitindicated by the surrounding box in the symbols and the letter F in the description of equipment

    78. Electrical Safety Standards for Medical Equipment Complex description, detailed in a series of International Standards - IEC 60601 Risk to Operator usually occurs when a wire within the device breaks, and contact is made with the metal casing Operator can ground the circuit from metal casing if he / she touches it, getting a shock

    79. Safety Standards As monitoring devices proliferated in ORs, awareness of leakage currents grew Because of capacitative and inductive linkage within medical devices, there will virtually always be some tiny current floating down wires to patients Moderate currents are not a big issue, and the maximum permitted level is below that which can be sensed, or cause harm

    80. Classification by maximum tolerated leakage currents

    81. Symbols indicating Class B Equipment

    82. Symbols indicating Class C Equipment

    83. Question (1998, 1999, 2002, 2004) What reduces the incidence of intra-operative fires with CO2 lasers? Using a red rubber ETT Wrapping a PVC ETT with lead foil Using FiO2 > 0.40 Using N2O/O2 mixture Using the CO2 laser in noncontiguous mode

    84. Question (2004) Which of the following is True about laser surgery? a) CO2 laser is absorbed by water and has deep penetration b) ND-YAG laser penetrates tissue to 200 um c) Nitrous oxide supports combustion d) PVC tubes are safe in laser surgery e) Rubber tubes are safe for CO2 laser

    85. Fire Risk Flammable anaesthetics largely abandoned No change in OR Fire incidence Diethyl Ether still widely used in dev countries Perceived need to prevent build up of static electricity has been progressively abandoned Risk of flammable skin prep is real, particularly with electrical OR beds Most equipment not marked any more

    86. Pause Show of hands: Who can tell me the location of the fire extinguisher in the OR they were working in this morning? At a recent meeting of NA hospital CEOs: More than 20 percent were aware of a recent OR fire Annual incidence ~100/year in USA Top priority for JCAHO in 2008 Im on Fire! OR Blazes on the Rise, Roane KR. US News & World Report, Aug 2003. Curr Opin Anaesthesiol 21:790795 How does that compare to epidural complications? Joint Commission on Accreditation of Healthcare OrganizationsCurr Opin Anaesthesiol 21:790795 How does that compare to epidural complications? Joint Commission on Accreditation of Healthcare Organizations

    87. OR Fire Staring a fire needs three factors: Oxidizer: O2, N2O Ignition source: (electric) spark in 100% of closed claims Combustible substances: ETT, circuit, drape, clothe etc Surgical Prep vapour

    88. Case 1 A 25-year-old man was admitted for laparoscopic appendectomy and general anesthesia was induced. The fiberoptic scope was assembled with the proximal end attached to the fiberoptic light source, and the scope was turned on with the distal end laid on the surgical drapes. Within 1 min, the anesthesiologist smelled smoke. Excerpted from Williams DH, et al. 2006 Flame retardant surgical drapesExcerpted from Williams DH, et al. 2006 Flame retardant surgical drapes

    89. Case 2 A 62-year-old man with copious body hair underwent tracheostomy in the operating room. The neck was prepared with DuraPrep surgical solution, and after drying for at least 3 min, the operative field was draped. Activation of electrocautery ignited a fire, and the patient was burned on his neck and shoulders. Excerpted from Weber et al. 2005Excerpted from Weber et al. 2005

    90. Fuel Sources Curr Opin Anaesthesiol 21:790795Curr Opin Anaesthesiol 21:790795

    91. Case 3 A 45-year-old man needed emergency tracheostomy. He was intubated with a cuffed oral polyvinylchloride endotracheal tube and ventilated with 100% oxygen prior to tracheal incision. During opening of the trachea using diathermy, a popping sound was heard and flames originating from the tracheal incision were observed. Excerpted from Niskanen et al. 2007Excerpted from Niskanen et al. 2007

    92. Case 4 A 73-year-old man was scheduled for bilateral parietal burr-holes to evacuate a subdural hematoma under monitored anesthesia care (MAC). The patient was brought to the OR and a clear plastic mask was loosely strapped to his face, and oxygen introduced at 6 L/min. The head was shaved, and the skin was prepared with a surgical solution of iodine in 74% isopropyl alcohol. After allowing at least 2min drying time as recommended in the manufacturers instructions, the surgical field was draped. The electrosurgical unit (ESU) was used to incise the pericranium. During the first activation of the ESU, a muffled pop was heard, which was followed almost immediately by the appearance of smoke from under the paper drapes. The entire drape was removed, the head was fully engulfed in a ball of flame, and the oxygen mask was also observed to be in flames. The paper drapes themselves were not on fire, and the surgeon used these to smother the flames while the anesthesiologist turned off the oxygen flow to the mask. Excerpted from Barker et al. 2001Excerpted from Barker et al. 2001

    93. Symbols of equipment safety in presence of flammable vapours

    94. Classification by electrocution risk from contact with chassis

    95. Question (2005) Regarding Pulse oximeter all true EXCEPT: A. Pulse oximeter function is not altered by low cardiac output state B. Normal saturation may be associated with carbon monoxide C. Function may be affected by ambient light D. Function may be affected by vasoactive drugs Answer: AAnswer: A

    96. Additional Reading: GD Parbrook: Basic Physics and Measurement in Anaesthesia PG Barash: Clinical Anesthesia Miller: Anesthesia Dorsch and Dorsch - Understanding Anesthesia equipment Current Anaes & Crit Care 2004;15 350-354, Electrical Safety in the operating theatre, Graham S Curr Opin Anaesthesiol 21:790-795, Fire safety in the operating room, Rinder CS BJA 1994 Jun;72(6):710-22, A short history of fires and explosions caused by anaesthetic agents, MacDonald AG

    97. Thank you Questions and Comments Welcome

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