Introductory Physics for Anesthesiologists

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Introductory Physics for Anesthesiologists

• T. Turkstra, M. Eng, P. Eng, MD, FRCPC
• April 15 2009

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Royal College of Physicians and Surgeons of
CanadaObjectives 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.

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Royal College of Physicians and Surgeons of
CanadaObjectives 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??

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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.

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Patient Scenario

• 52 yr old male, prev healthy, Ø 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?

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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.

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Physics? Why Bother?

• Fewer and fewer physics MCQs since about 2000
• Still fair game
• Fairly important aspect of much of our daily
  practice

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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

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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

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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.

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Force

• 1 Newton (N) of force applied to 1 kg of matter,
  will accelerate it by 1m/s2
• How much is that?

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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

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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.

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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. (FPA)
• As the pressure falls in the low pressure system,
  the spring pushes the diaphragm down, allowing
  more gas into the system

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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.

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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

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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

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The Ideal Gas Laws

• BOYLES LAW (First perfect gas law)

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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)

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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?

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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

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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

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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

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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)

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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)

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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?

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EXPLOSION?

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

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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)

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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

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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.

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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

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The Mole

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

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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

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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.

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20 g of Sevo in 224 litres 1
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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

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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 8.3144 J/K/mol

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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

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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

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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)

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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

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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

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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

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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

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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.

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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

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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

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• (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

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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

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The Hagen-Poiseuille Equation
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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

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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

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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

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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

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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
• ?

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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

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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

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Power

• Power is the rate of work, and is expressed in
  watts
• 1 watt is 1 joule / second

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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.

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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

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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

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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

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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.

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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

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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.

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

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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

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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.

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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

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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.

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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

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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

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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

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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

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Classification by maximum tolerated leakage
currents
Single fault condition condition in which one
means for protection against hazard is defective.
If a single fault condition results unavoidably
in another single fault condition, the two
failures are considered as one single fault
condition.
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Symbols indicating Class B Equipment
Class B Earthed Class BF, Floating Earth Class
BF, Floating Earth, defibrillators may be
used while equipment is connected
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Symbols indicating Class C Equipment
Class C Earthed Class CF, Floating
Earth Class CF, Floating Earth,
defibrillators may be used
while equipment is connected
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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 gt 0.40
• Using N2O/O2 mixture
• Using the CO2 laser in noncontiguous mode

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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

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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

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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.

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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

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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.

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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.

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Fuel Sources
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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.

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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.

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Symbols of equipment safety in presence of
flammable vapours
AP - Anaesthetic Proof
Unsafe to use in zone of risk where vapor is
mixed with an oxidising gas mixture (N2O is
better oxidiser than O2
Safe to use in zone of risk where vapor is mixed
with air
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Classification by electrocution risk from contact
with chassis
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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

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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 200415 350-354,
  Electrical Safety in the operating theatre,
  Graham S
• Curr Opin Anaesthesiol 21790-795, Fire safety in
  the operating room, Rinder CS
• BJA 1994 Jun72(6)710-22, A short history of
  fires and explosions caused by anaesthetic
  agents, MacDonald AG

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Thank you

• Questions and Comments Welcome