Physics of Anesthesia

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Physics of Anesthesia

• Gerard T. Hogan, Jr., CRNA, DNSc., ARNP
• Clinical Assistant Professor
• Florida International University

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Physics of Anesthesia

• Course Outline
• Pressure
• Gas Laws
• Volume
• Temperature
• Flow
• Osmosis
• Vaporization

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Physics of Anesthesia

• Course Outline
• Vapor Pressures
• Latent heat of vaporization
• Carrier gases
• Carbon Dioxide Absorption
• Electricity
• Electrical Safety in Anesthesia

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Physics of Anesthesia

• Course Outline
• Diffusion and Osmosis
• Humidification
• Gas Chromatography
• Mass Spectrometry
• Isotopes and Radiation

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Physics of Anesthesia

• Pressure
• Force is that which tends to change the state of
  rest or motion of any object
• SI the International Standard of Units
• Understanding pressure from the perspective of
  flow is essential for an understanding of
  Anesthesia

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Physics of Anesthesia
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Physics of Anesthesia

• Pressure
• The force that is exerted by atoms and molecules
• Generated by kinetic energy
• Movement of atoms and molecules is not uniform
• Gases expand indefinitely and can travel
• Pressure is exerted when molecules or atoms
  collide

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Physics of Anesthesia

• Pressure
• Gases are compressed for use in anesthesia
• Gases may be contained within any number of
  cylinders
• Cylinder pressure is measured by PSIG pounds
  per square inch gauge

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Physics of Anesthesia

• Pressure
• When any cylinder is empty, the atmospheric
  pressure inside the cylinder matches that outside
  the cylinder
• A full E cylinder of Oxygen contains 2200 PSIG
• A full E cylinder of Nitrous Oxide contains 745
  PSIG

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Physics of Anesthesia

• Bourdon Gauge
• Measures both cylinder and pipeline pressures of
  gases
• Aneroid gauge means it does not use Mercury for
  measurement
• ATSM standards the American Society for Testing
  and Measuring

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• Aneroid gauges are commonly found on all
  anesthesia gas machines
• They are inexpensive, accurate, and portable

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Physics of Anesthesia

• Aneroid Gauges
• They function by having a hollow metal tube that
  is bent into a curve and sealed
• This is linked to a clock like mechanism
• As the pressure in the cylinder increases, the
  tube straightens out
• This corresponds to a change in the position of
  the indicator

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Physics of Anesthesia

• Pressure
• Pressures other than gases are often measured in
• Millimeters of Mercury (mmHg) also known as torr
• Centimeters of Water (cm H2O)
• Atmospheric pressure is 760 torr or 1034 cm H2O

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Physics of Anesthesia
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Physics of Anesthesia

• Pressure
• Simple pressure equations
• Where ppressure, fforce, and aarea
• Consider the example of a bedsore

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Physics of Anesthesia

• Pressure
• Suppose 20kg of a patients weight is supported
  by an area 10cm x 10cm
• Gravity exerts a force of 9.81 m s-2
• 20kg x 9.81m s-2 196 N(ewtons)
• 196N/10-2m2 19.6 kPa
• Since systolic Blood Pressure is only 16 kPa,
  there is a loss of flow

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Physics of Anesthesia

• Pressure
• Important points are that
• Gravity influences the movement of atoms and
  molecules
• Energy forces operate within atoms or molecules
• Density is always a factor
• Dense substances exert more pressure than less
  dense ones

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Physics of Anesthesia

• Pressure
• Since we learned in chemistry that all atoms and
  molecules are in constant motion, the more you
  compress them, the greater the pressure becomes
• In other words, the sum of the forces exerted by
  a group of atoms or molecules when they are in a
  closed cylinder can be measured as pressure

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Physics of Anesthesia

• Ideal gas laws
• Interrelationship among temperature, pressure,
  and volume
• Boyles law
• Charless law
• Gay-Lussacs law

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Physics of Anesthesia

• Boyles law
• At a constant temperature, the volume of a gas
  varies inversely with the pressure
• P1V1P2V2
• This makes sense because if temperature remains
  constant, the more the pressure the less the
  volume of area it occupies

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Physics of Anesthesia

• Boyles law
• Example for you to work out
• At room temperature a volume of gas is 200ml at
  sea level. What is the volume of gas at an
  atmospheric pressure of 250 torr?
• Remember that P1V1P2V2

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Physics of Anesthesia

• Boyles law
• Is your answer
• A. 1250 ml
• B. 840 ml
• C. 608 ml
• D. None of the above

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Physics of Anesthesia

• Boyles law
• If the pressure of the gas decreases, then the
  volume of the gas (through natural movement of
  the molecules) would increase
• The correct answer was C, 608 ml

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Physics of Anesthesia

• Charless Law
• At a constant pressure, a volume of gas is
  directly proportional to the temperature
• V1/T1V2/T2
• This makes sense because we know that when you
  heat gases they expand, and when you cool them
  the contract within any closed space

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Physics of Anesthesia

• Charless law
• An example for you to work out
• At sea level a balloon filled with air at 15C is
  300ml. If we heated the room to 20C, what would
  the volume be?
• Remember V1/T1V2/T2

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Physics of Anesthesia

• Charless law
• Is your answer
• A. 350 ml
• B. 400 ml
• C. 445 ml
• D. 500 ml

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Physics of Anesthesia

• Charless law
• As the temperature in a closed space (such as a
  cylinder) increases, there is an increase in
  molecular movement
• This increase causes molecules to spread out
  increasing the pressure
• If the temperature is doubled, the volume doubles
  if expansion can occur
• The correct answer is B, 400 ml

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Physics of Anesthesia

• Gay-Lussacs Law
• At a constant volume, the pressure of a gas is
  directly proportional to the temperature
• P1/T1P2/T2
• This makes sense because we know that when gases
  are heated, pressure increases within a closed
  space

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Physics of Anesthesia

• Gay-Lussacs law
• Example for you to work out
• If a full cylinder of medical grade air is in a
  room where the temperature is 20C reads 1700 psi,
  what would the gauge read if we increased the
  room temperature to 30C?
• Remember that P1/T1P2/T2

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Physics of Anesthesia

• Gay-Lussacs Law
• Is your answer
• A. 2550 psi
• B. 2750 psi
• C. 2950 psi
• D. 3150 psi

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Physics of Anesthesia

• Gay-Lussacs law
• In any closed container, when the volume of a gas
  is kept constant, then the pressure of the gas is
  directly proportional to the temperature of the
  gas if the temperature doubles, the pressure
  doubles
• The correct answer is A, 2550 psi

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Physics of Anesthesia

• Any Questions?