Capnography

  • Capnometry is the measure of CO2 in exhaled gas
  • Capnography is the display of this information as a graph

Infrared Spectrometry

  • Most anaesthetic machines use infrared spectrometry to measure end-tidal CO2
  • Any molecule with two or more different atoms will absorb infrared radiation at a characteristic wavelength
    • For CO2 this is 4.3μm
  • The Beer-Lambert law states that the degree of absorption is proportional to concentration as well as distance the light has travelled
    • Beer = concentration
    • Lambert = distance
  • The emittor produces infrared light that passes through a sapphire lens that only permits 4.3micrometre wavelength light through
  • This passes through a test chamber and a reference chamber to a detector

What are the two different types of infrared capnography?

  • Sidestream
    • Draws 150 ml/min from the main breathing circuit near the patient end
    • Needs a water trap to prevent interference from water molecules
    • Has an exhaust port to return the gas to the circuit
    • Benefits are that the circuit is lighter, and the expensive delicate components are protected within the anaesthetic machine
  • Mainstream
    • Needs a bulky sampling chamber attached directly to the breathing circuit at the patient end
    • Needs heating to 41ºC to avoid water condensation

What are the problems with infrared spectroscopy?

  • Collision broadening
    • Mollecules that are in close proximity will exert forces upon one another that can change their respective energy levels and have a ‘blurring’ effect on the wavelengths absorbed, giving alterations in the amount of light received by the detector
  • Nitrous Oxide
    • If there is nitrous oxide present in the sampling chamber, CO2 can transfer some of the absorbed energy to the nitrous molecules, thereby giving a falsely higher absorption reading
    • Nitrous also has a similar absorption wavelength (4.5 micrometres)
      • Most machines have compensation mechanisms in place for this
  • Variation in emission and detection of infrared radiation by equipment
    • Double beam analysers employ a reference chamber that contains no CO2 allows subtraction of artefact signals, thereby correcting for
      • The amound of IR radiation emitted
      • The sensitivity of the detector
      • Changes in the lens behaviour
  • Water and alcohol can cause interference
    • Either the gas is dried
    • Or a specific wavelength chosen away from water’s maximal absorption frequency

A Normal ETCO2 Trace

  • During inspiration (phase 4), the CO2 sampling line detects the fresh gas flow as it flows into the lungs. This contains no CO2, and so the graph drops to zero
  • As the patient then exhales, the first part of the exhaled gas is from the trachea and upper airways not involved in gas exchange (dead space), so this has no CO2 either (end of phase 1)
  • As exhalation continues, gas from the alveoli with the shortest time constants begins to reach the sampling line, mixed with the dead space gas from other areas in the lung, and the concentration of detected CO2 starts to rise (phase 2)
  • As more alveoli empty, the CO2 level rises until it reaches a plateau (phase 3)
    • At this point there is no dead space gas remaining, and only alveolar gas emptying into the circuit
    • Those alveoli with the longest time constants will have picked up slightly more CO2 during this time, and therefore will have a higher concentration of CO2, meaning the plateau is not quite flat
  • This continues until the next inhalation

What might cause a rapid loss of end tidal CO2 trace?

  • Either there is circulatory failure, and CO2 is not reaching the lungs
    • Cardiac arrest
    • Profound Shock
    • Massive pulmonary embolism
  • Or there is ventilatory failure, and CO2 is not leaving the lungs
    • Airway obstruction
    • Bronchospasm
  • Or there is an equipment issue, and CO2 is not reaching the analyser
    • Disconnection
    • Kinking of sample tube

What is happening here and what should be done about it?

  • The patient is starting to make their own respiratory effort, causing dysynchrony with the ventilator
  • Either switch off the ventilator and allow patient to breathe spontaneously
  • Or administer sedation, opiates or muscle relaxation if not wanting patient to breathe spontaneously
    • The respiratory rate on the ventilator can also be increased to lower the PaCO2, and reduce the respiratory drive

What does rebreathing look like on the end tidal CO2 trace?

  • Trace appears normal in shape
  • Consistently raised baseline and raised peak ETCO2

What does airway obstruction look like on the end tidal CO2 trace?

  • Upward sloping plateau phase as CO2 takes longer to drain from obstructed small airways
  • May also have higher peak if CO2 is being retained and PaCO2 increasing (severe bronchospasm)

What might be the cause of a high end tidal CO2?

  • Inadequate ventilation
    • Tidal volume, respiratory rate, excessive dead space
  • Increased CO2 production
    • Sepsis
    • Malignant hyperthermia
    • After deflating tourniquet
    • ROSC
  • Rebreathing of CO2
    • Exhausted Soda lime
    • Inadequate fresh gas flow
    • Circuit failure eg inner tube dislocation in Bain Circuit

What might be the the cause of a low end tidal CO2?

  • Reduced ventilation
    • Airway obstruction
    • Bronchospasm
  • Reduced cardiac output
  • Excessive fresh gas flow

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