End-Tidal Gas Analysis

Describe the principles of pulse and tissue oximetry, co-oximetry and capnography, including calibration, sources of errors and limitations

Principles

Several mechanisms for ETCO2 measurement exist:

  • Infrared Spectroscopy
  • Colourimetric Methods
  • Rayman Scattering
  • Gas Chromatography

Infrared Spectroscopy

Infrared spectroscopy relies on the fact that:

  • Gases with two or more different atoms will absorb infrared radiation
  • Different gases absorbing different wavelengths to different degrees
  • Measuring the absorbed wavelengths and comparing with the likely composition of a mixture, a system can be designed using a specific wavelength to measure gas concentrations and avoid interference

End-tidal gas analysis using infrared light is used in the measurement of:

  • CO2
    Capnography is the continuous measurement and graphical display of the partial pressure of CO2 in expired gas. This is the most common method to measure ETCO2.
  • Anaesthetic agents

Measurement of CO2

Components:

  • Sapphire sampling chamber containing gas sample
    • CO2 absorbs infrared radiation at a peak wavelength of 4.28μm
    • The sapphire lens only allows 4.28μm light through
  • Emitter
  • Detector
  • Microprocessor
  • Display

Method:

  • Light is emitted and passes through the sampling chamber
    A lens is used to focus emitted light.
  • Levels of radiation are measured on the other side of the chamber
  • Levels correspond to the amount of gas present in the sample
  • The less radiation that reaches the detector, the more gas there is in the sample absorbing it

Equipment Errors

Errors can be classified into:

  • Specific to technique
    • The collision broadening effect
      Intermolecular forces vary depending on their proximity to other molecules in the gas mixture. A change in intermolecular forces may alter their bond-energy and the frequencies at which they absorb radiation. It can be overcome by:
      • Correcting for the presence of other gases
      • Manually adjusting the obtained values
    • Crossover with other gas mixtures
      CO2 and N2O have similar absorbance spectra, and may lead to error when a device is not designed to measure both wavelengths.
  • Failure of equipment
    These can be overcome by use of double-beam capnometer. This uses a reference chamber which contains CO2-free air, and the same emitter-detector system. All absorption from this system must occur due to artifact (as no CO2 is present). The artifactual component is then subtracted from the value detected in the main chamber. This corrects for:
    • Variable amount of infrared radiation released
    • Variable sensitivity of the detector
    • Variable efficacy of the crystal window and lens system
  • Relating to type of capnometer used
    ETCO2 may be either side-stream or in-line.
    • Side-stream CO2 involves a length of narrow tubing drawing gas from the expiratory limb of the breathing circuit (typically from the HME filter) to the capnograph
      • Side-stream requires a flow of 150 ml.min-1
      • Has a (pretty insignificant) delay (<1s) in measurement
      • May be blocked by water vapour, and require use of a water trap to remove condensation
    • In-line systems have a sampling chamber attached in-line with the ETT
      • The sampling chamber slightly increases the dead-space of the circuit
        May be relevant in children or very difficult to ventilate patients.
      • Adds weight to patient end of the breathing circuit
      • Require heating to 41°C to avoid condensation

Normal ETCO2 Waveform

The normal trace consists of four components:

  1. The baseline
    This consists of:
    • Inspiratory time
    • Early dead-space exhalation
      This is the period immediately before phase 2, where some gas with a PCO2 of 0 is exhaled.
  2. Alveolar exhalation, where PCO2 rises rapidly
  3. Alveolar plateau, where PCO2 flattens
    The highest-point of this curve is labelled ETCO2.
  4. Inspiration, where PCO2 returns to 0

ETCO2 Waveform Variations

Airway obstruction:

  • Occurs due to uneven emptying of alveoli with different time-constants

Hyperventilation:

  • Lower ETCO2 with shorter baseline
  • Plateau phase may not occur at very high respiratory rates

Rebreathing:

  • Baseline increases as inspired CO2 is measured from gas analyser

Changes in ETCO2

Normal ETCO2 is 32-42 mmHg, whilst normal PaCO2 is 35-45 mmHg.

High ETCO2

This may be from:

  • Decreased ventilation
    • Decreased RR
    • Decreased VT
    • Increased VD and therefore a greater VD:VT ratio
  • Increased production of CO2
    • Increased metabolic rate
      • Sepsis
    • Torniquet release
    • ROSC following arrest
  • Increased inspired
    • Rebreathing (i.e. equipment/ventilator malfunction)
    • External source of added CO2

Low ETCO2

Rapid Loss of ETCO2
  • Failure of ventilation
    • Circuit disconnect
    • Airway obstruction
    • Bronchospasm
  • Failure of circulation
    • Cardiac arrest
    • Shock
Gradual Loss of ETCO2
  • Increased VA (i.e. increased MV)
  • Decreased CO2 production
    • Hypometabolic state
      • Hypothermia
  • Increased VD, i.e. V/Q mismatch
    • Increased West Zone I physiology:
      • Hypotension
      • Increased RV Afterload:
  • Sampling error
    • Air entrainment into the sample chamber
    • Inadequate VT

Discrepancy between ETCO2, PACO2, and PaCO2

The normal gradient between PaCO2 and ETCO2 is 0-5 mmHg. Healthy and awake individuals should have essentially no (<1ml) alveolar dead space, and so essentially no gradient. This gradient is increased in patients with:

  • V/Q mismatch
    • ETCO2 will underestimate arterial CO2 as gas from un-perfused alveoli (with negligible CO2) will dilute CO2 expired gas

Colourimetric Methods

Litmus paper which changes colour when exposed to hydrogen ions (produced by CO2) can be used to confirm endo-tracheal intubation, though they may generate false-positive results due to gastric pH.


References

  1. Cross ME, Plunkett EVE. Physics, Pharmacology, and Physiology for Anaesthetists: Key Concepts for the FRCA. 2nd Ed. Cambridge University Press. 2014.
  2. Davis PD, Kenny D. Basic Physics and Measurement in Anaesthesia. 5th Ed. Elsevier. 2003.
  3. Leslie RA, Johnson EK, Goodwin APL. Dr Podcast Scripts for the Primary FRCA. Cambridge University Press. 2011.
Last updated 2017-10-08

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