Capnography and CO2 Detectors

USES

To detect and/or measure exhaled CO2 for 3 main reasons:

  • help confirm endotracheal intubation
  • monitor ventilation during procedural sedation (e.g. via Hudson mask) without mechanical ventilation
  • monitoring during mechanical ventilation

Monitoring uses include

  • airway disconnection alarm
  • detect ETT dislodgement
  • allows recognition of different wave forms types which can correspond to various pathologies (e.g. bronchospasm)
  • prediction of PaCO2
  • assess CPR quality during cardiac arrest (cardiac output) and detect ROSC
  • recognition of spontaneous breath during apnoea testing
  • to provide protection against unexpected hypercapnia in the neurosurgical patient
  • sudden decrease -> sudden decrease in pulmonary blood flow -> PE

DESCRIPTION

Qualitative colorimetric devices

  • e.g. Easy CapTM
  • detects breath to breath colour changes through a pH detector (metacresol purple on filter paper changes to yellow in the presence of CO2)

Quantitative devices

  • Capnometry devices provides measurement and numeric display of end tidal CO2 (ETCO2)
  • Capnography provides a display of the quantity of exhaled CO2 with time which produces a characteristic waveform
  • Infrared analysers are most commonly used

METHOD OF USE

  • placed in line with ventilation and patient’s airway
  • production of a quantitative ETCO2 value and allows assessment of waveform morphology (ETCO2 vs time)

Qualitative colorimetric devices

  • devices are inserted on the end of an endotracheal tube or tracheostomy tube and connected to a ventilator circuit (via universal connectors)
  • colour changes when ventilation occurs
  • can only be used once (colour does not change back, hence only useful for confirmation of ETT position in the trachea during intubation, not ongoing monitoring)

Quantitative devices

  • require calibration
  • Infra-red analysers
  • some are connected mainstream (flow through via an in-line adapter with universal connectors)
  • others are sidestream devices (with aspiration of gas that is transported to a sensor located a variable distance away)
  • the detected CO2 absorbance is converted to a quantitative amount and displayed with a waveform
  • older systems used Mass spectrometry

OTHER INFORMATION

  • normally ETCO2 is up to 5 mmHg/0.7kPa lower than PaCO2 as some alveolar dead space is always present
  • test quantitative devices by blowing on them and seeing am ETCO2 trace before placing in the patient circuit

Infrared analysers

  • Infra-red analysers use spectroscopic sensors to detect CO2 in a gaseous environment by its characteristic absorption wavelength
  • gas passes through a chamber made of material transparent to infrared light (e.g. sapphire)
  • light is focused through the chamber onto a photodetector and the amount detected quantified and converted to a calibrated quantity of CO2
  • single beam (sample gas passes through only) or double beam (additional beam with no CO2 to act as a control) systems

COMPLICATIONS/LIMITATIONS

  • False positive CO2 detection
    — oesophageal intubations after consumption of carbonated beverages or vigorous bag and mask ventilation (although this rapidly ceases after 20–30s or 6-8 ventilations; quantitative devices with waveforms reveal a characteristic decrementing, non-square pattern)
    — high levels of other gases that absorb the infrared light can lead to falsely high readings (e.g. nitrous oxide in operating theatres)
    — colorimetric CO2 detectors may change colour if exposed to acidic fluid (e.g. stomach contents, adrenaline solution from ampoules)
  • false negative results can occur (e.g. cardiac arrest states, a low pulmonary flow due to PE, or large alveolar dead space)
  • Mainstream devices
    — need time to heat up to avoid condensation on the heater
    — add dead space and weight to the circuit
  • prediction of PaCO2 from ETCO2 is variable (the major limiting factors = pulmonary blood flow and V/Q mismatch) and ETCO2 may be misleadingly different in conditions with significant V/Q mismatch
  • Sidestream devices
    — can result in a time delay
    — prone to blocking as not in line with gas flow
    — may be inaccurate if low tidal volumes as fresh gas may be entrained (e.g. paediatrics)
    — analysed circuit gases can also leak to the environment
  • utility in neonates and children may be impaired because of small tidal volumes

INCREASED ETCO2 – PaCO2 GRADIENT

This usually reflects an increase in alveolar dead space; about 5 mmHg is normal (ETCO2 should always be lower than PaCO2)

  • Low cardiac output or cardiogenic shock
  • Pulmonary embolism
  • Cardiac arrest
  • Positive pressure ventilation and use of PEEP
  • High V/Q ratios
  • inaccurate calibration of capnometer

INTERPRETATION OF CAPNOGRAPHY WAVEFORMS

References and Links

Journal articles

  • Cheifetz IM, Myers TR. Respiratory therapies in the critical care setting. Should every mechanically ventilated patient be monitored with capnography from intubation to extubation? Respir Care. 2007 Apr;52(4):423-38; discussion 438-42. PMID: 17417977.
  • Kodali BS. Capnography outside the operating rooms. Anesthesiology. 2013 Jan;118(1):192-201. PMID: 23221867.

FOAM and web resources


CCC 700 6

Critical Care

Compendium

Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also the Innovation Lead for the Australian Centre for Health Innovation at Alfred Health and Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of two amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.