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 Cap^TM
• 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

• See Capnography Waveform Interpretation

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

• Capnography.com
• HowEquipmentWorks.com — Capnography
• KI Doc — So much hot gas – ETCO2 for non-anaesthetists (2013)