Capnometry and the arterial-expired carbon dioxide gradient

The measurement of end-tidal CO₂ is a favourite topic of the examiners, eg. in  Question 6 from the second paper of 2005, "Critically evaluate  the use and limitations  of End-Tidal Carbon Dioxide measurement in Intensive Care practice".
Capnography is discussed in greater detail elsewhere:

• The normal capnograph waveform
• Abnormal capnography waveforms and their interpretation

There is also an excellent site by Prasanna Tilakaratna which explains infra-red absorption spectrophotometry using vividly colourful diagrams.

Use of capnography in the ICU

• Confirmation of ETT placement
• Airway disconnection alarm
• Monitoring during transport
• During CPR to assess adequacy of cardiac compression
• Recognition of spontaneous breath during apnoea test
• Neurosurgical patient to provide protection against unexpected hypercapnia
• Quick bedside assessment of bronchospasm
• Alert of sudden changes in pulmonary perfusion (eg. PE)
• Early alert of PEA in the absence of continuous BP monitoring
• More accurate monitoring of respiratory rate

Principles of capnometry

• The sensor is an infrared absorption spectrophotometer.
• CO₂ absorbs infra-red radiation (it being a greenhouse gas and all) and so expired air enriched with CO₂ will absorb more infra-red light than CO₂-poor air.
• There are two sensor types: mainstream and sidestream.
• In sidestream sensors, the respiratory gases are aspirated via a thin samling line, creating a 200ml/minute leak. The infrared sensor is at the end of that line. The disadvantage of this method, apart from the leak, is the delay in gas transport - up to 4 seconds might pass before you notice that the patient has stopped producing CO₂.
• In mainstream sensors, the sensor is positioned across the airway. The advantage of this is an immediate measurement, without delay. The disadvantage is increased dead space. Furthermore, humidity rain-out tends to confuse the sensor.

Advantages of end-tidal CO₂ monitoring

• Continuous monitoring
• Immediate feedback regarding cardiac output and ETT position
• Waveform analysis is possible
• Cheap
• Increased safety; decreased risk of undetected airway circuit disconnection

Disadvantages of end-tidal CO₂ monitoring

• Produces vigilance-impairing false alarms
• EtCO₂ values may not correlate with PaCO₂ values and the two may be substantially different
• The monitor in-line connector creates a small amount of apparatus dead space
• The adaptor fitted to the end of the ETT may be heavy, and may increase the risk of accidental extubation, particularly in children and neonates
• The gas sampling models of EtCO₂ monitors can diminish the delivered minute volume, as they access the circuit gas at a rate of about 200ml/min.
• Nitrous oxide can confuse some capnometers (i.e. be mistaken for CO₂)
• The presence of helium can cause the EtCO₂ measurement to be incorrectly elevated in some capnometers (i.e. those which use a reporting algorithm that assumes that the only gases present in the sample are those that the device is capable of measuring)

Evidence and Guidelines

• EtCO₂ rapidly detects lifethreatening complications in transported patients.
• American Heart Association Guidelines for Cardiopulmonary Resuscitation make the following recommendations
  • Use EtCO₂ to assess ETT position
  • Use EtCO₂ to assess efficacy of CPR
  • Use EtCO₂ to confirm the return of spontaneous circulation

Conditions which cause a persistent flat capnograph trace

• Ventilator disconnection
• Airway obstruction (eg. patient suddenly bit down on the tube)
• ETT perforation (the end tidal gas is escaping via the hole before it gets to the capnograph)
• Capnograph disconnection or obstruction
• Water droplet contamination of capnography module
• Cardiac / respiratory arrest
• Apnoea test in a brain dead patient

Conditions which increase the gradient between end-tidal and arterial PCO₂

This specific issue was asked about in Question 9.2 from the second paper of 2008.

• Pulmonary perfusion
  • Pulmonary embolism
  • Fat embolism
  • Air embolism
  • Cardiac failure (RHF)
  • Cardiac arrest
• Ventilation
  • Increased V/Q mismatch due to high PEEP
  • Increased alveolar dead space
  • High FiO2 (causing shunt into poorly ventilated alveoli)
• Artifact
  • The presence of helium can cause the EtCO2 measurement to be incorrectly elevated in some capnometers (i.e. those which use a reporting algorithm that assumes that the only gases present in the sample are those that the device is capable of measuring)
  • The presence of nitrous oxide can confuse some capnograph devices, and the NO2may be misinterpreted as CO2
  • The use of an inline HME filter can reduce the end-tidal CO2 concentration.