The above is a capnograph trace obtained from a patient in an intensive care unit.

a) What technology is used for the detection  of CO2  in expired breath in the ICU?

b) List 3 uses for capnography in intensive care

c) List 3 conditions  which may increase the gradient between end-tidal and arterial PCO2?

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College Answer

a) What technology is used for the detection  of CO2  in expired breath in the
ICU?

Infra-red absorption spectrophotometry

b) List 3 uses for capnography in intensive care

Airway disconnection alarm
Confirmation of ET tube placement in airway
During CPR to assess adequacy of cardiac compression
Recognition of spontaneous breath during apnoea test
Neurosurgical patient to provide protection against unexpected hypercapnia

c) List 3 conditions  which may increase the gradient between end-tidal and arterial PCO2?

Low cardiac output or cardiogenic shock
Pulmonary embolism
Cardiac arrest
Positive pressure ventilation and use of PEEP

High V/Q ratios.
Candidates who mention increased alveolar dead space should also get some credit.

Discussion

Capnography is discussed in greater detail elsewhere:

There is an excellent site by Prasanna Tilakaratna which explains infra-red absorption spectrophotometry using vividly colourful diagrams (how else?). In brief, CO2 absorbs infra-red radiation (it being a greenhouse gas and all) and so expired air enriched with CO2 will absorb more infra-red than CO2-poor air.

Uses of capnography in the ICU: some from the college, and some endogenously generated uses:

  • 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

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

  • 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 NO2 may be misinterpreted as CO2
    • The use of an inline HME filter can reduce the end-tidal COconcentration.

References

The best, most detailed review:

Walsh, Brian K., David N. Crotwell, and Ruben D. Restrepo. "Capnography/Capnometry during mechanical ventilation: 2011." Respiratory care 56.4 (2011): 503-509.

Whitaker, D. K. "Time for capnography–everywhere." Anaesthesia 66.7 (2011): 544-549.

Kodali, Bhavani Shankar. "Capnography outside the operating rooms." Anesthesiology 118.1 (2013): 192-201.

 

Yamauchi, H., et al. "Dependence of the gradient between arterial and end-tidal PCO2 on the fraction of inspired oxygen." British journal of anaesthesia (2011): aer171.

Razi, Ebrahim, et al. "Correlation of End-Tidal Carbon Dioxide with Arterial Carbon Dioxide in Mechanically Ventilated Patients." Archives of trauma research 1.2 (2012): 58.