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About

• Capnography is the non-invasive monitoring of the concentration or partial pressure of carbon dioxide in the respiratory gases
• Normally this is used as a measurement of the expired CO2, however inspired CO2 may increase when using a ventilator. A carbon dioxide scrubber or soda lime is used to remove carbon dioxide from the ventilation circuit, which is particularly important when using low flows. However, when the carbon dioxide scrubber is saturated and needs replacing, this can lead to an increase in the inspired concentration of CO2.
• Capnography is part of the gold standard of monitoring for any ventilated patient, patient undergoing sedation or anaesthesia and in cardiac arrests.
• Capnography does not require an endotracheal tube. This can be measure with supraglottic airways, face mask ventilation (appropriate fitted and with a good seal) or with nasal cannula.
• The capnograph can be depicted as a number, as with the Emma device (a portable CO2 capnograph), or waveform as seen on ventilator monitors.
• The end tidal CO2, or EtCO2, is the maximum partial pressure of CO2 measured at the end of exhalation. This is the number displayed on the capnograph.

Uses

• Capnography can provide a measurement of ventilation, perfusion and metabolism.
• CO2 capnography should be used in all intubated patients, patients undergoing conscious sedation and cardiac arrests.
• Capnography is the most accurate method to ensure there has not been an oesophageal intubation. Intubation of the trachea or the right main bronchus will produce similar initial capnograph waveforms.
• There may be additional uses for CO2 capnography in monitoring patients in other environments, such as sleep study or for obstructive sleep apnoea.
• Capnography can help to determine whether chest compressions are adequate during CPR. EtCO2 should be at least 10-20mmHg.
• Normal EtCO2 in an adult should be 35-45mmHg. A sudden increase in EtCO2 during resuscitation is indicative of a return of spontaneous circulation. This may be detectable before a palpable pulse.
• ETCo2 can add prognostic value during a cardiac arrest. During high quality compressions an EtCO2 of >20mmHg at 20 minutes suggests a higher chance of ROSC.
• It is vital to monitor EtCO2 and PaCO2 in traumatic brain injury patients. Low EtCO2 can be associated with a lower blood pressure and therefore a decreased cerebral perfusion pressure which is associated with mortality.
• There is some evidence of the use of EtCO2 for prognostication in sepsis, however, this is not routine.

Capnogram

• The graphical waveform can be split into four phases.
  • A: Start of expiration.
  • B: Expiration of a mixture of dead space and alveolar gas.
  • C: End of dead space gas and only expiration of alveolar gas.
  • D: The plateau of the curve represents partial pressure of CO2 exchanged at the alveolar. The degree of the slow indicates the degree of V/Q mismatch. For example in bronchospasm you would see a shark fin shape due to the significant V/Q mismatch.
  • E: This represents the EtCO2.
  • F: Start of inspiration.

Causes of high EtCO2

• Increased metabolism – malignant hyperthermia, shivering, fever, sepsis
• Ventilation – hypoventilation with failure to clear CO2.

Causes of low EtCO2

• Ventilation – hyperventilation, poor seal on facemask
Circulation – Cardiac arrest, PE, circulatory collapse Metabolic - hypothermia

No EtCO2

• Oesophageal intubation – although you may initially get some EtCO2 reading this will eventually stop.
• CO2 embolism – potentially during laparoscopic surgery with the insufflation port inserted into vasculature or solid organ, such as the liver. There is increased risk of this in bariatric surgery with high port placement and enlarged livers. CO2 embolism causes obstruction of the right ventricle and loss of EtCO2 trace.
• Equipment failure – it is vital to ensure that equipment checked prior to intubation.

References

• Thompson, John E., and Michael B. Jaffe. "Capnographic waveforms in the mechanically ventilated patient." Respiratory care 50.1 (2005): 100-109.
• Long, Brit, Alex Koyfman, and Michael A. Vivirito. "Capnography in the emergency department: a review of uses, waveforms, and limitations." The Journal of emergency medicine 53.6 (2017): 829-842.