Also known as end tidal CO2. Or, the maximum amount of carbon dioxide measured at the end of expiration. It's measured using infra-red capnography, and is used extensively in ICU- as capnography.com put it, 'Capnography is synonymous with patient safety during anesthesia and sedation' and it forms part of the key recommendations put forwards by the National Audit Project 4 (NAP4) on airway management way back in 2011. It can help to confirm the correct positioning of an ETT and is vital for monitoring during mechanical ventilation. It can be used as a guide to PCO2, and different capnography waveforms can be used as a diagnostic aid to identify different respiratory pathologies.
A normal ETCO2 reading is 4.5-6.0kPa, or 35-45mmHg. This is the same as the normal readings for PCO2 on a blood gas when the patient is generally cardiorespiratory stable and normothermic, and therefore an ETCO2 can be used to approximate PCO2 readings. A large difference in figures could indicate an increase in physiological dead space (ventilation of poorly perfused alveoli, meaning areas of functional lung tissue are not participating in gas exchange) or intra-pulmonary shunting (lack of ventilation to perfused alveoli, meaning gas exchange is not happening as blood passes alveoli)- these are both forms of V/Q mismatch.
An absent waveform can be indicative of an absent (extubated) airway or oesophageal intubation (there's no CO2 in the oseophagus), apnoea or respiratory arrest (there will be no end tidal CO2 if there is no tidal ) or a ventilator disconnection. A low ETCO2 reading can indicate a low metabolic production of CO2 (e.g. hypothermia) or increased removal of CO2 if someone is hyperventilating (there may be a progressive drop in CO2 as they blow it all off).
Conversely, a progressive rise in CO2 could indicate rebreathing CO2 (where an increase in baseline will be seen alongside overall increases), hypoventilation, increased CO2 production due to an increased metabolic rate (e.g. fever), or an airway obstruction leading to impaired removal of CO2 (this will usually also present with high airway pressures). A sudden rise in CO2 could be indicative of ROSC following CPR for cardiorespiratory arrest, as the patient starts breathing again. During CPR, there should be some type of waveform present (see pic by Medtronic below) which indicates good quality CPR.
Capnography
A capnography waveform has 4 phases- the inspiratory baseline, expiratory upstroke, alveolar plateau and the inspiratory downstroke.
1. INSPIRATORY BASELINE
Flat, as inspired gas at the beginning of expiration usually doesn't have any CO2. If baseline is high, could indicate rebreathing,
2. EXPIRATORY UPSTROKE
Rapid increase as alveolar gas exchange begins. O2 exchanged for CO2 and expiration of CO2-rich gas begins.
3. ALVEOLAR PLATEAU
Usually gradually increases as alveolar gas exchange continues during expiration. Airway obstruction will cause a greater increase (could appear as shark fin waveform, e.g. in asthma). ETCO2 reading taken at peak of plateau (as this is the end of expiration).
4. INSPIRATORY DOWNSTROKE
Rapid fall in CO2 concentration as inspiration starts and O2 rich air arrives.
This image shows a normal capnogram, and then some alternative abnormal presentations including shark fins seen in airway obstruction, hypo and hyperventilation, and CPR vs ROSC.
Devices such as EasyCap can be used. These are subjective measures, therefore less accurate, but can be used in a pre hospital setting or to assess ETCO2 in emergency intubations if full monitoring is not available for whatever reason. These use colour changing pH paper to assess CO2, which is acidic and so will change the colour of paper from purple to yellow if present.
These aren't intended for long term use so once used to confirm ETT placement, they will need to be changed out for an infra-red capnography monitor connected into the ventilator circuit for ongoing monitoring.
All gifs kindly borrowed from capnography.com, thank you!
Thank you for reading through this post, see references below and don't hesitate to message me through the blog, or on instagram/twitter (@christienursing) for any questions, comments or suggestions.
Love, Christie x
References-
Whitten, C. (2017) Ventilation perfusion mismatch. https://airwayjedi.com/2017/01/06/ventilation-perfusion-mismatch/
LITFL (2020) Capnography and CO2 detectors. https://litfl.com/capnography-and-co2-detectors/
LITFL (2020) Capnography waveform interpretation. https://litfl.com/capnography-waveform-interpretation/
National Audit Projects (2011) NAP4: Major complications of airway management in the United Kingdom. https://www.nationalauditprojects.org.uk/NAP4_home
Galvagno, S., Kodali, B. (no date) Capnography in Intensive Care Unit. https://www.capnography.com/icu/capnography-in-icu
Capnography.com (no date) https://www.capnography.com/
an awesome resource for understanding the science behind capnography and for examples with explanations!
Baid, H., Creed, F., Hargreaves, J. (2016) Oxford Handbook of Critical Care Nursing (2nd edn). OUP: Oxford