A Beginner’s Guide to Capnography

What are capnography and capnometry?

Capnography is the measurement of the concentration of carbon dioxide (CO₂) in respiratory gas. It demonstrates this graphically via the capnogram waveform and numerically via capnometry. This method offers the advantage of being non-invasive and providing information on arterial blood carbon dioxide concentration (PaC0₂) as opposed to other methods of measuring carbon dioxide in the blood.

There are also other qualitative methods of detecting expired CO₂ which include pH-sensitive chemical indicators that change colour with different levels of CO₂.

Why is capnography useful in sedation and mechanical ventilation?

Capnography is useful in sedation, intubation and in mechanical ventilation. Capnography provides a non-invasive predictor of arterial carbon dioxide (PaCO₂), can give us early information about the child’s ventilation, act as an apnoea monitor, help to identify airway obstruction, and can indicate pulmonary blood flow.

For intubation, capnography can help us to identify whether the tube is in the right place and whether we are adequately ventilating the child. For sedation, capnography can help us to monitor the level of sedation and the child’s ventilation throughout this (even with the best intentions conscious sedation can end up as deep sedation with hypoventilation, laryngospasm, or general anaesthesia).

How do we interpret the normal capnogram?

The normal shape of the capnogram waveform is almost rectangular in appearance. It is composed of four main phases that illustrate the movement of carbon dioxide in our airways and alveoli.

Phase 1 (the blue line): This is the inspiratory baseline and reflects the inspired gas (which has only a minuscule amount of CO₂). The lack of CO₂ detected in this phase results in a flat waveform.

Phase 2 (the pink line): At the beginning of expiration, exhaled CO₂ rapidly rises and so does the slope of the capnogram. CO₂ travels from the alveoli through the bronchi and trachea (the conducting airways) where gas is present but not able to be exchanged (anatomical dead space). The speed at which the CO₂ is exhaled determines the slope of this part of the curve.

Phase 3 (the red line): This phase is the alveolar plateau. The gently sloping plateau represents late expiration where alveolar gas which is rich in CO₂ is detected. The angle between phase 2 and phase 3 is called the Alpha Angle, the change from airway gas to alveolar gas. The value at the end of the slope is called the End-Tidal CO₂ (ETCO₂), the maximal expired CO₂ concentration. The ETCO₂ is the numeric value on the monitor and is normally 4.5-6 kPa (35 – 45 mmHg).

Phase 4 also known as phase 0 (the yellow line): During this phase, CO2 values drop sharply as inspiration begins.

How can capnography help us to detect abnormalities?

The height, shape, frequency, rhythm, and speed of change of the waveform and numeric ETCO₂ can help us to detect a number of abnormalities.

No trace = wrong place

This is something you don’t want to see. If there is no trace, you are not getting any CO₂ back. Immediately after intubation, this would suggest that the tracheal tube is in the wrong place (oesophageal intubation). A few waves of decreasing height during oesophageal intubation can temporarily cause a false positive. It may be caused by alveolar gas being forced into the stomach during mask ventilation or fizzy drinks being joyfully drunk before intubation. The ventilator could also be disconnected or the capnograph disconnected if there is no trace. In cardiorespiratory arrest, there should still be an attenuated trace – watch the UK Royal College of Anaesthetists’ No Trace = Wrong Place video.

The slow slope of bronchospasm

Partial obstruction of the airways in bronchospasm slows down the passage of CO₂. A ‘shark’s fin’ shape is seen in phases 2 and 3 of the capnogram. The more serious the obstruction, the slower the slope. In some cases, the alpha angle cannot be seen, indicating that the dead space has not yet been emptied. Bronchospasm will improve as treatment is administered and the alpha angle will return to normal.

Too little

Many factors can lead to a decrease in waveform amplitude and a low ETCO₂. These include a decrease in CO₂ production, reduced or lack of pulmonary perfusion (lack of blood reaching the alveoli means a lack of gas exchange), changes in alveolar ventilation (such as apnoea or hyperventilation), cardiac arrest, hypothermia (which cases decreased CO₂ production) or faulty apparatus (such as disconnections, leaks, or ventilator malfunction).

Hypotension, hypovolemia, reduced cardiac output, or pulmonary embolism can cause a decrease in pulmonary perfusion.

Too much!

A high ETCO₂ and an increase in the amplitude of the waveform can be observed for the opposite reasons. Fever can increase CO₂ production. Increased blood pressure and cardiac output may increase blood flow to the lungs. Hypoventilation may allow CO₂ to build up within the alveoli.

Too leaky

A waveform with a peaked, triangular appearance suggests that there is a significant leak around the tracheal tube. An air leak can also have other appearances on the capnogram including sudden drops in the waveform and a stepped appearance to phase 3.

Capnography can also detect a variety of other abnormalities, but some are more relevant to anaesthesia.

The hats and caps of capnography

A quick [and maybe even fun] way of recognising common capnography abnormalities is by thinking about the hats and “caps” of capnography. Imagine you are going to watch the races at Royal Ascot, you need to look your best and wear your very fanciest hat. The fanciest hat you could wear would be a top hat, which resembles the normal capnogram seen with an unobstructed airway.  Although a side hat is ok, it might fall off – this looks like the trace seen with a partially obstructed airway. A party hat at Royal Ascot is bad, and very much out of place, the appearance of this hat looks similar to the trace seen with a significant air leak. And finally, the worst thing you could do is turn up without a hat at all! You’re in the wrong place if you turn up without a hat.

Take-Home Points:

1. Always use capnography for sedation and intubation
2. No trace = wrong place
3. Hypoventilation occurs before hypoxia
4. Be able to interpret the normal waveform
5. Pattern recognition can help identify abnormal waveforms

References:

Cook TM, Kelly FE, Goswami A. ‘Hats and caps’ capnography training on intensive care. Anaesthesia. 2013; 68 (4): 421

Kodali BS. Capnography. 2022. Accessed online at https://www.capnography.com

Nickson C. Capnography Waveform Interpretation. Life In The Fast Lane. 2020. Accessed online at https://litfl.com/capnography-waveform-interpretation/

West JB. Respiratory physiology: the essentials. 9th ed. Baltimore, MD, USA: Lippincott Williams & Wilkins, 2012.

Yarstev A. The normal capnography waveform. 2018. Accessed online at https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%205592/normal-capnograph-waveform

Yarstev A.  Abnormal capnography waveforms and their interpretation. 2019. Accessed online at https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%205593/abnormal-capnography-waveforms-and-their-interpretation