The monitoring of expired carbon dioxide from our patients is a very important part of ensuring that we are adjusting their ventilation to suit their needs.

It ensures that we can be aware very quickly of any problems they may be encountering, allowing us to intervene early enough.

Understanding the capnograph is very important and recognition of some of the differing wave forms is a very useful tool.

The Normal Capnograph

There are five phases to the capnograph which you can see in the diagram.

The start and ends of these are noted by the letters A-E

You can see above the graph where inspiration and expiration take place and the 'normal' shape of the graph which we would expect to see.

Lets break it down to understand it.

Phase I A-B

This is the start of expiration. This is where it is mainly the dead space gas within the circuit (physiological and mechanical) is moved and there is very little carbon dioxide here, so the line remains relatively flat.

Phase II B-C

As the patient continues to breath out the levels of carbon dioxide detected rises rapidly until it reaches a peak.

Phase III C-D

This is as the end tidal carbon dioxide level is measured. As the patients breath reaches its end, we come to the end tidal point and this is the value that will be displayed alongside the capnograph.

This value can be displayed in Kilopascals (Kpa) or millimetres of mercury (mmHg). If you want to go from mmHg to Kpa then divide the value by 7.5

Phase IV D-E

As the patient starts inspiration there will be a rapid drop back to the baseline as the carbon dioxide is replaced by the oxygen that the patient breaths in. 

Capnograph showing leak

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The carbon dioxide levels slowly begin to rise as seen in the diagram. 

The end tidal starts to climb which could be due to low tidal volumes of a low respiratory rate.

This could just mean a change to the ventilator settings, but if the patient is awake and not mechanically ventilated it might indicate that their level of consciousness is dropping.

capnograph showing hyperventilation


The levels of carbon dioxide now start to fall.

The level on the capnograph gets lower and is also reflected in the end tidal carbon dioxide number.

This could be a result of the patient just breathing too quickly, as evidenced in a panic attack.

It could also mean that the patient has a metabolic compromise which means they are having to breathe more quickly to compensate. A patient with a diabetic ketoacidosis would be a good example of this.

Gas trapping/Rebreathing

The patient may not be breathing out all of their carbon dioxide when they exhale.


In this case the capnograph will not return to baseline and indeed this may gradually become worse with the lowest value becoming higher and higher.

This can happen with a patient having a bronchospasm or if they are a patient with severe COPD. It could also be that the mandatory respiratory rate is set too high not giving them sufficient time to breathe out before the next breath.

Capnograph showing apnoea


If the carbon dioxide suddenly falls, or seems to stop recording completely then this could be for a number of reasons which you need to check.

Has the respiratory circuit become disconnected somewhere? Or has the sidestream become disconnected from the circuit or the monitor?

Is the circuit occluded or kinked or has the patient actually stopped breathing for some reason?

It could also be that there has been an acute pulmonary embolus which means that the cardiovascular system is no longer returning carbon dioxide to the lungs because of the obstruction.

Capnography showing obstruction


When the carbon dioxide is exhaled normally the level in the capnograph goes up almost as a straight line to the end tidal value.

However, if there is some kind of obstruction to that exhaled gas then the rise will be slower over time leading to a more sloped appearance.

Patients with chronic obstructive airways disease will have this kind of waveform or those that are having a bronchospasm.

Capnography during cardiac arrest

Cardiac Arrest

The effectiveness of cardiac compressions can be assessed using the capnograph.

As blood is returned to the heart and then pushed out and into the lungs so it will carry the carbon dioxide to the lungs.

If the cardiac compressions are poor or not effective then the amount of carbon dioxide carried to the lungs will be low and so reflected in the capnograph.

Capnograph showing leak

Normally the end tidal appears as a plateau on the capnograph.

If there is a leak in the system however it will look like the diagram here.

You can see that the plateau is not there and the return to the baseline happens rather quickly.

Because of the leak in the circuit the carbon dioxide is escaping rather than being measured.