Carbon Dioxide Monitoring

by Dr. David Sainsbury
Snr. Staff Anaesthetist, Dept.of Paediatric Anaesthesia
Women & Childrens Hospital, N.Adelaide, Australia.
email:david.sainsbury@adelaide.edu.au

Why monitor CO₂?

Global well-being.
The excretion of CO₂ is the final common pathway of metabolism. As such it provides a useful global indication that all is well.

ventilation must be sufficient to carry oxygen into the lungs
oxygen is being transported to the mitochondria (cardiovascular function)
aerobic metabolism is consuming this oxygen and producing carbon dioxide
CO₂ is being transported to the lungs (cardiovascular function)
CO₂ in the expired air gives an indication of adequate ventilation

Note: CO₂ monitoring gives no information on regional blood flow. Cerebral or coronary ischaemia is possible in the presence of normal capnography.

Risk management
The Dutch looked at preventable anaesthetic deaths in healthy people having minor surgery. They demonstrated on the way that there is no such thing as minor anaesthesia! They found the commonest problems were with the airway and that the earliest detection would be with CO₂ analysis. Their response was to make CO₂ analysis compulsory in 1980. Their work has been demonstrated again by the AIMS data.

The college of anaesthesia in Australia took an interesting approach in recommending that a CO₂ analyser be available for every intubated AND ventilated patient by 1997.

Specific diagnostic patterns
Carbon dioxide analysis can assist diagnosis of a number of patient problems. These will be described below.

How do CO₂ monitors work?
A light is shone through the expired air and the degree of absorption of a certain frequency of infra-red light is proportional to the concentration of CO₂. The light may be split with half passing through a reference cell. The light may also be 'chopped' so that it is not continuously heating the gas in the reference cell.

The analyser may be placed in one of 2 places: in-line, and out of circuit at the end of a sampling tube.

In-line analyser

Advantages	Disadvantages
No sampling tube to block	bulk
	needs to be heated
	windows fogging up
	can't be used on non-intubated patients

Sampling
The sampling variety has the big disadvantage of a sampling tube which tends to get blocked with condensation. Some companies offer a special tube that allows water to escape. They are an expensive gimmick.

CO₂ Waveforms(Capnograms)
There are two main graphs that we look at which are a function of the sweep speed.
These are waveforms and CO₂ trends.

At high sweep speed we get a wave form of the CO₂ from each breath which is known as the capnogram. There is only one normal shape:

At first there is a rapid rise as the dead space gas comes out of the major airways. Then there is a plateau which is allowed to have a slow rise.

Finally there is a rapid decline as the next breath enters the patient.

Arterial pCO₂
The plateau is essential for accurate analysis.
The normal end tidal value is about 40 mm Hg or 5%. In the absence of significant cardiac shunts there is no significant alveolar to arterial CO₂ gradient so what you are seeing is also the arterial CO₂ concentration.

In a patient with chronic airways disease the slope may be increased and the end tidal value somewhat higher. This is useful to record at the beginning of an anaesthetic to prevent over ventilation. It is surprising how easy it is to hyperventilate an elderly patient. The standard 15 ml/kg 12 breaths /Min will drop their CO₂ to 25 mm Hg. This will make them slow to breath at the end of the case. The other real danger is the cerebral vasoconstriction caused by the low CO₂. Perhaps that has been the reason that granny is sometimes 'not quite herself' after the operation.

Intubation
First of all if you get a capnogram you can be sure you have intubated the trachea. It is said you may get three or four breaths of CO₂ from the stomach. We attach the CO₂ analyser to the ET tube before intubation. This is very comforting when I am supervising someone else intubating a patient. There are case reports of oesophageal intubation where a supervising consultant performed all the known tests to verify tube placement.

CO₂ Trends
There are two main graphs that we look at which are a function of the sweep speed.
These are CO₂ waveforms and trends.
At slower speed we get a trend image of the peeks and troughs of expired CO₂.
The trend of end tidal CO₂ is the most useful graph to watch to follow ventilation. and there are a few patterns that are diagnostic.