5. Measurement of CO2 Flashcards
Intro
The capnograph is an essential monitor which is used in all but the briefest of
anaesthetics. Capnography is also now viewed as an essential standard in every area
where there are intubated patients or patients with laryngeal mask airways in place.
There is not a vast amount to ask about the principles of the commonest method of
CO2 measurement (infrared absorption), and unless you are unfortunate enough to
encounter an examiner who has a passion for Raman scattering, the oral is likely to
concentrate equally on clinical uses. Make sure that you are able to interpret the range
of capnograph traces that you may commonly encounter. It will help if you can
draw them.
Methods of Measuring End-Tidal CO2
Infrared absorption: this is the main method of measuring CO2 in the operating
theatre.
Its principle is that a molecule will absorb infrared radiation (wavelength 1–40 μm)
as long as it contains at least two different atoms. This applies to CO2, as well as to
N2O and to all other inhalational agents.
The system comprises an infrared source, a filter to ensure that only radiation of the
desired wavelength is transmitted, a crystal window (glass absorbs infrared), a sample
chamber and a photodetector.
The fraction of radiation absorbed is compared with a reference gas (so regular
calibration against zero and known CO2 concentrations is essential) before the value
is displayed.
The infrared wavelength absorbed varies with the gas, thereby allowing its identification.
For CO2, this absorption is maximal at 4.28 μm. There is some overlap
between CO2 and N2O for which modern instruments can compensate; collision
broadening would otherwise falsely elevate the CO2 readings.
Other Methods
Colorimetric: carbonic acid forms from CO2 and water and will change a pHsensitive
colour indicator. This principle is used in portable devices intended to
confirm correct tracheal tube placement in emergency situations in which formal
capnography is not available
Mass spectrometry: this technique is extremely accurate, has a very rapid response
time and allows the simultaneous measurement of different compounds. The instruments, however, are very large and expensive and are not used for routine gas
monitoring in the UK. The gas sample is introduced into an ionization chamber in
which some of its component molecules pass through an electron beam and become
charged. The ionized particles are then accelerated out of the chamber into a strong
magnetic field, which deflects the particles according to their mass.
Raman effect: the interaction of electromagnetic radiation with a molecule may
result in a partial, as opposed to a complete, transfer of energy. Intermolecular
bonds absorb the energy, and some is then re-emitted at different wavelengths.
There is usually a decrease in wavelength which is characteristic of the individual
molecule.
Information that can be obtained from a capnograph trace
Cardiovascular information: CO2 production can occur only if the patient has a
cardiac output. A falling CO2 may indicate a decreasing cardiac output, a sudden fall
may be a sign of pulmonary embolus and a flat trace will be seen if there is complete circulatory arrest. Normal end-tidal CO2 usually reassures the anaesthetist that
ventilation is adequate.
Draw traces
Respiratory information:
there are many possible variations of a capnograph trace,
some of which may be quite subtle,
such as the waveform you may see with intermittent
malfunction of an inspiratory valve,
or the small rise at the end of the rising
plateau phase that may be seen in obese and pregnant patients.
(This is attributed to fast- and slow-emptying alveoli with different time constants.)
You are more likely to be asked about the traces which convey more commonly important information.
No CO2 trace
No CO2 trace: this may indicate oesophageal intubation, tracheal tube displacement
or disconnection of the breathing system.
Low or falling end-tidal CO2
High or rising end-tidal CO2
Low or falling end-tidal CO2:
this may be due to overventilation if IPPV is being used,
or to hyperventilation in a patient breathing spontaneously.
High or rising end-tidal CO2:
This may be caused by inadequate ventilation,
respiratory depression, rebreathing or exhaustion of the soda lime.
It may rarely be a sign of a hypermetabolic state,
of which the most extreme example is malignant hyperpyrexia,
in which there is a massive increase in CO2 production.
Abnormal capnography waveforms:
Abnormal capnography waveforms: a slow upstroke and slowly rising plateau
indicates chronic or acute airway obstruction. The obstruction can be anywhere in
the system – either in the upper or lower airway or in the breathing circuit. A trace
that shows inspiratory dips in the waveform may be indicating partial recovery
from neuromuscular blockade. A raised baseline indicates rebreathing.
