Capnography

Question 1

What is capnography?

Answer 1

Also known as infrared spectography, rapid and accurate to measure CO2 concentration throughout the cycle.

Question 2

What type of molecules absorb in the infrared region of the spectrum?

Answer 2

Gases with molecules that have at least two dissimilar atoms eg. Carbon and oxygen atoms

Question 3

At what wavelength does CO2 absorb radiation?

Answer 3

4.3 micrometers

Question 4

How does Beer-Lambert law apply to capnography?

Answer 4

The amount of radiation absorbed is proportional to the number of CO2 molecules (partial pressure CO2) present in gas mixture.

Question 5

What are the components of capnography?

Answer 5

1. Light source - emits infrared radiation
2. Interference filter -wavelengths not transmitted are removed by interference phenomena. Made to transmit narrow band of wavelengths.
3. Sample chamber and reference chamber - infra red passes simultaneously through both chambers, and the absorption of both chambers is compared to calculate CO2 values.
4. Photodetector - the greater the radiation absorbed, the less monitored by the detector and vice versa.

Question 6

How does infrared spectrometry work?

Answer 6

• infrared light is passed across the gas sample to fall on a thermopile detector/ sensor.
• presence of CO2 in the gas leads to a reduction in the amount of light falling on the detector.
• the thermopile detector produces heat which is measured by a temperature sensor and is proportional at the partial pressure of CO2.
• This leads to changes in the electrical output and voltage in a circuit.

Question 7

Other than capnography, what methods can be used to measure CO2?

Answer 7

1. Raman spectrometry (argon laser, can measure all gases in breathing system- small and portable)
2. Mass spectrometry (ionised and exposed to magnetic field in vacuum chamber, highly accurate, bulky and expensive equipment)
3. Photo acoustic spectrometry (irradiated with suitable wavelength pulsatile infrared radiation- accurate with rapid fluctuations)
4. Chemical colorimetric analysis (CO2 hydrated to make carbonic acid, measured by pH sensitive measures)

Question 8

What types of capnographs are there?

Answer 8

Side stream and Main stream.

Question 9

What are the advantages of each type of capnograph?

Answer 9

Side stream:
- easy to connect, no need for sterilisation, can be used in awake patients, easy to use in unusual positions, multiple gas analysis, calibration gas needed

Main stream:

• no sampling tube
• less delay to reading
• suitable for neonates/ children
• no effect due to changes in water vapour pressure

Question 10

Why is ETCO2 less than alveolar CO2?

Answer 10

Because ETCO2 is always diluted with alveolar dead space gas from unperfused alveoli (not perfused so no CO2)

Question 11

Why is alveolar CO2 less than arterial CO2?

Answer 11

Blood from unventilated alveoli and lung parenchyma (both have higher CO2 contents) mixes with the blood from ventilated alveoli.

End tidal kPa is 0.3-0.6 less than arterial CO2 in a healthy adult.

The difference is reduced if the lungs are Ventura later with large tidal volumes.

Question 12

What delay is acceptable in side stream capnograph?

Answer 12

A delay of less than 3.8 seconds.

Question 13

In a side stream capnograph, what dimensions are the sampling tube? Sampling rate?

Answer 13

Internal tube diameter 1.2mm, shirt as possible and a sampling rate of about 150-200ml/ min.

Question 14

What can over estimate end tidal partial pressure of CO2?

Answer 14

Presence of nitrous oxide if measured with infra-red absorption and mass spectrometry.

Nitrous oxide absorbs infra red radiation and can increase the reading on a capnograph. It also has a the same molecular weight of as CO2 hence mass spectrometry can be over estimated.