Designed, Produced and maintained by

Bhavani Shankar Kodali MD

header1

Physics of Capnography

Bhavani Shankar Kodali MD

 

Factors affecting IR Spectrography

 

Nitrous oxide absorbs IR light.



Absorption is minimal at IR 4.3 µ m

 

Collision Broadening Phenomenon

 

 

Effect of n2o on C02 measurements

Use of 4.3 µ m IR light does not affect C02 measurements.
Collision broadening phenomena does increase C02 values.

 

Correction factors for the presence of nitrous oxide.

Percent Nitrous oxide

Corrected reading =

70 %

Observed PC02 x 0.90

50%

Observed PC02 x 0.94



Because nitrous oxide absorbs IR (IR absorption spectra of N20 = 4.5 µm whereas C02 = 4.3 µm), the presence of N20 therefore can give falsely high C02 readings. This problem can be eliminated by using a narrow band IR filter that only transmits the the wavelength most strongly absorbed by C02 (about 4.3 µm). Another problem relates to n2o concerns the interaction between N20 molecules and C02 molecules. This produces a "collision broadening effect" that affects the sensitivity of the IR analyzer and causes an apparent increase in C02 reading. "Collision broadening" is a phenomenon in which the spectral absorption peaks of a gas (C02) are broadened owing to the collision or proximity of molecules of another gas (N20).1 The correction factors for the presence of various concentrations of N20 have been studied and range from 0.90 at 70% n2o (corrected PC02 = observed PC02 x 0.90) to 0.94 at 50% n2o.2 Most monitors provide some system of electronic compensation to reduce this effect. Alternatively, the simplest method of eliminating this error is to calibrate the instrument with a gas mixture which contains the same background gas concentration as that to be analyzed.3,4


References:

1. Raemer DB, Calalang I. Accuracy of end-tidal carbon dioxide tension analyzer. J Clin Monit 1991;7:19-208.

2. Kennell EM, Andrews RW, Wollman H. Correction factors for nitrous oxide in the infrared analysis of carbon dioxide. Anesthesiology 1973;39:441-3.

3. Carbon dioxide monitors. Health Devices 1986;15:255-85.

4. Paloheimo M, Valli M, Ahjopalo H. A guide to C02 monitoring. Finland:Datex Instrumentaitrium, 1988.

previous        main        next