Physics of Capnography

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