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A Comprehensive Educational Website (established 1998)

Designed, Produced, and maintained (Edition 10, March 2019) by 

Bhavani Shankar Kodali MD

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Bhavani Shankar Kodali MDBostonResponse to Derek Flaherty BVMS, DVA, DipECVAA, MRCA, MRCVS, FHEA, Professor of Veterinary Anaesthesia and Analgesia.

Thank you for your question. Actually, both the groups may be correct.
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Classically, the Beer–Lambert law was first devised independently where Lambert's law stated that absorbance is directly proportional to the thickness of the sample, and Beer's law stated that absorbance is proportional to the concentration of the sample. The modern derivation of the Beer–Lambert law combines the two laws and correlate the absorbance to both, the concentration as well as the thickness (path length) of the sample.
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Beer Law does apply to CO2 absorption as it is concentration based. How far the components of Lambert's Law applies here is the debate. Although the thickness of the sample physically fixed (chamber), the thickness can vary depending on the concentration of a given gas.
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Professor James Philip MD, our Bio-medical Engineering Director and the architect of GASMAN gives his view as follows:
Thank you for inviting me to comment.

If the Beer-Lambert law is not the mathematics of IR light attenuation, how do the conflicting anesthesiologists explain the math?

If it is whether the path length variation counts, it counts in the sense that path length must be constant after a capnometer is calibrated with a known gas sample.

Indeed, as my esteemed colleague Dr. Kodali and capnography.com explains, there are techniques other than NDIR (non-dispersive IR spectroscopy) that can be used for CO2 monitoring, Mass Spectrometer, Raman Spectrometer. They so not use the Beer-Lambert law. However, none of the current clinical products use this.