Physics of Capnography
Chemical method of CO2 measurement
A pH-sensitive chemical indicator changes color on exposure to CO2
FEF end-tidal detector:
False negative results may occur even with correct endotracheal tube placement in patients in cardiac arrest, in whom sufficient CO2 may not be present in the lungs. It is also more relevant to point out the possibility of color change in the device due to agents other than exhaled carbon dioxide (false positive results). Gastric contents, mucus, and drugs such as epinephrine can cause false positive results. It is imperative that clinicians using these devices be aware of this limitation. One way to avoid this pitfall is to observe the change in color in the device with each breath. A false positive result causes a permanent color change in the device; hence, the color does not vary with ventilation.2,3
1.O'Flaherty D, Adams AP. The end-tidal carbon dioxide detector. Assessment of new method to distinguish oesophageal from tracheal intubation. Anaesthesia 1990;45:653-5.
2.Srinivasa V, Kodali BS. Video on orotrachal intubation. NEJM 2007;357:6:620
3.Srinivasa V, Kodali BS. Caution when using colorimetry to confirm endotracheal intubation. Anesth Analg 2007;104:738