ETCO2..The colorimetry

EtCO₂ can be measured by colorimetry and capnography.
Colorimetric devices provide continuous, semi-quantitative EtCO2 monitoring. A typical device has the following three color ranges:

Purple—EtCO₂ is less than 0.5%
Tan—EtCO₂ is 0.5–2%
Yellow—EtCO₂ is greater than 2%

Normal EtCO₂ is greater than 4%; hence, the device should turn yellow when the endotracheal tube is inserted in patients with intact circulation.(1)
False positives may occur when the device is contaminated with acidic substances, such as gastric acid, lidocaine or epinephrine. The device will not provide an accurate reading if it is expired or if the tube is clogged with secretions.

1-Galvagno SM, Kodali BS. Critical monitoring issues outside the operating room. Anesthesiology Clin. 2009;27(1):141–156

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