End-tidal carbon dioxide monitoring in emergency medicine, Part 1: Basic principles

End-tidal carbon dioxide (PetCO2) monitoring is becoming more common in both the ED and the out-of-hospital setting. Its main use has been as an aid when confirming endotracheal intubation. However, since CO2 is intrinsically coupled with states of metabolism, circulation, and ventilation, PetCO2 monitoring along with analysis of its capnographic component is becoming increasingly valuable for other uses in the ED. This article reviews the physiology of CO2, the means by which end-tidal CO2 may be monitored, and the components and analysis of the capnogram.