End-tidal CO2 monitors can be qualitative or quantitative
Qualitative devices indicate presence or absence of EtCO2
Colorimetric EtCO2 devices change color from purple (CO2 absent) to yellow (CO2 present)
Quantitative devices give numeric values
Mainstream (or inline) CO2 monitors are placed within the closed circuit of the ventilator (or BVM)
Sidestream CO2 monitors use special NC to detect CO2
EtCO2 is ~3-5 mmHg less than PaCO2 - see EBQ:End-Tidal CO2 PaCO2 correlation
Due to dilution of CO2 in alveolar dead space
Processes that increase alveolar dead space:
Pulmonary embolism
Decreased cardiac output
Qualitative devices indicate presence or absence of EtCO2
Colorimetric EtCO2 devices change color from purple (CO2 absent) to yellow (CO2 present)
Quantitative devices give numeric values
Mainstream (or inline) CO2 monitors are placed within the closed circuit of the ventilator (or BVM)
Sidestream CO2 monitors use special NC to detect CO2
Colorimetric EtCO2 devices change color from purple (CO2 absent) to yellow (CO2 present)
Mainstream (or inline) CO2 monitors are placed within the closed circuit of the ventilator (or BVM)
Sidestream CO2 monitors use special NC to detect CO2
Due to dilution of CO2 in alveolar dead space
Processes that increase alveolar dead space:
Pulmonary embolism
Decreased cardiac output
Pulmonary embolism
Decreased cardiac output
Capnography
Compartment pressure measurement
Thromboelastography 
Glasgow-Blatchford Bleeding Score
Tourniquet
Tourniquet 
Age adjusted D-dimer
D-dimer