ETCO2 is a representation of 1) The concentration of CO2 in the blood, and 2) Perfusion of blood to the lungs, with the second being the primary determinant of your ETCO2 numerical value in the prehospital field. Without taking this into consideration, mistakes in diagnosis and treatment in PHEM are likely. Listen to how you can, in conjunction with your clinical and physical assessment, use ETCO2 to your advantage.