Capnometry in emergency medicine

Capnometers measure carbon dioxide (CO₂) in expired air and provide clinicians with a noninvasive measure of systemic metabolism, circulation, and ventilation. If two of these systems are held relatively constant, changes in CO₂ excretion will reflect the third. CO₂ measurement has been advocated as a method of ensuring endotracheal intubation. Because the air in the esophagus has very low levels of CO₂, the use of capnometers may help prevent unrecognized esophageal intubation in prehospital and emergency department settings. In patients with normal perfusion and ventilation, end-tidal CO₂ measurements closely reflect alveolar PCO₂. Capnometry may decrease the need for frequent arterial blood gas measurements. A sudden change in end-tidal CO₂ measurement may indicate decreased lung perfusion and an early shock state. Thus, capnometry may be useful to monitor critical patients in the ED. In addition, capnometry has potential usefulness as a noninvasive indicator of the efficacy of on going CPR efforts. End-tidal CO₂ has been shown to correlate with cardiac output, perfusion pressures, and successful resuscitation in experimental models of cardiac arrest. Further clinical studies are needed to define the role of capnometry in emergency medicine.