Capnometer transport delay: measurement and clinical implications.

The sidestream capnogram is delayed behind real time by transport delay (TD; time to aspirate gas through the sampling tubing) and by the dynamic response (DR) of the measurement cuvette. In six capnometers, we measured TD and DR by plunging the end of the sample tubing into a flask containing CO2 and then digitally analyzing the capnogram. TD ranged from 0.6 to 5.0 s and accounted for 89% or more of the total response time (TD + DR) of the capnometer. TD was generally not reported in the manufacturers' specifications. TD was further prolonged by low aspiration rates or by sampling tube extensions. During a series of quick breaths after endotracheal intubation, long TD can delay the appearance of CO2 and result in a false diagnosis of esophageal intubation. Also, long TD can prolong DR, which can result in underestimation of end-tidal PCO2 during rapid ventilation in pediatric anesthesia.