Mechanical influences on the capnogram

We found that altering aspirating flow rate (ASR) and sample tubing length (STL) affected the response time and capnogram of 2 different side-stream capnometers. Increased flow rate shortened rise time, transit time, and total delay time (TDT), whereas increased STL increased transit and TDTs but not rise time. Increasing rise time pushed the ascending limb of the capnogram to the right. Peak and baseline PCO2 values were accurate only when TDT did not exceed respiratory cycle time (RCT). Awareness of the potential for artifactual changes in the capnogram will avoid their misinterpretation as physiologic events.