Comparison of a sidestream and mainstream capnometer in infants.

This study was designed to compare capnograms obtained from a sidestream and mainstream capnometer during rapid mechanical ventilation in infants. Ten infants were studied during elective anesthesia and surgery. Simultaneous recordings of exhaled CO2 were made with a sidestream and a mainstream machine; arterial blood gases were measured for comparison. The mean arterial to end-tidal CO2 difference (PaCO2-PetCO2) was approximately 5 torr with either technique; the correlation of PetCO2 with PaCO2 was slightly better with the mainstream analyzer (r2 = .895 vs. .654). The recordings obtained with the sidestream machine were grossly distorted, with flattening of the ascending limb (slope 37.3 vs. 153.3 torr/sec, sidestream vs. mainstream, p less than .001) and absence of the alveolar plateau. The mainstream analyzer gives a more accurate representation of the expired CO2 waveform in small children at rapid respiratory rates.