Accuracy of expiratory carbon dioxide measurements using the coaxial and circle breathing circuits in small subjects

Mass spectrometry is widely used to measure the end-tidal concentrations of inhalation anesthetics and other gases during surgery in order to estimate their arterial concentrations. When certain breathing circuits are used in newborns, however, fresh gas or ambient air may contaminate the expired sample, introducing a systematic error in the measurement of any end-tidal gas concentration. We estimated this error in newborn piglets using carbon dioxide as an indicator substance of expired gas. The capnograms and the difference between arterial carbon dioxide tension (PaCO2) and peak-expired carbon dioxide tension (PeCO2) were compared when either a coaxial (Bain) or circle breathing circuit was used. Gas was sampled from the proximal airway and distal trachea. No combination of circuit and sampling site produced a flat alveolar phase until the circle circuit was modified with diversion valves to reduce gas mixing. The mean PaCO2-PeCO2 gradients using the coaxial/proximal sampling, coaxial/distal sampling, and modified circle/proximal sampling circuits were 12.4, 9.2, and 8.8 mm Hg, respectively. The mean PeCO2 in each of these combinations was significantly different from the corresponding mean PaCO2 (p less than 0.05). Using the modified circle circuit with distal sampling, mean PeCO2 was not significantly different from mean PaCO2: the mean PaCO2-PeCO2 gradient was 2.2 +/- 0.2 mm Hg (SEM), range, 0 to 6 mm Hg, with 95% confidence limits less than or equal to 8 mm Hg. When a coaxial breathing circuit is used in small subjects, PaCO2 may be significantly underestimated regardless of sampling site, although the circle breathing circuit with distal tracheal sampling yields accurate results.