208 ACCURACY OF EXPIRATORY CO2 MEASUREMENTS IN SMALL SUBJECTS USING THE COAXIAL AND CIRCLE BREATHING CIRCUITS

Abstract

Mass spectrometry is widely used to measure end-tidal concentrations of inhalation anesthetics and other gases during and after surgery in order to estimate their arterial concentrations. When certain breathing circuits are used in newborns, however, fresh gas flow may contaminate the expired sample, introducing a systematic error in the measurement of any end-tidal gas concentration. This error was estimated using CO2 as an indicator substance of expired gas. The capnograms and the difference between PaCO2 and peak-expired CO2 (PeCO2) were compared when either a coaxial or circle circuit was used to ventilate paralyzed newborn piglets. Gas was sampled from proximal and distal tracheal sites. No combination of standard circuit and sampling site produced a flat alveolar phase until the circle circuit was modified slightly to reduce gas mixing. Mean PaCO2-PeCO2 using the coaxial/proximal sampling, coaxial/distal sampling, and modified circle/proximal sampling circuits were 12.4, 9.2, 8.8 mm Hg, respectively. Mean PeCO2 of each combination was significantly different from PaCO2 (p<.05). Using the modified circle circuit with distal sampling, mean PaCO2 in 17 piglets was 2.2 ± 0.2 mm Hg (SEM), range 0–6 mmHg, with 95%, confidence limits for each point ≤ 8 mmHg. The modified circle system with distal tracheal sampling produces accurate results, although the standard coaxial and circle circuits do not.