Comparison of mainstream (Capnostat 5) and two low-flow sidestream capnometers (VM-2500-S and Capnostream) in spontaneously breathing rabbits anesthetized with a Bain coaxial breathing system

Abstract Objective To evaluate agreement with PaCO2 of two low sampling rate sidestream capnometers and a mainstream capnometer in rabbits and the effect of using high fresh gas flow from a Bain coaxial breathing system. Study design Prospective, crossover study. Animals A total of 10 New Zealand White rabbits, mean ± standard deviation (SD) body weight 3.4 ± 0.3 kg. Methods Two sidestream analyzers (Viamed VM-2500-S and Capnostream 35) with a sampling rate of 50 mL minute−1 and a mainstream capnometer (Capnostat 5) were tested. All capnometers use infrared spectroscopy and advanced microprocessor technology. Rabbits were anesthetized and intubated with noncuffed endotracheal tubes of 3 mm internal diameter and adequate seal. A sidestream sampling adapter or the mainstream capnometer was attached to the endotracheal tube and connected to a Bain coaxial breathing system. Oxygen, 1.5 L minute−1, delivered sevoflurane to maintain anesthesia. An auricular artery catheter allowed blood sampling for analysis of PaCO2 corrected to rectal temperature. Inspired and end-tidal CO2 (P e′ CO2) measurements were recorded during blood sample withdrawal. From each rabbit, 10 paired PaCO2/P e′ CO2 measurements were obtained. The rabbit was recovered from anesthesia and was anesthetized again with an alternate capnometer after one week. Data were analyzed using Bland–Altman and two-way anova for repeated measures. Results Analysis included 100 paired samples. Negative bias reflects underestimation of PaCO2. Bland–Altman mean (±1.95 SD) for Capnostat 5 was –16.7 (–35.2 to 1.8) mmHg; Viamed was –27.9 (–48.6 to –7.2) mmHg; Capnostream was –18.1 (–34.3 to –1.9) mmHg. Viamed PaCO2 – P e′ CO2 gradient was greater compared with both other capnometers. Conclusions All three capnometers underestimated PaCO2. The Capnostat 5 and Capnostream performed similarly. Clinical relevance These capnometers underestimate PaCO2 in spontaneously breathing rabbits anesthetized using a Bain coaxial breathing system with high fresh gas flows.