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

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 weighing 3.4 +/- 0.3 kg [mean +/- standard deviation (SD)]. 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 used 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 PaCO2 analysis corrected to rectal temperature. Inspired and end-tidal carbon dioxide (PE'CO2) measurements were recorded during blood sample withdrawal. From each rabbit, 10 paired PaCO2/PE'CO2 measurements were obtained. Each rabbit was recovered from anesthesia and was anesthetized again with an alternate capnometer after 1 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) was -16.7 (-35.2 to 1.8) mmHg for Capnostat 5, -27.9 (-48.6 to -7.2) mmHg for Viamed, and -18.1 (-34.3 to -1.9) mmHg for Capnostream. Viamed PaCO2-PE'CO2 gradient was greater than other two capnometers. Conclusions All three capnometers underestimated PaCO2. Capnostat 5 and Capnostream performed similarly. Clinical relevance These capnometers underestimated PaCO2 in spontaneously breathing rabbits anesthetized using a Bain coaxial breathing system with high fresh gas flows.