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 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.

Keywords: anesthesia circuits; capnography; capnometry; carbon dioxide; monitoring; rabbit.