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Ventilatory Frequency Influences Accuracy of End-Tidal CO₂ Measurements Analysis of Seven Capnometers

Received from the Department of Anesthesia, University of Iowa College of Medicine, Iowa City, Iowa.

An accurate high-frequency response is mandatory when end-tidal CO₂ (PETCO₂) is monitored during pediatric general anesthesia. The purpose of this study was to assess the accuracy of six infrared-based capnometers and one multiplexed mass spectrometer available at our institution at increasing frequency. Capnometers studied were the Data-scope Accucap, Hewlett-Packard 47210A capnometer, Narkomed 3 Capnomed, Novametrix Capnogard model 1250, Perkin-Elmer Advantage, Puritan-Bennett Datex CO₂ monitor, and Traverse Medical Monitor model 2200 capnometer. Changes in CO₂ concentration were generated by a solenoid valve switching between 100% O₂, and 7% CO₂, in O₂, Frequencies, 8–101 cycles/min were chosen to stimulate a range that might be generated by children during general endotracheal anesthesia. At every rate the displayed PET_CO2 was recorded. Differences in displayed PET_CO2, from known CO₂ ranged from –16.4 to + 6.6. At or below frequencies of 31 cycles/min, four capnometers over-reported and three underreported PET_CO2. At frequencies above 31 cycles/min, six capnometers underreported and one over-reported PET_CO2. Errors may be clinically significant if they influence ventilator settings for patients.

Key Words: EQUIPMENT—capnometers • Monitoring—capnometry • MEASUREMENT TECHNIQUES—capnometry • CARBON DIOXIDE—measurement

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