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Bymixer system can measure O₂ uptake and CO₂ elimination in the anesthesia circle circuit

[Un système mélangeur (bymixer) peut mesurer le captage d’O₂ et l’élimination du CO₂ dans le circuit anesthésique en cercle]

From the Department of Anesthesiology, University of California-Irvine, Orange, California, USA.

Address correspondence to: Dr. Peter H. Breen, Department of Anesthesiology, UCI Medical Center, Building 53, Room 227, 101 The City Drive South, Orange, CA 92868, USA. Phone: 714-456-6652; Fax: 714-456-7702; E-mail: pbreen{at}uci.edu

Background: The ability to measure carbon dioxide elimination (CO₂), oxygen uptake (O₂), and R (respiratory exchange ratio, CO₂/O₂) during anesthesia may help the non-invasive detection of critical events (e.g., abrupt decrease in cardiac output) and metabolic upset (e.g., onset of anaerobic metabolism).

Methods: We have developed a new clinical bymixer (inline mixing chamber) that can measure mixed inspired and expired gas fractions in the anesthesia circle circuit. The addition of a standard anesthesia gas analyzer and flowmeter, and a new airway temperature and humidity sensor, allow determinations of CO₂ and O₂ at the airway opening of the circle circuit. Over a range of tidal volume and frequency, CO₂ and O₂ were compared to reference values generated by the combustion of metered liquid ethanol in a new metabolic lung simulator.

Results: By linear regression, bymixer-flow measurements of CO₂ (slope = 1.02, Y-intercept = –5.31, coefficient of determination, R² = 0.998) and O₂ (slope = 1.05, Y-intercept = –4.34, R² = 0.993) correlated closely to the reference values generated by the metabolic lung simulator. Limits of agreement analysis generated percent errors (mean ± 1.96 SD) of –1.2 ± 7.2% for CO₂ and 2.5 ± 9.8% for O₂.

Conclusions: The new clinical bymixer is compact, lightweight, disposable, inexpensive, and has a fast and adjustable response time (time constant about 14 sec). Anesthesia circle circuit integrity is maintained. Bymixer-flow measurements of CO₂ and O₂ are accurate and may add to clinical monitoring under anesthesia and surgery.