New Clinical Bymixer for Fast-Response, Adjustable Measurement of Mixed Expired Gas Fractions in the Anesthesia Circle Circuit
Background: The volume of O2 consumption (VO2) and the volume of CO2 production (VCO2) can give vital information about a patient under anesthesia. For example, monitoring VO2 can detect non-steady state critical events and changes in tissue metabolism in anesthetized patients. However, VO2 and VCO2 are typically NOT measured during anesthesia because collection of mixed expired gas is nearly impossible in the circle circuit. Instead, an inline bypass mixing chamber, or bymixer measures mixed VO2 and VCO2 fractions. However, the response time is long using this system, and the mixing chamber is difficult to fabricate, clean and sterilize. Technology: University of California, Irvine researchers have developed a new bymixer from common components that solves these problems. The technology incorporates a novel, parallel design into a new clinical bymixer, which by-passes a constant fraction of total flow through a mixing chamber. The bymixer provides accurate measurement of mixed expired gas fractions in the anesthesia circle circuit, over a wide range of tidal volume (300-1200 ml), frequency (6-20 breaths/mm), and partial pressure of carbon dioxide (PCO2, 6-50 mm Hg). Simple changes in mixing chamber volume provide adjustable bymixer response time. Fast bymixer response will allow VO2 and CO2 elimination measurements to be updated in less than 1 min (bymixer time constant of 12 sec allows 95% bymixer response by 36 sec). Application: The new clinical bymixer is constructed from standard anesthesia circuit components, attaches easily to the anesthesia machine's inspired outlet and expired inlet ports, is simple to clean and sterilize, and has no reservoir to trap condensed water vapor from expired gas. Introduction of this new clinical bymixer will facilitate indirect calorimetry during anesthesia and the non-invasive detection of metabolic upset (e.g. onset of anaerobic metabolism) and critical events (e.g. onset of pulmonary embolism).
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