BYMIXER PROVIDES ONLINE CALIBRATION OF MEASUREMENT OF CO2 VOLUME EXHALED PER BREATH

The measurement of CO2 volume exhaled per breath (V-CO2'br) can be det ermined during anesthesia by the multiplication and integration of tid al flow (V) and P-CO2. During sidestream capnometry, P-CO2 must be adv anced in time by transport delay (TD), the time to suction gas through the sampling tube. During ventilation, TD can vary due to sample Line connection internal volume or flow rate changes. To determine correct TD and measure accurate V-CO2'br during actual ventilation, TD can be iteratively adjusted (TDADJ) until V-CO2'br/tidal volume equals P-CO2 measured in a mixed expired gas collectlon (P<(E)over bar(CO2)>) (J. P Appl. Physiol. 72:2029-2035, 1992). However, P<(E)over bar(CO2)> is difficult to measure during anesthesia because CO2 is absorbed in the circle circuit. Accordingly, we implemented a bypass flow-mixing chamb er device (bymixer) that was interposed in the expiration limb of the circle circuit and accurately measured P<(E)over bar(CO2)> over a wide range of conditions of ventilation of a test lung-metabolic chamber ( regression slope = 1.01; R(2) = 0.99). The bymixer response (time cons tant) varied from 18.1 +/- 0.03 sec (12.5 l/min ventilation) to 66.7 /- 0.9 sec (2.5 l/min). Bymixer P<(E)over bar(CO2)> was used to correc tly determine TD,, (without interrupting respiration) to enable accura te measurement of V-CO2'br over widely changing expiratory flow patten s.