ASSESSMENT OF ERRORS WHEN EXPIRATORY CONDENSATE PCO2 IS USED AS A PROXY FOR MIXED EXPIRED PCO2 DURING MECHANICAL VENTILATION

Objectives, We designed a series of experiments to determine whether e xpiratory water condensate (PconCO(2)) can be used as a proxy for mixe d expired gas collection. Methods, in 18 adult mechanically ventilated patients with ARDS (40 samples), simultaneous collections of arterial blood, expiratory water trap condensate, mixed expired gas, and minut e ventilation were used to calculate VCO2 and V-D/V-T. To assess the e ffect of temperature, a constant gas flow (PCO2 10-30 mm Hg) was bubbl ed through water at temperatures of 19.5-37 degrees C. Gas and water s amples were collected, immediately analyzed for PCO2, and a temperatur e correction factor was calculated. A lung model was constructed using a 5 L anesthesia bag connected to a mechanical ventilator with a heat ed humidifier. Temperature at the Y-piece was set to approximate to 37 degrees C and CO2 was injected into the bag to establish an end-tidal PCO2 oi 20-70 mm Hg. After equilibration, condensate was collected, P CO2 was measured, and the temperature-corrected PCO2 was compared to P ECO2. The capnogram at points along the expiratory limb, circuit was u sed to evaluate mixing. Results, There was an over-estimation of PECO2 by PconCO(2) (p < 0.001) for the patient data, resulting in an undere stimation of V-D/V-T (p < 0.001) and an overestimation of VCO2 (p < 0. 001). The temperature correction factor for PCO2 in water was -0.010 ( about half of the factor used for whole blood), The bias between tempe rature-corrected PconCO(2) and PECO2 was 0.3 +/- 3.2 mm Hg ill the lun g model. Mixing in the expiratory limb was poor, as evaluated by the c apnogram. Conclusions. Even with temperature correction, we failed to precisely predict PECO2 from PconCO(2). For measurement of V-D/V-T and VCO2, we do not recommend methods that use PconCO(2).