A simple method to determine mixed exhaled CO2 using a standard circle breathing circuit.

INTRODUCTION

Mixed exhaled CO2 and CO2 excretion (VCO2) are not routinely monitored during anesthesia because they are difficult to measure using standard equipment. We conducted two experiments to test the hypothesis that the ventilator bellows contains mixed exhaled CO2.

METHODS

In the laboratory experiment, a lung model was assembled with a CO2 source. The gasses were sampled from the bellows and VCO2 was calculated. These values were compared to VCO2 measured by the NICO monitor (Novametrix Medical Systems), a commercially available mainstream capnometer. In the clinical experiment, gasses were sampled from the ventilator bellows in nine subjects under general anesthesia. VCO2 was calculated and compared to values obtained from the NICO monitor.

RESULTS

VCO2 measured from the bellows connected to the lung model resulted in an overall precision of 26 mL/min, bias of -24 mL/min, and average error of -12.0% compared to set CO2 flow. In the clinical trial, calculated VCO2 sampled from the bellows compared to measured VCO2 from the NICO monitor had a bias of -0.15 mL/min, and precision of 33.16 mL/min.

CONCLUSION

VCO2 calculated from the bellows was not as accurate as NICO VCO2 in the laboratory; however, it can be done using standard anesthesia equipment. Furthermore, in nine anesthetized subjects, the bellows method of VCO2 determination had an overall percent error of 1.34% from NICO VCO2. Given this small percent error between the bellows method and the NICO monitor, we believe that the bellows method of VCO(2) determination is suitable for clinical practice.