A Systematic Method for Validation of Gas Exchange Measurements

Abstract

The measurement of gas exchange is useful but, thus far, has not been practical during the mechanical ventilation of critically ill patients. To validate 2 new commercial instruments (Siemens-Elema Servo Ventilator 900B, Beckman Metabolic Cart), a lung model was constructed into which is delivered CO2 and N2 at precise rates to simulate CO2 production (.ovrhdot.VCO2) and O2 consumption (.ovrhdot.VO2. The model consisted of 13.5-1 glass jar with an attached 1-l anesthesia bag. The lung model was ventilated at preset tidal volumes and frequencies. The measured respiratory quotient (RQ) with the known RQ of burning methanol (RQ = 0.67) in the jar, were compared. When the model was ventilated with levels of tidal volume and gas exchange applicable to adults, both instruments measured .ovrhdot.VO2 within 5-13% of predicted values. Varying the FLO2 [fractional inspiratory O2] did not significantly affect this accuracy. At tidal volumes < 350 ml, the difference increased between predicted .ovrhdot.VCO2 and measured .ovrhdot.VCO2. The difference between measured vs. the actual RQ of methanol was 5 and 1.5% in the Siemens-Elema and Beckman Systems, respectively.