A Systematic Method for Validation of Gas Exchange Measurements

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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.

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