Evaluation of methods for indirect calorimetry with a ventilated lung model

A combined lung and ventilator model was built, validated and used to test commercial systems for indirect calorimetry. It simulates O₂ uptake and CO₂ excretion under ventilator treatment conditions. In the model inspiratory gases are diluted with N₂ and CO₂ to give the desired expiratory concentrations. Minute volume, F_I O ₂, ventilatory pressure, $\dot VO_2 , \dot VCO_2 $ and consequently RQ can be altered to simulate the adult clinical situation. A selected respiratory pattern is maintained by the lung model. Equipment for indirect calorimetry can then be connected to it and the results compared. Reference values are derived from measurements with a mass spectrometer and a Godart spirometer. Three commercially available instruments (Beckman MMC, Horizon MMC and Engström MC) were evaluated with this system. The limits of agreement with the reference values under different conditions (F₁ O ₂ 0.4–0.7, ventilatory pressure 0–50 cmH₂O) were determined. Differences as high as 15% from the true values of $\dot VO_2 $ and $\dot VCO_2 $ were observed. The pattern of mechanical ventilation and the intrinsic properties of the analyzers in the equipment used for indirect calorimetry influence measurements to a significant extent.