Evaluation of methods for indirect calorimetry with a ventilated lung model

Abstract

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, FIO2, ventilatory pressure, {Mathematical expression} 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 (F1O2 0.4-0.7, ventilatory pressure 0-50 cmH2O) were determined. Differences as high as 15% from the true values of {Mathematical expression} and {Mathematical expression} 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. © 1989 Springer-Verlag.