[Indirect calorimetry in artificially respirated children. 2. A measuring procedure and its evaluation in a newly developed metabolism-lung model].

A new device for continuous measuring oxygen uptake (VO2) and carbondioxide elimination (VCO2) in artificially ventilated or CPAP-system breathing children has been developed. A dual-channel analyzer system based on zirconiumoxyd cells measures oxygen fractions. This allows not only single measurements but also continuous determination of fraction differences for oxygen (delta FO2) (accuracy +/- 0.003 Vol%). Carbondioxide is measured by infrared absorption. A mixing device for inspiratory gas was designed to smooth fluctuations of inspired oxygen fractions almost completely. The reliable sampling system for expiratory gases has already been described [19]. The breathing-e.g. ventilating-system was modified in such a way that the total gas flow is independent of mode and breathing volume (equally 15 l/min). For which purpose we use a self aspirating, time-cycled, volume limited respirator or a high-flow-CPAP-system. The prototype described ran test performances on a specially developed pneumatic metabolic-lung-model. This new lung model enables free choice of respiratory quotient (R) by independent setting of O2-uptake and CO2-elimination. Under these controlled laboratory conditions gasvolume-balances correspond to expected values in children really as shown during simulated trials. In the metabolic-lung-model accuracy and reproduction averaged +/- 1% for VCO2. Both were independent of the mode of ventilatory support, FIO2, and R. Determinations of VO2 were more dependent on FIO2 and R. After computing primary data according to a special formula which equalizes differences between VI and VE the maximal error was +/- 7%. Maximum difference between preset and measured R-values ranging from 0.769 to 1.429 was -4.6%, determined at R = 1.429. By means of this independent test series insights into clinically expected measurement errors and dimensions of limits of accuracy could be demonstrated. It seems to be justified that our newly developed device for accurate measuring O2-uptake and CO2-elimination is highly recommendable for use in extremely difficult conditions as in ventilated children.