New metabolic lung simulator: Development, description, and validation

Abstract

Objective: Indirect calorimetry, the determination of airway carbon dioxide elimination (VCO3) and oxygen uptake (VCO3), can be used to non-invasively detect non-steady state perturbations of gas kinetics and mirror tissue metabolism. Validation of monitoring instruments in patients is difficult because there is no standard reference measurement, a wide range of physiologic values is required, and steady state is difficult to achieve and confirm. We present the development, critical details, and validation of a practical bench setup of a metabolic lung simulator, to generate a wide range of accurate, adjustable, and stable reference values of VCO3 and VCO3, for development, calibration, and validation of indirect calorimetry methodology and clinical monitors. Methods: We utilized a metered alcohol combustion system, which allowed safe, precise, and adjustable delivery of ethanol to a specially designed wick system to stoichiometrically generate reference VCO3) and VCO3. Gas was pumped through a circular circuit between the separate metabolic chamber and mechanical lung, to preserve basic features of mammalian gas kinetics, including a physiologic ventilation waveform and the ability to induce non-steady state changes. Accurate and precise generation of VCO3 and VCO3 were validated against separate measurements of gas flow and gas fractions in a collection bag. Results: For volume control ventilation, average error for VCO2 and VO2 was -0.16% ± 1.77 and 1.68% ± 3.95, respectively. For pressure control ventilation, average error for VCO2 and VO2 was 0.90% ± 2.48% and 4.86% ± 2.21% respectively. Low values of measured ethanol vapor and carbon monoxide supported complete and pure combustion. Conclusions: The comprehensive description details the solutions to many problems, to help future investigations of metabolic gas exchange and contribute to improved patient monitoring during anesthesia and critical care medicine. © Springer Science+Business Media, Inc. 2007.