New metabolic lung simulator: development, description, and validation

Abstract

Objective: Indirect calorimetry, the determination of airway carbon dioxide elimination (V(CO2),and oxygen uptake (V(O2)), 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 V(CO2) and V(O2), 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 V(CO2) and V(O2). 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 V(CO2) and V(O2) were validated against separate measurements of gas flow and gas fractions in a collection bag.

Results: For volume control ventilation, average error for V(CO2) and V(O2) was -0.16% +/- 1.77 and 1.68% +/- 3.95, respectively. For pressure control ventilation, average error for V(CO2) and V(O2) 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.