New metabolic lung simulator: development, description, and validation

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.

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.

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.

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.