Continuous non-invasive monitoring of energy expenditure, oxygen consumption and alveolar ventilation during controlled ventilation: validation in an oxygen consuming lung model

Background: We have developed a combined indirect calorimetric and breath-by-breath capnographic device (GEM) for respiratory monitoring: oxygen consumption (V?O₂), carbon dioxide excretion (V?CO₂), respiratory quotient (RQ), energy expenditure (EE), alveolar ventilation (V?_A) and dead space/total ventilation (V_D/V_T). Methods: The device was tested in a lung model in which V?O₂ was achieved by combustion of hydrogen. V?CO₂ was achieved by delivering CO₂ into the single alveolus combustion chamber. V?O₂, V?CO₂, compliance, and anatomical dead space could be varied independently. Results: Measured V?O₂ was 101±3% (SD) of set value at a F₁O₂<0.6 and 101±7% at a F₁O₂>0.6 during 15 hours of testing. The corresponding V?CO₂ values were 99±2% and 102±7%. The GEM could with good accuracy measure accumulated energy expenditure (EE) during simulated unstable patient conditions up to a F₁O₂ of 0.8. At F₁O₂ above 0.8 V?CO₂ and V?O₂ could be estimated using a default RQ value of 0.85. On-line estimated V?_A and V_D/V_T values could be obtained at any F₁O₂ up to 1.0. In a test sequence with stable V?O₂ and V?CO₂ the GEM adequately followed changes in V?_A, induced by changes in anatomical dead space, breathing frequency and compliance. Conclusion: The overall performance of the device is satisfactory and well comparable with any equipment tested. It allows near-continuous non-invasive monitoring of EE, V?O₂, V?CO₂, V?_A, V_D/V_T in ventilated, critically ill patients, providing a rationale for ventilator settings and nutritional support.