An approach to continuous estimation of CO_2 production during non-steady state exercise using stable isotope

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A new method was developed for continuous isotopic estimation of human whole body CO_2 rate of appearance (Ra) during non-steady state exercise. The technique consisted of a breath-by-breath measurement of ^<13>CO_2 enrichment (E) and a real-time fuzzy logic feedback system which controlled NaH^<13>CO_3 infusion rate to achieve an isotopic steady state. Ra was estimated from the isotope infusion rate and body ^<13>CO_2 enrichment which was equal to E at the isotopic steady state. During a non-steady state incremental cycle exercise (5w/min or 10w/min), NaH^<13>CO_3 infusion rate was successfully increased by the action of feedback controller so as to keep E constant. It was concluded that the newly developed system was potentially useful for studies in dynamic aspects of human metabolism during exercise although further developments would be needed in methods of corrections for the tracer recovery at rest.