A calorimetric performance monitoring system includes an analyzer that is fluidly connected to an in-stream respiration flow sensor. The system includes a controller that is configured to determine a flow rate of respiration flow and at least a portion of a composition of the respiration flow on a breath-by-breath basis and temporally associate the respiration flow value and the determined portion(s) of the composition of the respiration flow and segregate non-steady state respiration performance data from steady state respiration performance data and determine a calorimetric performance from the steady state respiration performance data.
