Estimating VO2

• An automated machine check could calculate the amount of machine leak you have in the circuit, separate from any patient leak. Just occlude the Y-piece and run the vent while comparing your VTI - VTE. Average 20 breaths or so and likely there is a delta of a few ml with each breath accounting for air lost from circuit connections, internal machine leaks, tubing, etc. Variability may also be due to measurement error from the flow meters used.
• Once connected to the patient, VTI - VTE gives the total volume lost per breath. Subtract the machine leak found above and this gives you the patient side leaks (air lost from around the ETT cuff, volume lost to chest tubes, etc)
• FiO2 * FGF = V̇O₂
• You can calculate the V̇O₂ using the Fick equation: <math chem>\ce{\dot VO2} = Q \times\ (C_a\ce{O2} - C_v\ce{O2})</math>, where Q is the cardiac output.
• Reference values for V̇O2 max
• In 2016, the AHA published a scientific statement recommending that CRF – quantifiable as V̇O₂ max/peak – be regularly assessed and used as a clinical vital sign; ergometry (exercise wattage measurement) may be used if V̇O₂ is unavailable. source