The Metabolic (Lung) Simulator

The Metabolic (Lung) Simulator as a Teaching Instrument

If you are teaching physiology, metabolism and/or the use of Metabolic Measurement Systems, perhaps one of the most important subjects you should teach is to understand what factors affect the accuracy of VO2, VCO2 and Flow (volume) measurements and how to find and correct such problems.

All too often we hear that a metabolic measurement system is validated by comparing it to another one that is presumed accurate.

VacuMed’s Metabolic Simulator (MS) not only provides a known, accurate VO2/VCO2 mass flow, but may also be used to study many factors that affect accuracy:

  • The effect of gas analyzers linearity on VCO2/VO2, i.e., R.

Using the MS, R should not change for a given mass flow while changing the respiratory rate.

  • The effect of the location of the gas sampling site.

Sampling from near mid-stream in a B x B system and improper placement in a mixing chamber system can affect accuracy.

  • The effect of the dead space.

Using the MS, add dead space and study results.

  • The effect of a leak in a breathing valve.

Most valves, even new ones, have some back flow that causes inaccuracy at low flow.

  • The effect of gas temperature on the flow sensor.

Most flow sensors are calibrated at room temperature, but measure near body temperature, how large is the resulting error?

  • The effect of changing humidity on the flow sensor.

Most flow sensors are calibrated at room humidity, but measure gas that is fully saturated, how large is the resulting error?

  • The effect of respiration rate on the accuracy of the flow sensor.

Set the MS for, say, 1.5-liter stroke volume, then vary the breathing frequency. What variations do you see?

  • The effect of varying the flow rate on the accuracy of the flow sensor.

Keep the breathing rate constant but change the stroke volume of the MS, how accurate are the results?

  • How to find leaks in the gas sampling circuit with the MS.

One of the most difficult problems to find or even suspect. Depending on the type of leak, it may have different effects on low ventilation vs. high ventilation.

  • The effect of respiration rate on the accuracy of the VO2/VCO2.

Adding up the errors determined from the above list, how do they affect overall accuracy?