Ve is measured either by collection of exhaled gas into a device such as a Douglas bag or by an in-line flow detector. V, is usually assumed to be equivalent to Ve, although there is a slight difference between the two because of the addition of water vapor from the lungs and the difference between O 2 consumed and CO2 produced. As it is technically difficult to measure differences between V, and Ve, V, is often calculated using the Haldane equation in which it is assumed that insoluble nitrogen is not consumed:

Substituting, the equation for Vo2 becomes

Since FiCO2 is zero during mechanical ventilation and usually no greater than 0.03 per cent when breathing room air, the equation for VCO2 can be simplified to

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