Consequently, under a situation where we have no cuff leakage and a perfect volume measurement the expired tidal volume shall always be larger than the inspired one since the inspired gas is colder and dryer. The net result from each contribution, temperature and humidity, is multiplicative, 1.058 × 1.062 = 1.123. Accordingly, the volume expansion of an initial dry gas by vapour is about 6.2%. Water vapour saturation is temperature dependent and at 37 ☌ vapour saturation pressure is 47 mmHg which is 6.2% of the normal barometric pressure (760 mmHg). From the descriptive universal gas law, PV = nRT, it can be seen that an increase of the number of molecules must be accompanied by an increase in either pressure (P) or volume (V). The number of molecules (n) in the initial dry gas is increased. Water molecules (vapour) are added until saturation. The rising temperature reflects an augmented kinetic energy of the molecules in the gas which expands the inherent volume. The increase in temperature from 20 to 37 ☌, which is equal to an increase from 293 to 310 K, expands the volume by 5.8% (310/293 = 1.058) according to Charles’s law (V~T). How come? The explanation is that in the lungs dry gas is humidified and heated to body temperature. He, or she, will continue to have an intact lung function and not losing any lung volume.
A patient connected to an ICU-ventilator without humidifier and with flawless Y-piece volume measurements who inspires 500 ml repeatedly expires about 560 ml.
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