Tidal volumes: cold and dry or warm and humid, does it matter?
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EDITORIAL
Tidal volumes: cold and dry or warm and humid, does it matter? Mats Wallin1,2 · Göran Hedenstierna3 Received: 9 July 2019 / Accepted: 31 October 2019 © The Author(s) 2019
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. He, or she, will continue to have an intact lung function and not losing any lung volume. How come? The explanation is that in the lungs dry gas is humidified and heated to body temperature. The increase in temperature from 20 to 37 °C, 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). The rising temperature reflects an augmented kinetic energy of the molecules in the gas which expands the inherent volume. The gas is also humidified in the lung. Water molecules (vapour) are added until saturation. The number of molecules (n) in the initial dry gas is increased. 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). Water vapour saturation is temperature dependent and at 37 °C vapour saturation pressure is 47 mmHg which is 6.2% of the normal barometric pressure (760 mmHg). Accordingly, the volume expansion of an initial dry gas by vapour is about 6.2%. The net result from each contribution, temperature and humidity, is multiplicative, 1.058 × 1.062 = 1.123. 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. This everyday occurrence is not always obvious. The reason is that manufacturers compensate for the increase in temperature and humidity and present a reduced expired volume according to the applied
* Mats Wallin [email protected] 1
Department of Physiology and Pharmacology, Karolinska Institute, Stockholm, Sweden
2
Maquet Critical Care AB, Röntgenvägen 2, 171 54 Solna, Sweden
3
Department of Medical Sciences, Clinical Physiology, Uppsala University, Uppsala, Sweden
dry and cold reference condition, but also the generally limited accuracy in volume measurement, in both ICU-ventilators and anaesthesia machines contributes. Both factors conceal the gas expansion phenomenon within the lung. The reasoning above elucidates why the conversion factor between the two common reference conditions used during tidal volume measurements, ATPD [ambient temperature (here 20 °C) and pressure, dry] and BTPS (body temperature ambient pressure, saturated), is 12%. In previous anaesthesia machine and ICU-ventilator standards, manufacturers were allowed to freely choose which reference condition (ATPD, BTPS) to use. A device from a manufacturer using BTPS as the reference delivers a tidal volume from its outlet that is 12% smaller than the one delivered by a device based on ATPD when an ATPD calibrated flowmeter is use
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