![]() Heat and moisture exchange devices: are they doing what they are supposed to do? Anesth Analg. The effect of heat and moisture exchanger on humidity and body temperature in a low-flow anaesthesia system. Humidification during mechanical ventilation in the adult patient. Particular requirements for basic safety and essential performance of critical care ventilators. ![]() Particular requirements for basic safety and essential performance of an anaesthetic workstation. Delivery of tidal volume from four anaesthesia ventilators during volume-controlled ventilation: a bench study. Only during true dry and ambient conditions “the volume entering the patient respiratory tract” will be the same as the volume delivered by the ventilator. Accordingly, the gas volume entering the patient’s respiratory tract differs in clinical practice from the volume delivered by the ventilator due to ordinary clinical humidification and heating. Both ventilators will however present the same exhaled volume (V Te), 500 ml, since the ATPD-ventilator converts and presents the volume at dry and ambient conditions (ATPD). The exhaled volume, when measured in the close proximity (no cooling) of the airway, corresponds to the enlargement of the lung. Within the patients’ lungs the inspired gas further expands and the patients’ lungs will be enlarged with 560 ml (in the ATPD ventilator) and 500 ml (in the BTPS ventilator) respectively (Fig. ![]() Both present V Ti as 500 ml on the display. An ATPD-calibrated ventilator delivering 500 ml dry gas from the outlet supplies the lungs with about 540 ml due to humidification and rise in temperature by the HME whereas a for a BTPS calibrated ventilator the post-HME volume entering the tracheal tube is 480 ml when an ATPD calibrated spirometer is used. However, inhaled gas volume is still 4–5% smaller than the corresponding exhaled volume which has body temperature and is saturated (BTPS). Thus, in an ICU-ventilator a HME expands the dry gas coming out from the ventilator outlet by 7–8% so the actual gas volume entering the patient’s respiratory tract is 7–8% larger than the volume delivered at the ventilator outlet. A HME humidifies the inspired dry gas to a relative humidity of about 80% but the surrounding environment of the HME has a cooling effect and as a consequence the gas entering the tracheal tube has a temperature of about 33 ☌. A constant delivery of dry fresh gas will drain out the patient’s mucous membrane and consequently the gas must be humidified which is commonly performed by a passive humidifier, a heat moisture exchanger (HME), or sometimes by an active humidifier. In a non-rebreathing wall-gas dependent ICU-ventilator, the CO 2 produced by the patient is eliminated by the fresh gas. Both factors conceal the gas expansion phenomenon within the lung. The reason is that manufacturers compensate for the increase in temperature and humidity and present a reduced expired volume according to the applied dry and cold reference condition, but also the generally limited accuracy in volume measurement, in both ICU-ventilators and anaesthesia machines contributes. This everyday occurrence is not always obvious. 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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