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The efficiency of HMEs decreases with increasing tidal volumes. HMEs always result in an elevation of the inspiratory and expiratory airway resistances; this should be considered especially in cases that involve spontaneous respiration. The pressure drop across HMEs should be less than 2 hPa for a flow of 60 L/min, a level that also has been measured for cascade humidifiers.HMEs with a hygroscopic coating of CaCl2 should be given preference over LiCl-coated ones, especially because products of the same efficiency are available with CaCl2 coating. Lithium is a potentially toxic substance that can be taken up by way of bronchopulmonary resorption after accidental washing out [37]. Therefore, a possible safety hazard cannot be eliminated, especially in neonates and babies. Not least for these reasons HMEs must never be combined with active humidification systems or medication nebulizers. Even if the reduction in functional efficiency of the HME that is caused by washing off of the coating of hygroscopic substances is disregarded, the presence of medication aerosols in the HME, in particular, can result in a dangerous increase in resistance to gas flow. The internal volumes of HMEs should be as small as possible so that they do not increase the effective deadspace too much. A combination of HMEsand catheter mounts results in a further increase in the deadspace, and there-fore, must be considered critically, especially in cases that involve spontaneous respiration. If a catheter mount is necessary to add flexibility to the breathing system, the HME preferably should be connected directly onto the tracheal tube with the catheter mount behind it; otherwise, the humidification efficiency of the HME will be reduced by condensation in the catheter mount. Children should be ventilated with special HMEs that have a small internal volume. Caution is required in patients who have elevated sputum production, pulmonary trauma with bleeding, pulmonary edema, or a similar condition;in such patients a partial obstruction of the HME with a resulting elevation of the airway resistances might occur. In patients who have very viscous secretions (eg, as a result of a primary pulmonary disease or long-term dehydration therapy), heated humidifiers with a set temperature of greater than 370 degrees C should be given preference. Several recent investigations showed that not every device that is designated as an HME is appropriate for conditioning respiratory gases (ie, it effectively humidifies the inspiratory air). Most of the products that are available on the market are, at best, adequate for anesthetic ventilation or short-term postoperative follow-up ventilation. Generally, this is true of all HMEs that have not been coated with hygroscopic substances. CoatedHMEs have a much better humidification efficiency; however, here too, the existence of substantial differences in quality among the products prohibits an uncritical application.