Equipment review: Mechanical effects of heat-moisture exchangers in ventilated patients

In intubated patients the humidification and warming of inspired gases requires the addition to the ventilator circuit of a heated humidifier, or of an artificial nose [1,2]. Artificial noses were introduced as disposable devices more than 10 years ago, and since then they have been used in anaesthesia. In the most recent years the use of artificial noses has been increasingly extended into the intensive care unit (ICU) setting for patients undergoing long-term mechanical ventilation. The market has increased considerably in parallel with this widespread use, and presently offers a wide range of artificial noses. More than 30 models (for adult patients) are now available on the Italian market alone, making the choice of artificial nose difficult for the clinician. Indeed, the choice of artificial nose requires consideration of different parameters, which often have conflicting merits.Most studies [3,4,5,6] have compared artificial noses and heated humidifiers in terms of humidification efficiency and antimicrobial properties. Only in recent years has attention been paid to the mechanical effects of artificial noses. Although few clinical studies on this aspect are available [7,8,9,10], their results are nevertheless consistent and in agreement. The present review examines the available data on the mechanical effects of artificial noses, in order to provide the clinician with some help in making a choice of artificial nose.It is first necessary to consider how artificial noses modify the geometry of the ventilation circuit. These devices are placed distally to the Y-piece in the circuit, in line with the endotracheal tube. Unlike artificial noses, heated humidifiers are inserted in the inspiratory pathway of the ventilation circuit. Hence, artificial noses are not just part of the ventilation circuit, but are also part of the artificial airway. If we remove a heated humidifier and add an artificial nose, we can expect the following mechanical changes in the entire ve