Mechanical effects of heat-moisture exchangers in ventilated patients

Although they represent a valuable alternative to heated humidifiers, artif icial noses have unfavourable mechanical effects. Most important of these i s the increase in dead space, with consequent increase in the ventilation r equirement. Also, artificial noses increase the inspiratory and expiratory resistance of the apparatus, and may mildly increase intrinsic positive end -expiratory pressure. The significance of these effects depends on the desi gn and function of the artificial nose. The pure humidifying function resul ts in just a moderate increase in dead space and resistance of the apparatu s, whereas the combination of a filtering function with the humidifying fun ction may critically increase the volume and the resistance of the artifici al nose, especially when a mechanical filter is used. The increase in the i nspiratory load of ventilation that is imposed by artificial noses, which i s particularly significant for the combined heat-moisture exchanger filters , should be compensated for by an increase either in ventilator output or i n patient's work of breathing. Although both approaches can be tolerated by most patients, some exceptions should be considered. The increased pressur e and volume that are required to compensate for the artificial nose applic ation increase the risk of barotrauma and volutrauma in those patients who have the most severe alterations in respiratory mechanics. Moreover, those patients who have very limited respiratory reserve may not be able to compe nsate for the inspiratory work imposed by an artificial nose. When we choos e an artificial nose, we should take into account the volume and resistance of the available devices. We should also consider the mechanical effects o f the artificial noses when setting mechanical ventilation and when assessi ng a patient's ability to breathe spontaneously.