Abstract
Although they represent a valuable alternative to heated humidifiers, artificial noses have unfavourable mechanical effects. Most important of these is the increase in dead space, with consequent increase in the ventilation requirement. 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 design and function of the artificial nose. The pure humidifying function results in just a moderate increase in dead space and resistance of the apparatus, whereas the combination of a filtering function with the humidifying function may critically increase the volume and the resistance of the artificial nose, especially when a mechanical filter is used. The increase in the inspiratory load of ventilation that is imposed by artificial noses, which is particularly significant for the combined heat-moisture exchanger filters, should be compensated for by an increase either in ventilator output or in patient's work of breathing. Although both approaches can be tolerated by most patients, some exceptions should be considered. The increased pressure and volume that are required to compensate for the artificial nose application 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 compensate for the inspiratory work imposed by an artificial nose. When we choose an artificial nose, we should take into account the volume and resistance of the available devices. We should also consider the mechanical effects of the artificial noses when setting mechanical ventilation and when assessing a patient's ability to breathe spontaneously.
Highlights
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]
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
The major unfavourable mechanical effect of artificial noses is the increase in dead space, with consequent increase in the ventilation requirement
Summary
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]. The alveolar hypoventilation that results from lack of compensation for the artificial nose dead space may be significant, especially in patients with the most severe alterations in respiratory mechanics, in whom the ventilatory treatment relies on low levels of minute ventilation It has been observed [12] that the removal of a combined HME and filter, and the consequent reduction in dead space, allowed for a remarkable decrease in PaCO2 in ARDS patients subjected to controlled hypoventilation. In order to avoid an increase in patient respiratory work, we should compensate for the additional ventilatory requirement imposed by the artificial nose with an increase in the mechanical support This compensation can be achieved in different ways, depending on the mode of partial ventilatory support in use, generally it is sufficient to increase the inspiratory pressure support level [9,10]. This latter effect could better be explained by the increase in tidal volume that results from the increased need for ventilation imposed by the artificial nose,
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