The Performer, a new type of active HME, when used as a passive device, provided airway conditioning at least comparable to that with other HMEs. When used as an active device, however, the efficiency of the Performer increased beyond that of a purely passive HME.
The optimal level of conditioning remains debatable [1, 2]. For example, Williams and coworkers [20] suggested that heating and humidifying the inspired gas to the natural targets of core temperature (i.e. 37°C) and AH of 44 mgH2O/l reduces heat and moisture exchange with the mucosa and maximizes mucociliary clearance. However, Tsuda and coworkers [21] found airway damage after only 3 hours inhalation of gas at 35°C with AH of 39 mgH2O/l.
In normal conditions the temperature of expired gases ranges between 28 and 32°C with an AH of 27–33 mgH2O/l [22, 23], and thus a temperature of 29–33°C with an AH of 28–35 mgH2O/l should be adequate for inspired gases [2]. Two previous studies [12, 13] showed that a HME that is able to deliver a mean AH of 30 mgH2O/l could safely be used for up to 7 days in mechanically ventilated patients. These data suggest that, in general, it is not necessary to provide an AH greater than 30 mgH2O/l.
The Performer as a 'passive' device
When used as a passive device, the Performer provided an average absolute humidity of 28.9 ± 0.9 mgH2O/l in vitro during normothermic conditions and 30.7 ± 1.6 mgH2O/l in vivo. The passive Performer was consistently more efficient than the Hygroster and was comparable to the Hygrobac. Moreover, in all conditions tested, the Hygroster delivered a temperature and AH significantly lower than that with the Hygrobac – a feature others have noted [18].
Several clinical studies have found a satisfactory AH (i.e. ≥ 30 mgH2O/l) when using HMEs in patients with high minute ventilation (between 10.5 and 16.5 l/min) [19, 23, 24]. In the present study we found that increasing or decreasing the minute ventilation above or below 10 l/min resulted in a marked reduction in AH, to even below the commonly suggested limits [1, 2].
We also investigated the effects of severe hypothermia on HME efficiency. We found that hypothermia markedly reduced the efficiency of HMEs. These findings confirm that HMEs should be used with caution in severely or moderately hypothermic patients.
The Performer as an 'active' device
When the Performer was used as an 'active' humidifier it provided higher levels of humidification (AH range 30–36 mgH2O/l), independently of minute ventilation and expiratory AH, unlike the other HMEs. Active Performer also showed good stability in patients without any loss of efficiency after 12 hours of continuous use, and reached a steady state in terms of temperature and humidity after only 15 min of use.
To improve the efficiency of HMEs, use of two other different devices – the Booster (TomTec, Kapellen, Belgium) and the Humid-Heat (Gibeck, Upplands wasby, Sweden) – has been proposed. The Booster is a small heating element that is placed between the HME and the patient. The heating element, powered electrically, is covered by a Gore-Tex membrane, in which water (added from the outside) vaporizes and thus increases the AH of inspired gases [25]. Patients ventilated with Booster for 96 hours had higher temperature and AH of inspired gases (2–3°C and 2–3 mgH2O/l more than with a standard HME), and there was no bacterial colonization of the ventilatory circuit [25]. Similar in design to the Performer is the Humid-Heat, in which external water and heat are added to the patient side of a HME circuit. The Humid-Heat can boost temperature and AH up to 37°C and 44 mgH2O/l, which are close to the levels achieved with conventional hot water humidifiers [8, 26, 27]. In addition, if the water supply runs out, all of these devices continue to work as passive HMEs, avoiding the possibility that dry gases will be delivered.
In severe hypothermic conditions, active Perfomer was more efficient than the Hygroster, although the AH was lower than the minimum required levels. In these extreme conditions, hot water humidifiers should be used.
Airflow resistances and dead space
The presence of any HMEs in the ventilatory circuit increases the airflow resistance [28]. We found similar low inspiratory airflow resistances with the Performer, Hygrobac and Hygroster, with no difference between the beginning of the experiment and after 1 hour of use. After increasing the peak inspiratory flow to a very high level (1 l/s) the airflow resistance was still low, with an average value of 2.3 ± 0.6 cmH2O/l per s. This additional resistance, which is lower than that with an endotracheal tube, is not likely to play any significant role during controlled mechanical ventilation [29] and can be considered acceptable during assisted ventilation [28].
Because of the internal volume of HMEs, ranging from 50 to 90 ml, the dead space of the ventilator circuit is increased [16, 30], causing an increase in carbon dioxide levels, especially during low tidal volume ventilation [31]. HMEs also cause an increase the inspiratory work of breathing, with an increase in intrinsic PEEP [16, 32]. Consequently, because they increase the resistive dead space load, use of HMEs cannot be recommended in patients who are weak or difficult to wean, unless the level of ventilator assistance is increased [16].
Limitations
Potential limitations of the study must be addressed. First, we did not examine the effects on gas exchange, respiratory mechanics, secretions, or microbiological contamination of the ventilator circuit. However, during the study we did not observe any obstruction of the endotracheal tube. Second, we tested the Performer in vivo only at a single minute ventilation and for a relatively brief period of only 12 hours. Third, we did not have any data from a heated humidifier because the heated humidifier, being an active system, can deliver gas at a broad range of temperatures and AHs (i.e. with a relative humidity of 100%), independent from the ventilatory settings.
Possible indications and advantages of the Performer
Although HMEs may be safely used during long-term ventilation [12, 13], many centres do not routinely use HMEs for fear of tube obstruction and insufficient humidification [33]. In the presence of thick secretions, the use of HMEs, because of water loss from the airways, may increase the risks for tube occlusion, air trapping and hypoventilation [2]. Because the Performer can deliver higher AH than other HMEs, it may be useful in patients in whom the use of HMEs appears to worsen the clinical characteristics of secretions and in hypothermic patients who would otherwise require the use of heated humidifiers.
Although we did not directly evaluate the cost, in agreement with a previous study that evaluated a similar active HME (Humid-Heat) [8], the Performer should allow a reduction in daily sterile water consumption, avoidance of condensate in the ventilator circuit, a decrease in changes of ventilator circuits, and a reduction in nurses' workload.