Hospital Universitario de Canarias La Laguna
The use of mechanical ventilation with an artificial airway requires conditioning of the inspired gas. This is because medicinal gases are cold and dry and when the upper airway is bypassed it cannot contribute to the natural heat and moisture exchange process of inspired gases. At low levels of inspired humidity, water is removed from the mucus and periciliary fluid by evaporation, causing increased viscosity of mucus and loss of the periciliary fluid layer. In this way, mucociliary clearance decreases, since thick mucus is difficult for cilia to remove and, besides, mucociliary transport is impaired due to decreased cilia beat rate. Continuous desiccation of the mucosa causes cilia paralysis, cell damage and decreased functional residual capacity. In addition, atelectasis may develop.
Artificial humidification of medicinal gases may be active or passive. In active humidifiers, called heated humidifiers (HHs), the inspired gas passes across or over a heated water bath. Passive humidifiers, called artificial noses or heat and moisture exchangers (HMEs), trap heat and humidity from the patient’s exhaled gas and return some of it to the patient on the subsequent inhalation.
There is controversy about both the optimal humidity level of the inspired gas and the appropriate humidification system. Some authors have advocated absolute humidity levels of 26–32 mg of water vapour/l of gas and recommend the use of an HME because they provide these levels. However, others advocate an absolute humidity level of 44 mg of water vapour/l of gas and recommend the use of HH because they can condition inspired gas to this humidity level (programmed to deliver medicinal gas at a temperature of 37ºC and a relative humidity of 100%).
In the review by Williams et al, the data from 200 relevant studies on respiratory tract physiology and humidification were plotted on a humidity exposure map.(1) It reveals that there are few humidity, temperature and mucosal function studies of human subjects and that the duration in most of them was only 12 hours. The map data developed by the review authors suggests that mucociliary dysfunction can occur after 24–48 hours with an absolute humidity level of <32 mg water vapour/l (the humidification delivered by HME devices). The optimal humidification model of inspired medicinal gas should be at body temperature and 100% relative humidity, containing approximately 44 mg water vapour/l of gas (which can be achieved with HH devices). However, further research with exposure times longer than 24 hours is needed for full verification of this proposition.
Hurni et al developed a randomised study with 115 patients who needed mechanical ventilation for 48 hours or more.(2) The patients received inspired gas conditioned either by HH at 32ºC and a relative humidity of 100% or by an HME. The morphologic integrity of the respiratory epithelium was evaluated in 41 patients receiving mechanical ventilation for five days or more. In both patient groups, the scores significantly decreased from day one to day five (in the HH group from 787 ± 104 to 745 ± 88; and in the HME group from 813 ± 79 to 739 ± 62; p < 0.01 for both groups). In the group with HME, the reduction of the epithelium score was greater – although not significantly so – than in the patient group with HH. In my opinion, there are two causes for the absence of significant differences. First, the limited sample size of only 41 patients; and, secondly, in the HH group the inspired gas was conditioned to a relative humidity of 100% and a temperature of only 32ºC. (But what should the result be with a temperature of 37ºC which ensures delivery of approximately 44 mg of water/l of gas?)
There is also controversy about the possible influence of these systems on the incidence of VAP. While one study reported a lower incidence of VAP associated with the use of HME, several studies found no significant differences between the two systems, and some studies found a lower incidence of VAP associated with HH.
A recent meta-analysis by Kola et al, which enrolled 1,378 patients from nine trials, found that the use of HME decreased VAP rate (relative risk = 0.7; 95% CI = 0.50–0.94).(3) Only one of the studies included in the meta-analysis, the study by Kirton et al,(4) reported a significantly lower incidence of VAP with HME compared to HH. The meta-analysis did not include the nonrandomised studies developed by Cohen et al(5) and by Blin et al(6), which found significantly decreased VAP rates using HH compared with HME.
After the meta-analysis, in two randomised studies developed by Lacherade et al(7) and by Boots et al,(8) it was found that in fact there were not significant differences in VAP incidences associated with the use of HH or HME.
In 2006, our group published a randomised study(9) of 104 patients requiring mechanical ventilation for more than five days. We analysed the incidence of VAP associated with the use of HH or HME and found it to be lower in the HH group (eight of 51 [15.69%] versus 21 of 53 [39.62%]; p = 0.006). In addition, multivariate Cox regression analysis showed HME as a risk factor for VAP (hazard rate = 16.2, 95% CI = 4.54–58.04, p < 0.001).
We believe that the reduction of VAP found in our study when using HH, as compared with HME, may be because we analysed patients on mechanical ventilation for more than five days with a mean duration of mechanical ventilation (20 days), higher than in previous studies (4–14 days). It is possible that HH has a protector effect with mucociliary clearance and VAP in patients undergoing prolonged ventilation.
Our study had several limitations. First, we did not perform the direct assessment of gas heating and humidification in the patient, so airway temperature and humidity were not monitored (the reliability of the data reported by the manufacturers was assumed). Secondly, we did not perform an indirect assessment of gas heating and humidification in the patient, so secretion characteristics or possible epithelial bronchial damage was not assessed.
More research is necessary to establish the optimal humidification level and system. However, I recommend the use of HME in patients who are expected to need mechanical ventilation for 24–48 hours, and HH in patients expected to require more prolonged ventilation.
- Williams R, et al. Crit Care Med 1996;24:1920-9.
- Hurni JM, et al. Chest 1997;111(3): 686-91.
- Kola A, et al. Intensive Care Med 2005;31(1):5-11.
- Kirton OC, et al. Chest 1997;112:1055-9.
- Cohen IL, et al. Crit Care Med 1988;16: 277-9.
- Blin F, et al, and the members of the Nosocomial Infections Research Group. Intensive Care Med 1996;22 Suppl 3:S324.
- Lacherade JC, et al. Am J Respir Crit Care Med 2005; 172(10):1276-82.
- Boots RJ, et al. Crit Care Med 2006;34(3):687-93.
- Lorente L, et al. Crit Care 2006;10(4):R116.