Review
Humidification policies for mechanically ventilated intensive care patients and prevention of ventilator-associated pneumonia: a systematic review of randomized controlled trials

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Summary

The Dutch Working Party on Infection Prevention (WIP) aimed to determine whether certain humidification policies are better than others in terms of prevention of ventilator-associated pneumonia (VAP) in mechanically ventilated intensive care unit (ICU) patients. Publications were retrieved by a systematic search of Medline and the Cochrane Library up to February 2006. All (quasi-) randomized trials and systematic reviews/meta-analyses comparing humidification methods in ventilated ICU patients were selected. Two reviewers independently assessed trial quality and extracted data. If the data was incomplete, clarification was sought from original authors and used to calculate the relative risk of VAP. Data for VAP were combined in the analysis, where appropriate, using a random-effects model. Ten trials were included in the review. In general, the quality of the trials and the way they were reported were unsatisfactory. The results did not show any benefit from specific humidification techniques in terms of reducing VAP. WIP do not recommend either passive or active humidifiers to prevent VAP, nor the type of passive humidifiers to be used. Regarding active humidification, WIP recommends using heated wire circuits. This is due to the theoretical consideration that less condensate reduces colonization and subsequent risk of spread throughout an ICU when condensate is removed.

Introduction

The upper airways filter, heat and moisten the inspired air so that it reaches the lower airways at body temperature with added water. In mechanically ventilated patients the ventilator tube bypasses the upper airways and inspired gases require artificial conditioning to prevent mucosal injury and ventilator-associated pneumonia (VAP). To date there has been much debate about the optimal heating and moistening method in long-term ventilated patients.

The most common technique is active humidification by heated humidifiers (HHs) or a combination of a heated humidifier and a heated wire circuit (HH-HWC). By heating a water reservoir, both heat and moisture are added to the dry, cold gases before administering them to the patient. The warmed gas is cooling when travelling from the HH to the patient resulting in rainout within the circuit. Condensate collecting in the tubing is known to attract bacterial contamination.1 Condensate may impede good airflow, cause VAP when draining toward the patient and contaminate environment and healthcare workers' hands during manual removal from the respiratory circuit. Using heated wires in the circuit may reduce condensate formation.

The alternative is to use passive humidification with warming by heat and moisture exchangers (HMEs), also called artificial noses. HMEs mimic the process that occurs in the upper airways. These devices are placed between the Y-connector of the ventilator tubing and the tracheal tube, by which condensate accumulation in the tubing is reduced to a great extent. Hydrophobic and hygroscopic HMEs have to be distinguished. Hydrophobic HMEs are mainly characterized by bacterial filtration properties and not so much by humidification. Hygroscopic HMEs and a variant of hygroscopic HMEs, i.e. hygroscopic condenser humidifiers (HCHs), are mainly characterized by humidification properties and not so much by bacterial filtration. HCHs are similar to hygroscopic HMEs. Their surface, however, is coated with lithium chloride or calcium chloride which improves chemically heat and moisture exchange. Some hygroscopic HMEs and HCHs also have bacterial filter membranes in addition to their humidification compounds (HMEF or HCHF).

The Dutch Working Party on Infection Prevention (Werkgroep Infectiepreventie or WIP) provides recommendations and guidelines for infection prevention in healthcare. With a view to making guidelines evidence-based, the WIP conducted a systematic review of the literature on whether certain heating and moistening policies are better in terms of prevention of VAP. The WIP planned to compare the following: (i) Active versus passive humidification methods, resulting in six comparisons: hydrophobic HME versus HH; hygroscopic HME versus HH; hydrophobic HME versus HH-HWC; hygroscopic HME versus HH-HWC; hygroscopic HMEF versus HH; and hygroscopic HMEF versus HH-HWC; (ii) Active humidification without heated wire circuit versus active humidification with heated wire circuit (HH versus HH-HWC); (iii) Different types of HMEs for passive humidification: hydrophobic HME versus hygroscopic HME; hydrophobic HME versus hygroscopic HMEF; and hygroscopic HMEF versus hygroscopic HME.

The scope of this review was not to assess the impact of humidification policies on artificial airway occlusion, nor cost-effectiveness or environmental and healthcare workers' hand contamination.

Section snippets

Searching

Publications were retrieved by a search of Medline and the Cochrane Library up to February 2006. Terms included were “ventil*, pneumonia, humid*, heat and moisture exchanger* and circuit*. The search strategy in Medline was: (humid* OR humidification OR circuit* OR humidity OR humidifier OR humidifiers OR heat and moisture exchanger* OR artificial nose) AND ((((ventilator associated pneumonia) OR (VAP AND (pneumonia OR pneum*)) OR (“Respiration, Artificial”[MAJR] AND pneumonia) OR (ventilated

Selection

By judgement of abstracts, 12 studies appeared to fulfil the selection criteria. Of these, one paper was excluded after reading the whole article because it did not address the questions posed in this review.2 Another paper was excluded because the complete data could not be obtained despite multiple attempts to contact the authors.3 Ten trials were included in the review.4, 5, 6, 7, 8, 9, 10, 11, 12, 13

Quality assessment

All trials were described as parallel group randomized controlled trials but just two had

Discussion

This systematic review identified 10 eligible randomized controlled trials that addressed humidification policies in mechanically ventilated ICU patients. The results did not show a benefit associated with a certain humidification technique in terms of reducing VAP.

Nine trials compared passive with active humidification techniques. A single trial comparing hydrophobic HME with HH-HWC showed a significant difference. Eight trials comparing various passive and active humidification techniques did

References (14)

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