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Does the use of heat and moisture exchangers rather than heated humidifiers affect the incidence of ventilator associated pneumonia?

Citation: Boots RJ, et al. Clinical utility of hygroscopic heat and moisture exchangers in intensive care patients. Critical Care Medicine 1997; 25:1707-1712
 

Lead author's name: RJ Boots

Three-part Clinical Question:
Patients:  intensive care patients requiring mechanical ventilation > 48h

Intervention: hydroscopic heat & moisture exchanger v. heated humidifier

Outcome: incidence of ventilator-associated pneumonia

Search Terms: Intensive care, critical care, mechanical ventilation, ventilator associated pneumonia, nosocomial pneumonia, heat & moisture exchanger, heated humidifiers, controlled trial

The Study: Randomised controlled trial, with “blinded” analysis. Analysis depended on completion of the trial and not on intention-to-treat.

The Study Patients: 116 intensive care patients requiring mechanical ventilation > 48h
 

Control group: 41 patients had a heated humidifier, with a circuit change every 2 days
 

Experimental group 75 patients had heat & moisture exchanger. 42 had been randomised to having a circuit change every 2 days and 33 to a 4 day change. 

The Evidence:

Comments:

Patients were randomised to one of 3 treatments. It is not stated how long the follow up to the diagnosis of VAP was made; it would have been useful to have compared nosocomial infection rates during the ICU stay. An unstated number of patients were excluded from analysis because they were ventilated for less that the time to first circuit change. This is not therefore an intention-to-treat analysis.

Microbiologists were blinded, but it is not clear that the diagnosis of VAP was indeed a wholly blinded decision. It is not stated that treatment of patients was otherwise equal in all groups. The groups appeared similar at the start of the trial.

The power calculation was based on differences in circuit colonisation and not on VAP rates. 

Hydroscopic HMEFs are not as effective bacterial filters as hydrophobic devices and may not provide sufficient protection from the aspiration of contaminated ventilator circuit contents.

In summary, this is a poorly designed study, which failed to focus on the clinically important question of whether HMEFs prevent VAP.
 
 

Appraised by: Dr David Swann, Consultant, Intensive Care, Royal Infirmary of Edinburgh.

 
Email: d.g.swann@ed.ac.uk

Kill or Update By: December 2004

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