Key Point

Compared to heated humidifiers, heat and moisture exchangers greatly increase the risk of ventilator-associated pneumonia in critically ill patients who are on a ventilator for more than five days.

TENERIFE, SPAIN—Comparisons of ventilator-associated pneumonia (VAP) risk with a heated humidifier or heat and moisture exchanger (HME) have resulted in controversy. While a number of studies found that VAP incidence was lower when HMEs were used, enough data have shown no difference in VAP rates with either type of humidification system to cast doubt on the favorable HME findings.

Furthermore, the latest study of this issue came out strongly against HMEs.¹ "Patients mechanically ventilated for more than five days had a lower incidence of VAP with a heated humidifier than a heat and moisture exchanger," principal author Leonardo Lorente, MD, PhD, told Pulmonary Reviews. Also, the use of HMEs was a risk factor for VAP caused by gram-positive cocci or gram-negative bacilli and for primary and secondary endogenous VAP, said Dr. Lorente, Attending Physician in the Intensive Care Unit at the University Hospital of Canarias in Tenerife, Spain.

The rates and risk of VAP were compared among 104 critically ill patients who required mechanical ventilation for more than five days. The patients were randomized to receive humidification with a heated humidifier or HME at the time of intubation. The heated humidifiers were set to deliver a temperature of 37¼C and 100% relative humidity at the proximal airway, and the HMEs were changed every 48 hours.

The same measures were taken in both groups to prevent nosocomial pneumonia. These measures included maintaining a closed tracheal suction system, semirecumbent body positioning, continuous enteric nutrition, periodic verification of residual gastric volume, ranitidine prophylaxis for stress ulcers, oral washing with chlorhexidine, no routine ventilator circuit changes, no selective digestive decontamination, and no aspiration of subglottic secretions.

All of the following criteria were required for a diagnosis of pneumonia: new onset of bronchial purulent sputum, a body temperature above 38¼C or lower than 35.5¼C, a white blood cell count of greater than 10,000/mm³ or less than 4,000/mm³, new or progressive infiltrates on chest radiography, and a culture of respiratory secretions by tracheal aspirate showing more than 106 CFU/mL of an infectious organism. VAP was diagnosed when pneumonia was detected after 48 hours of mechanical ventilation.

The heated humidifier and HME groups were not significantly different in terms of sex, age, APACHE II score, use of antibiotic therapy before development of VAP, number of days on a ventilator, or diagnostic group. However, the rate of VAP in the two groups was 15.7% and 39.6%, respectively. "Kaplan-Meier analysis confirmed a significantly lower incidence of VAP in the [heated humidifier] group than in the HME group," the authors related. The median amount of time without VAP was 20 days in the heated humidifier group and 42 days in the HME group, the investigators noted.

A multivariate Cox regression analysis associated HME use with a hazard ratio for VAP of 16.2. VAP incidence in the HME group was similar to the 36% rate found in a previous study by Lorente and colleagues.²

Infections with gram-positive cocci resulted in three cases of VAP in the heated humidifier group and eight cases in the HME group. In those two groups, gram-negative bacilli were responsible for five and 13 cases of VAP, respectively. The gram-positive organism most commonly isolated from patients with VAP was methicillin-susceptible Staphylococcus aureus; Pseudomonas aeruginosa was the most frequently isolated gram-negative bacillus.

The reduced VAP risk with heated humidifiers versus HMEs was due in part to the improved design of the heated humidifier used in the study, which minimized the possibility of exogenous organisms entering the ventilator circuit. In addition, heated humidifiers can deliver higher levels of humidity to the airway, and those levels may facilitate maximal mucociliary clearance of infectious organisms, Dr. Lorente said. HMEs, on the other hand, do not maintain optimal humidification and mucociliary transport after 24 to 48 hours of mechanical ventilation.

—Timothy Begany

Reference
1. Lorente L, Lecuona M, Jimenez A, et al. Ventilator-associated pneumonia using a heated humidifier or a heat and moisture exchanger: a randomized controlled trial. Crit Care. 2006;10:R116 [Epub ahead of print].
2. Lorente L, Lecuona M, Jimenez A, et al. Tracheal suction by closed system without daily change versus open system. Intensive Care Med. 2006;32:538-544.

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