Infection Issues with Heated Humidification

Physicians frequently express concerns that heated humidifiers may become contaminated and place patients at risk for respiratory infections. Yet despite long term use of heated humidifiers for intubated patients and 4 years of clinical use for patients using nasal CPAP, there is no evidence that patients develop excess numbers of respiratory infections or infections due to unusual organisms.

There are a number of possible reasons for this which are discussed below.

Heat

There is a misconception that hot water humidifiers act as a culture medium so that any pathogens reaching the humidifier chamber encounter an environment that supports rapid growth and multiplication. In fact the environment in the humidifier chamber under most operating conditions is such that the majority of pathogens are rapidly killed. Goularte et al (1) found that 4 species of Gram -ve pathogens and S. aureus were rapidly killed by the conditions found in hot water humidifiers. At 44 degrees Celsius there was a rapid reduction in the number of viable organisms with levels of bacteria reducing 100 - 10,000 fold reduction at 2 hours and by more than 1,000,000 fold after 24 hours. At 50 degrees the number of viable organisms decreased more than 100,000 fold after 2 hours and all organisms were killed by 4 hours. At 37 degrees there was no killing of bacteria but neither was there an increase in numbers. Bacteria in water at normal body temperature can survive but water does not contain the nutrients required for multiplication.

Transport

To cause disease, pathogens require a means of transport from the humidifier chamber to the patient. Old heated humidifiers produced aerosols that were capable of transporting bacteria, but modern humidifiers produce molecular humidity only and molecules of water are too small to transport bacteria or other contaminants. Pathogens reaching the chamber are trapped there. Orec et al found that despite heavily contaminating a humidifier chamber no bacteria were found outside the chamber after 8 hours of operation. Bacteria were found to move around a ventilator circuit by traveling with condensate but could not leave a humidifier chamber as there was no means of transport.

Nasal Defenses

As opposed to intubated patients, those on nasal CPAP have their normal respiratory defenses intact. Any pathogens carried in the inspiratory airflow are subject to the same defenses as during normal breathing. Heated humidifiers do not encourage bacterial growth and do not provide a method of transpon for bacteria, so normal physical defenses are sufficient to prevent respiratory infections.

References

1. Goulart TA, Manning M, Craven D.Bacterial colonisation in humidifying cascade reservoirs after 24 and 48 hours of continuous mechanical ventilation. Infection Control 1987 Vol 8. No 5.

2. Orec R, Richards GN, Cornere B,Dove B, Morris A. Movement of bacteria contaminants is reduced in heated ventilator circuits Respiratory Care (Abstract) 1995 AARC