Heated Humidifiers
Risks and Concerns Over
Contamination and Infection
Many people frequently express concerns that heated humidifiers may become contaminated, placing the patient at risk for respiratory infection. Yet despite long-term use of heated humidifiers for intubated patients and years of clinical use for patients using nasal CPAP, there is little or no evidence that patients develop numbers of respiratory infections or infections due to unusual problems.
There are a number of possible reasons for this, which are discussed below.
Heat
There is a common 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 chamber, under most operating conditions, is such that the majority of pathogens are rapidly killed.
Goularte et al1 found that four 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 two hours and by more than 1,000,000 fold after 24 hours. At 50 degrees Celsius, the number of viable organisms decreased more than 100,000 fold after two hours, and all organisms were killed by four hours.
At 37 degrees Celsius, 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. Most modern heated humidifiers produce molecular humidity only, and molecules of water are too small to carry bacteria or other contaminants. Pathogens reaching the chamber are trapped there.
Orec et al2 found that despite heavily contaminating a humidification chamber, no bacteria were found outside the chamber after eight hours of operation. Bacteria were found to move around a ventilator circuit by traveling with condensate, but could not leave a humidification 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 humidification chambers do not encourage bacterial growth, and do not provide a method of transport for bacteria, so normal physical defenses are sufficient to prevent respiratory infections, the two studies conclude.
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.
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