(Chest. 2005;128:2138-2140.)
© 2005
American College of Chest Physicians
Sterile Water Is Unnecessary in a Continuous Positive Airway Pressure Convection-Type Humidifier in the Treatment of Obstructive Sleep Apnea Syndrome*
Markus Wenzel, MD;
Matthias Klauke;
Frank Gessenhardt;
Dominic Dellweg, MD;
Peter Haidl, MD;
Bernd Schönhofer, MD and
Dieter Köhler, MD
* From Krankenhaus Kloster Grafschaft (Drs. Wenzel, Dellweg, Haidl, and Köhler, Mr. Klauke, and Mr. Gessenhardt), Zentrum für Pneumologie, Beatmungs- und Schlafmedizin, Schmallenberg; and Klinikum Hannover (Dr. Schönhofer), Oststadt, Hannover, Germany.
Correspondence to: Markus Wenzel, MD, Annostr. 1, Krankenhaus Kloster Grafschaft, Zentrum für Pneumologie, Beatmungs- und Schlafmedizin, D-57392 Schmallenberg, Germany
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Abstract
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Objective: We investigated the necessity of using sterile water in humidifiers for avoiding respiratory tract infections during nasal continuous positive airway pressure (nCPAP) therapy.
Methods: Water in a convection-type humidifier (Sirius; Heinen and Löwenstein GmbH; Bad Ems, Germany) was labeled with 99mTc-diethylenetriamine penta-acetic acid. Low-flow (2 L/min, 4 L/min, or 6 L/min) and high-flow (31 to 46 L/min) rates were applied, rates typical for nCPAP. Heat and moisture exchange filters were placed behind the start of the tube to measure any radioactive aerosol.
Results: We demonstrated that no radioactive aerosols were produced, either with low or high flows.
Conclusions: The convection-type humidifier produces water vapor but does not aerosolize the water. We conclude that bacteria, other microorganisms, or even solutes that may be contained in the water cannot be transported into the air and thus will not be deposited in the lung. In order to avoid respiratory tract infections, sterile water is not required, at least in this particular humidifier. We suggest that nonsterile tap water is probably a safe alternative.
Key Words: humidifier • nasal continuous positive airway pressure • nonsterile water
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Introduction
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Sterile water is commonly used to fill humidifiers during nasal continuous positive airway pressure (nCPAP) use. This is based on the assumption that a contaminated chamber would increase the risk of respiratory tract infections, as bacteria might be aerosolized during therapy.1234 This assumption is untested, and patients and physicians often wonder if tap water is a safe alternative. We decided to test the hypothesis that water contaminants are aerosolized with such humidifiers. We used radioactively labeled contaminant as a surrogate for bacteria and measured its deposition on a filter placed at the end of the circuit. Our study only tested an nCPAP humidifier with a convectional air stream flow over the surface of the water, and not a "bubbling-through" humidifier, which is known to aerosolize water contents.3
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Materials and Methods
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As a first step and as positive control, we examined the efficiency of the filters and their ability to collect aerosolized radioactive particles. We used a nebulizer (PARI TurboBOY; PARI; Starnberg, Germany) and two heat and moisture exchange (HME) filters (Type BB 50, Pall Biomedical Corporation; East Hills, NY) attached in sequence at the mouthpiece as filters (Fig 1
).
The nebulizer was filled with 400 MBq 99mTc-diethylentriamine penta-acetic acid (DPTA) in 5 mL of water. The size of aerosols being produced by this nebulizer has a mass median aerodynamic diameter of 3.5 µm with a geometric SD of 2.8. We nebulized the entire liquid and afterwards measured the activity of the filters with a gamma camera (type Dyna 4/15, with software GMS-586, version 2.1; Gaede Medizinsysteme; Freiburg, Germany). For determination of zero activity and lowest detection, we made 10 native measurements each of 10 minutes.
Next we tested our hypothesis that particulate matter in the water could be aerosolized by the nCPAP humidifier. The water in the humidifier chamber (Sirius; Heinen and Löwenstein GmbH; Bad Ems, Germany) [Fig 2
] was radioactively labeled (400 mL with 400 MBq 99mTc-DTPA). In this humidifier, the air stream flows convectionally over the surface of the water and does not create bubbles. At the beginning of the tube, we put two HME filters (Fig 3
).
Because this type of humidifier is sometimes used to humidify oxygen, we first tested the system with low oxygen flow (2 L/min, 4 L/min, and 6 L/min). Next, we tested it with two different settings of high airflow (with nCPAP): (1) at a pressure of 10 millibars (mbar) with a resulting flow of approximately 31 L/min, and (2) at a pressure of 20 mbar with a resulting flow of 46 L/min. Seven humidifiers were tested at each flow rate and running time of 10 min. The radioactivity of the filters was measured with a scintillation camera.
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Results
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The positive control with aerosolized 99mTc-DTPA showed that downstream of the nebulizer, the first of the two HME filters collected all of the aerosols; the second filter did not collect any radioactivity. The measurement of background activity and lowest detection at 10 times for 10 min shows as mean value of 0.11 counts per minute per pixel (5,952/52,200) with an SD of ± 0.005. After subtraction of background, no radioactivity was detected on the filters that tested the humidifier, either at low-flow or at high-flow rates (Table 1
).
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Discussion
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Our results suggest that the Sirius convection-type humidifier (and possibly other devices that work by air convection) does not aerosolize solutes contained in the water. This is not a surprising finding, since it has been previously shown that water vapor contains no soluble ingredients.567 Since the solute 99mTc-DTPA is of much smaller size compared to bacteria, it is highly unlikely that bacteria would become aerosolized by this method.
Demers8 points out that there are some bacteria of very small size, for example, Pseudomonas diminuta, with a size of 0.7 x 0.3 µm, that are able to pass many different filters. The Pall Medical Corporation type BB 50 filters we used are tested by the European Norm EN 13328 for filters in mechanical ventilation (International Committee for Standardization; Brussels, Belgium). They are able to capture particles not only by the size of P diminuta but also bacteriophage MS-2 with 0.02 µm with an efficiency of > log 5.9
Concerning fungi and molds, it is remarkable that their spores usually have a size of several micrometers, the mycelia even larger so that crossing into the air is even more impossible than bacteria. We want to mention that our data are only valid for patients with an intact immune system.
Aerosols have to be generated with high active energy, eg, with a nebulizer. This also holds true for humidifiers that act by bubbling air through the water chamber, as this has been shown to aerosolize particles contained in water.6 Regarding sedimentation of calcium carbonate, we have had good experience, insofar as regular cleaning of the humidifier by the patients offers satisfying prevention, even in regions with high amounts of calcium carbonate in the tap water.
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Conclusion
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We conclude that bacteria present in the water of a convection-type humidifier cannot become airborne as a primary aerosol during nCPAP therapy. It is our opinion that it is not necessary to use sterile water in such humidifiers in order to avoid respiratory tract infections, and that tap water is a safe alternative.
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Acknowledgements
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We thank G. Oberdörster, MD, University of Rochester, NY, for helpful review and comments.
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Footnotes
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Abbreviations: DTPA = diethylentriamine penta-acetic acid; HME = heat and moisture exchange; mbar = millibar; nCPAP = nasal continuous positive airway pressure
Received for publication April 5, 2004.
Accepted for publication April 25, 2005.
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References
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Abstract
Introduction
