Respiratory gas humidification

= Basics =

Respiratory gas humidification is a method of artificial warming and humidifying of respiratory gas for the patient during mechanical ventilation. By the term respiratory gas conditioning, also the cleaning of respiratory gas is understood in addition to warming and humidification. These three essential functions of respiratory gas conditioning constitute the preparation of inspired respiratory gas for the sensitive lungs. If natural respiratory gas humidification fails, pulmonic infections and damaging of lung tissues may be the consequence.

When a patient is requiring mechanical ventilation over a longer period of time, it is imperative to take measures to compensate the losses of heat and moisture in order to avoid the aforesaid complications. Basically there are two methods available for that purpose: active or passive respiratory gas humidification. Experiments on a combination of both methods have not yet gained any significance in practice.

Active respiratory gas humidifiers

An active respiratory gas humidifier ensures that mechanical ventilated patients are supplied with respiratory gas conditioned to the optimum. In active humidifying processes, moisture and heat is input to respiratory gas, for example by use of electric energy. Performance data and safety-related requirements for active respiratory gas humidifiers are specified by the standard ISO 8185. According to that standard, the minimum water content of inspired respiratory gas is ca. 33 mg/dm³ and the maximum respiratory gas temperature is ca. 42 °C.

Depending on the state of aggregation of water (aerosols or water vapor), active respiratory gas humidifiers are subdivided into nebulizers, evaporators, and bubble humidifiers.
* "Nebulizers"Nebulizers generate aerosols consisting of droplets of various sizes that are admixed to the inspired respiratory gas. There is distinguished between nozzle nebulizers and ultrasonic nebulizers. Since nebulizers carry the risk of overwatering the patient, today the are only still used for the administration of medicamentous aerosols.
* "Evaporators"Evaporators enrich the inspired respiratory gas with water vapor. In a throughflow evaporator, the inspiration flow is lead through a warmed up water bath, in case of a surface evaporator however the inspiration flow is guided along the surface of the water level. Consequently a surface evaporator transports only water vapor and no water droplets into the patient. The advantage of it is, water vapor doesn't carry any germs. Therefore the risk of passing on germs by surface evaporators is a minimum.
* "Bubble respiratory gas humidifiers"In a bubble respiratory gas humidifier, the inspiration flow is guided through a capillary system. In this capillary system warmed up water is circulating. The humidifying capacity of a bubble respiratory gas humidifier is rather slight, however can be improved by increasing the water temperature. A bubble respiratory gas humidifier is mostly used in oxygen therapy with high flow rates via a mask or nasal cannula in order to prevent drying out of mucous membranes in nose and mouth.

Passive respiratory gas humidifiers

Passive respiratory gas humidifiers are independent from external energy sources or external water supply. They function as Heat and Moisture Exchangers (HME) and are placed like an artificial nose between tube and Y piece. There they withdraw heat and moisture from expired respiratory gas, which they resupply to the inspiratory gas at the next following inspiration. Due to significant qualitative differences, only HMEs should be used that ensure effective respiratory gas humidification. Thereby HMEs with high reversible water retention capacity, small internal volume, and low flow resistances are to be preferred.

To enable the absorption of sufficient amounts of water and heat, the expiratory stream of respiratory gas must be fully and thoroughly sifting through the HME. In case of leakages as, for instance, with patients having bronchial fistulas, this precondition is not fulfilled. Also a risk exists with patients having increased secretory production, bleedings, etc. Here clogging up of HMEs may occur. In such cases, the application of active respiratory gas humidifiers is recommend.

References

* W. Oczenski, H. Andel und A. Werba: "Atmen - Atemhilfen." Thieme, Stuttgart 2003, ISBN 3-13-137696-1
* J. Rathgeber: "Grundlagen der maschinellen Beatmung." Aktiv Druck, Ebelsbach 1999, ISBN 3-932653-02-5
* S. Schäfer, F. Kirsch, G. Scheuermann und R. Wagner: "Fachpflege Beatmung." Elsevier, 2005, ISBN 3-437-25182-1
* A. Schulze: "Respiratory Gas Conditioning and Humidification." In: Clin Perinatol, 2007; 34: 19-33, ISSN 0095-5108
* M.P. Shelly, G.M. Lloyd und G.R. Park: "A review of the mechanism and methods of humidification of inspired gases." In: Intens Care Med, 1988; 14:1, ISSN 0342-4642
* F. Kapadia, M. Shelly, J.M. Anthony, et al.: "An active heat and moisture exchanger." In: Br. J. Anaest. 1992; 69: 640-642, ISSN 0007-0912

External links

* http://www.wilamed.com