Respiratory gas humidification

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Respiratory gas humidification is a method of humidifying the breathing gas in mechanically ventilated patients. In addition to humidification, the term breathing gas conditioning also includes heating and cleaning of the breathing gas. These three essential functions of breathing gas conditioning serve to prepare the inspired breathing gas for the sensitive lungs. If the natural breathing gas conditioning fails to take place, pulmonary infections and damage to the lung tissue can result.

If a patient is ventilated over a longer period of time, measures must be taken to compensate for the loss of heat and moisture in order to avoid such complications. There are basically two methods available for this: active or passive respiratory gas humidification. Experiments to combine both methods have not yet achieved any practical importance.

Active respiratory gas humidifiers

An active respiratory gas humidifier ensures that mechanically ventilated patients are supplied with optimally conditioned respiratory gas. In the active humidification process, moisture and heat are supplied to the breathing gas using, for example, electrical energy. According to the standard ISO  8185 the performance and safety requirements for active humidifier are fixed. Thereafter, the minimum water content are of the inspired breathing gas at 33 mg / l and the maximum breathing gas temperature at 42  ° C .

Depending on the physical state of the water ( aerosols or water vapor ), the active respiratory gas humidifiers are divided into nebulizers, evaporators and bubblers.

Nebulizer

Nebulizers generate aerosols in different droplet sizes that are added to the inspired breathing gas. A distinction is made between nozzle nebulizers and ultrasonic nebulizers. Since nebulizers pose the risk of overhydrating the patient, they are only used today to administer aerosols for medication.

Evaporator

Evaporators enrich the inspired breathing gas with water vapor. While with the flow-through evaporator the inspiration flow is passed through a heated water bath, with the surface evaporator ("Drawover humidifier") the inspiration flow is guided along the water surface. As a result, the surface evaporator only transports water vapor and no water droplets to the patient. This has the advantage that water vapor does not transport germs. The risk of germ transmission is minimal with the surface evaporator.

A new type of evaporator is the countercurrent evaporator, which works on the natural principle of the nasopharynx. Since the air is passed over a large, damp surface, which is at body temperature, the air is always optimally heated and humidified. Normal surface evaporators, on the other hand, have to work with overheating of the water, which means that the humidification cannot be optimal, especially with the fluctuating gas flows of today's forms of ventilation. In addition, due to the overheating during brief interruptions in ventilation, there is a risk of a so-called hot shot, that is, a brief very hot and humid amount of air, which represents a risk for the patient.

Bubbly

With the bubbler (principle of the continuous evaporator, English "Bubble through humidifier") the inspiratory flow is directed through a capillary system. Heated water circulates in the capillaries. The humidification capacity of the soda maker is only small, but can be improved by increasing the water temperature. A bubbler is mostly used for oxygen therapy with high flow via a mask or nasal probe to prevent the mucous membranes from drying out or blockages in the nose and mouth.

Passive humidifiers

Passive humidifiers, also known as Heat and Moisture Exchangers (HME) , are independent of external energy sources or an external water supply. They work as heat and moisture exchangers (special form of surface evaporator) and are placed as an "artificial nose" between the tube and the Y-piece . There they withdraw both heat and moisture from the exhaled air and supply both again when inhaled. Due to significant qualitative differences, only HMEs that guarantee effective respiratory gas humidification should be used. HMEs with a high reversible water retention capacity, low internal volume and low flow resistance are preferred.

In order to absorb enough water and heat, the HME must be completely permeated by the expiratory breathing gas flow. In the case of leaks, such as those that occur in patients with bronchial fistulas, this requirement is not met. There is also a risk in patients with increased secretion production, bleeding, etc. Here the HME can be relocated. In these cases, the use of active respiratory gas humidifiers is recommended.

literature

  • W. Oczenski, H. Andel, A. Werba: Breathing - breathing aids. Thieme, Stuttgart 2003, ISBN 3-13-137696-1 .
  • J. Rathgeber: Fundamentals of mechanical ventilation. Aktiv Druck, Ebelsbach 1999, ISBN 3-932653-02-5 .
  • S. Schäfer, F. Kirsch, G. Scheuermann, R. Wagner: Specialist care ventilation. Elsevier, 2005, ISBN 3-437-25182-1 .
  • A. Schulze: Respiratory Gas Conditioning and Humidification. In: Clin Perinatol. 34, 2007, pp. 19-33, ISSN  0095-5108
  • MP Shelly, GM Lloyd, GR Park: A review of the mechanism and methods of humidification of inspired gases. In: Intens Care Med. 14, 1988, p. 1, ISSN  0342-4642
  • F. Kapadia, M. Shelly, JM Anthony et al .: An active heat and moisture exchanger. In: Br. J. Anaest. 69, 1992, pp. 640-642, ISSN  0007-0912
  • W. Mauritz, K. Steinreadyhner: Humidification of the breathing gas, physical therapy. In: J. Kilian, H. Benzer, FW Ahnefeld (ed.): Basic principles of ventilation. Springer, Berlin a. a. 1991, ISBN 3-540-53078-9 , 2nd, unchanged edition, ibid. 1994, ISBN 3-540-57904-4 , pp. 304-313.