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Individualized respiratory gas conditioning.

Between dogmas, secretion, and excessive formation of condensation.

The development of HME filters (heat and moisture exchanger) and, in turn, the possibility of passive respiratory gas conditioning almost 50 years ago sparked a kind of philosophical war on the type of respiratory gas conditioning.

In favor of active respiratory gas conditioning.

The advocates of active respiratory gas conditioning point to the contraindications of the HME solutions and cite the negative effects during ventilation in relation to a reduction of the alveolar ventilation with additional dead space, additional flow resistance, and increased respiratory effort in the weaning process.

In favor of passive respiratory gas conditioning.

The supporters of passive respiratory gas conditioning cite an increased rate of pneumonia associated with ventilation due to frequent manipulation of the breathing tubes as a downside of the active respiratory gas conditioning. In addition, this technology is much more expensive and technically more complicated than the HME filter technology.

There is, however, general consensus regarding the need for respiratory gas conditioning. In the case of intubated and tracheostomized patients, the upper airways are bypassed. Consequently, the nasal, oral, and pharyngeal cavities cannot fulfill their physiological task of cleaning and heating the inhaled air.

As the nasal mucous membrane is well perfused and moist, the inhaled air is warmed and humidified by evaporation and convection. Condensation causes moisture during exhalation that is reabsorbed and stored. During the next inspiration, the stored moisture can be released. Cold and dry gases absorb a large portion of the existing heat and moisture. This can cause a substantial moisture imbalance in the airways and substantially impair respiratory gas conditioning.

Situation with mask ventilation and high-flow O₂ therapy.

The aforementioned problems occur even with mask ventilation and high-flow O₂ therapy (HFOT). Although the upper airways (nasal, oral, and pharyngeal cavities) are not eliminated, the high gas flows and the system and mouth leakages regularly cause the airways to dry out. In the case of HFOT, the continually high respiratory gas flow causes a similar effect. Even after a short time, the mucous membranes dry out, disrupting the mucociliary clearance function with demonstrable histological damage to the mucociliary and epithelial cells, thus promoting bacterial colonization.

The blocking of the tracheal cannula or main bronchi by viscous secretion constitutes a much-feared complication of the ventilation therapy. As a result, the ventilation can be severely compromised and requires rapid intervention, through aspiration for instance. The respiratory gas conditioning that heats and humidifies the administered respiratory gases is designed to maintain the mucociliary clearance and prevent damage to the cilia.

Active respiratory gas conditioning.

Active respiratory gas conditioning uses surface humidifiers in many cases. The mixture of gas for inspiration is directed over a heated water surface and saturated with heat and water vapor in the process. The aim is to achieve a respiratory gas temperature below the tip of the tube of almost 37 °C. The requirements for active respiratory gas conditioning systems have been specified as performance data in a binding standard since 2009. In accordance with the standard, the water content of the inhaled air must be at least 33 mg/l and the maximum inspiration temperature must not be more than 42°C. The individual setting of the active respiratory air humidifier must take into account both the bronchial secretion situation of the person being ventilated and the condensation in the circuit. Ambient factors such as room temperature, direct sunlight, heat output of other devices, and the placement of the respiratory air humidifier directly next to a radiator or air-conditioning unit also influence the amount of liquid in the ventilation circuit.

Starting at a certain quantity of condensation, the flow resistance in the circuit is increased and so too the respiratory effort of the spontaneously breathing patient. In an extreme case this can lead to devicerelated malfunctions of the ventilator. To prevent this, integrated tube heating is used. As a result, the moisture is transported across the entire length of the tube without significant temperature loss. This prevents the gas in the circuit from cooling down and no significant condensation is generated.

Condensation often collects in unheated circuits. The water is removed regularly by draining intermediate “water traps” in the tube.

The frequent manipulation of breathing tubes was recognized in the 1990s as the main cause of a higher rate of pneumonia. With modern devices used for active respiratory gas conditioning, there is no evidence of ventilating-associated pneumonias.

If the patient is ventilated with dry and warm respiratory gases due to the failure to top up the water, reference is made to the “Sahara effect,” which damages the epithelium. To identify this phenomenon in good time, modern respiratory air humidifiers have a water shortage alarm. Just like excessively dry respiratory gases, excessively moist respiratory gases compromise the person being ventilated. The effects range from a reduction of the mucociliary clearance function, changes to the surface of the mucous droplets, to undesired washing away of contaminated secretion from the upper tracheal area into the deep lung, which may impair gas exchange and cause infections.

Passive respiratory gas conditioning.

Passive respiratory gas systems (sometimes called “artificial noses”) are often described as a heat and moisture exchanger (HME). They remove heat and moisture from the patient’s exhaled air, store it reversibly in the inner material, and feed it into the dry respiratory gases with the next inspiration. At the same time, they act as an antimicrobial barrier for microorganisms. The use of tube heating and the prevention of condensation with the earlier need of frequent draining of the water traps eliminates the hygienic benefit of HME filters.

Modern respiratory air humidifiers allow personal settings of the temperature profile. This also substantially reduces the amount of condensation generated. Due to the design of an HME filter and the associated shelf life, use with long-term ventilation is contraindicated. Likewise, the use of HME filters must be seen as potentially critical in the case of acute respiratory insufficiency. The increased additional anatomic dead space reduces CO₂ washout and alveolar ventilation , which has been shown to increase mortality in ARDS. Ventilation with lung-protective parameters also proves more difficult as a result. An increased secretion load and tracheobronchial bleeding constitute exclusion criteria for the use of an HME filter. The increase in respiratory effort renders usage counterproductive in the context of more difficult weaning. Quality differences of the various manufacturers have an influence on ventilation.

The greater flow resistance of the HME filter substantially increases the patient’s required respiratory effort. HME filters with flow resistances of less than 2 mbar with a flow of 60 liters per minute are ideal.

Given the current data available, there is no clear recommendation for or against the usage of passive or active systems. Rather, there is a need to estimate the planned usage time, the current situation of the lung, and possible contraindications of HME filters. Such systems eliminate the need to simultaneously use bedside filters in combination with active humidification.


The Individualized respiratory gas conditioning is a key component in ventilation therapy. Thanks to technical innovations, active respiratory air humidifiers have become established over the past few years. The setting of individual temperature profiles, tube heating, and the alarm technology put the major criticisms of these systems into perspective.

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