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The conditioning of the gases in mechanically ventilated patients
Short Review Istituto di Anestesia e Rianimazione, Universita' degli Studi di Milano, Ospedale Maggiore Policlinico-IRCCS, Milano, Italy; 2Dipartimento di Scienze Cliniche e Biologiche, Universita' degli Studi dell'Insubria, Varese, Italy; 3Cattedra di Anestesia e Rianimazione, Universita degli studi di Brescia, Spedali Civili, Brescia, Italy


When the nose and the upper airways are bypassed due to a tracheostomy or endotracheal tube, the normal function of heating and humidifying of the inspired gases is alterated. Therefore in these situations a correct heating and humidification of inspired gases is mandatory. The goal of any heating and humidification system is to provide inspired gases with a water content similarly to that usually provided by the nose or the upper airways.


The water present as vapor in a gas mixture is defined humidity. The maximal amount of water vapor that can be held by a gas mixture directly depends on the temperature of the gas1. When a gas mixture holds all the water vapor that is capable is defined "saturated".

AH is the mass of water vapor held in a given volume of gas at a particular temperature (mg/L or g/m^3) (i.e, at 100% of RH for 32°C AH is 36 mg/L while for 37°C AH is 44 mg/L).

RH is the ratio between the actual amount of water vapor and the maximal capacity of water vapor for the same temperature (i.e., at 32°C with 50% of RH AH is 18 mg/L).


Not intubated patients breathing ambient air

The standard ambient air presents a temperature of 22°C, with RH of 50% and AH of 10 mg/L while the alveolar air usually presents a temperature of 37°C, with RH of 100% and AH of 44 mg/L. During inspiration the gases are progressively heated and humidified along the nose and the upper airways, until they are fully saturated at the body temperature. In table 1 are presented the different values of temperature, RH and AH of the inspired and expired gases in not intubated patients breathing ambient air.

The point of the airways in which the inspired gases reach the body temperature (i.e., 37°C) and 100% of RH is called the Isothermic Saturation Boundary (ISB)1. Normally the ISB is 5-6 cm below the carina and after this point the inspired gases do not further change temperature and humidity.

The position of the ISB can change according to the volume, temperature and AH of the inspired gases2. The ISB never reaches the bronchioles or alveoli in physiological conditions3. However during severe hyperventilation in extreme cold and dry conditions the ISB can shift towards alveoli4.

The difference between the alveolar and ambient air water content is called the humidity deficit1. Usually the humidity deficit above the carina (i.e, carina AH minus airway opening AH) is around 27 mg/L while the humidity deficit below the carina (i.e., alveoli AH minus carina AH) is around 7 mg/L. This suggests that the majority of humidification during inspiration is provided by the upper airways.

During expiration, the gases leaving the alveoli are cooled and loose heat and humidity until they reach a temperature of around 33°C maintaining 100% of RH (i.e, AH of 37 mg/L).

Intubated patients breathing medical gases

Contrary to common knowledge, the inspired medical gases are not cold and fully dry. In fact we have evidence that the temperature of the medical gases delivered by mechanical ventilation are usually around 20-25°C. However the AH ranges between 3 and 10 mg/L and inversely dependents on the inspired oxygen concentration (FIO2) (i.e., for FIO2 of 21% AH is 5.2±1.2 mg/L, for FIO2 of 50% AH is 4.1±0.9 mg/L and for FIO2 of 100% AH is 3.6±0.6 mg/L) thus the real problem with the medical gases is not the temperature but the poor content of moisture, strongly dependent on the FIO2 used4.

In the intubated patients breathing not conditioned medical gases, at the carina are present similar levels of temperature compared to physiological conditions but with a marked reduction in RH (table 2). This is due to the fact that the endotracheal tube allows the heat exchange but is not able to provide more than 3-4 mg/L of AH as previously condensed during the expiratory phase.



In this condition the ISB is markedly shifted towards the alveoli. In fact in intubated patients there is a reversal in the humidity deficit compared to not intubated patients. The upper carina humidity (i.e., endotracheal tube plus lower trachea) deficit is around 5 mg/L while the lower carina humidity deficit is around 34 mg/L. This suggests that the majority of humidification during inspiration is provided by the lower airways, which are not physiologically appropriated to conditioning the gases.

Thus, the most important problem in intubated patients breathing not conditioned medical gases is not the heat and water losses, but on the contrary, the shift of ISB towards anatomical zone not specified to condition the inspired gases.


Heat loss

The heat loss from the upper airways is due to the increase of temperature and humidity of the inspired gases passing through the airways. The air has a low specific heat (1998 J/Kg) compared to the heat of vaporization for water (2450 J/Kg), so most of the heat lost from the upper airways is used to humidify the inspired gases5. This heat loss may decrease the body temperature. So in all the situations in which there is an impairment of the body thermoregulation, such as prolonged surgery6,7, critically ill patients8, an adequate conditioning is strictly necessary to avoid further heat loss.


Moisture loss

The moisture loss, beside to cause a substantial loss of water from the airways9, causes a dehydration of the nasal and the trachebrochial mucosa10. The tracheobronchial mucosa is more sensitive than the nasal mucosa to dehydration and just 10 minutes of ventilation with dry gases, are sufficient to damage the cilia function9. The most important damages are the impairment and destruction of the mucociliary activity11, the reduction of mucous production with an increase in viscosity12 and a difficult to cough or to expectorate13.


Heat gain

Beside to the heat gain due to the higher inspired gas temperature compared to the body temperature, side effects such as thermal injury14 or airway burns can develop15. It has been shown that thermal injury develops when the tracheal temperature is above 40°C16. So it is suggest, to always deliver gases at a temperature less than 40°C17.

Moisture gain

Breathing overhumidified gases will cause a water deposition in the airways, and can induce cellular damage5. Furthermore this water deposition may mechanically obstruct the small airways leading to alveolar collapse16 and inactivate the pulmonary surfactant leading to alveolar collapse18.


The heat and humidity of any gas delivered to a patient should have the same inspiratory characteristics occurring, physiologically, at the level of entry into the respiratory system, to correct the humidity deficit and to avoid the risk of an inadequate or excessive conditioning17.

There are different humidification standards for not intubated and intubated patients breathing medical gases. In not intubated patients a minimum level of AH of 10 mg/L has been suggested19 while a minimum of 3019 to 33 mg/L20 of AH has been proposed for intubated patients.

In order to choose the adequate conditioning of the inspired gases we reasoned following the physiological data of the inspiratory and expiratory phase (tables 1, 2). As shown before, in the intubated patients the AH delivered by the tube itself is around 3-4 mg/L and the remaining part of the trachea up to the carina can deliver another 1 mg/L. Thus the AH delivered by the endotracheal tube and the lower trachea up to carina, is around 5 mg/L. In order to reach the physiological AH of 37 mg/L at the carina (table 1), around 32 mg/L of H2O should be reached at the tip of the tube (27 mg/L directly by water of humidifier and 5 mg/L by the medical gases). This means that a mixture of gases with a temperature of 31-32°C with an RH of 100% (i.e., AH of around 32 mg/L) should be given at the tip of the tube. The amount of water actually given by the humidifier inversely depends on the AH of the medical gases (i.e., if the medical gases have AH of 3 mg/L the humidifier will provide 29 mg/L while if the medical gases have AH of 10 mg/L the humidifier will provide 22 mg/L).

Since the catheter mount causes a drop in gas temperature, between 1-2°C, the temperature at the Y piece of the ventilator circuit should be around 33-34°C with an RH of 100%. Using this setting (endotracheal tube plus the humidifier) the AH that we give to the patient (i.e., 37 mg/L) is comparable to the amount of AH expired (i.e., 37 mg/L).

In our opinion, heating and humidifying the inspired gas to 37°C and 100% of RH (AH 44 mg/L) as recently proposed21 is absolutely uncorrected and possibly dangerous, because in this way is given an amount of AH greater than the real necessities leading to a fluid overload in the airways of about 70-100 ml per day, which could cause epithelial or alveolar damage or bronchial irritation16,17.


The ideal humidifier should enclose the properties listed in table 3.



Heat and moisture exchangers

The passive humidifiers include the heat and moisture exchanger (HME) and the HME plus an antimicrobiological filters (HMEF)1.

The heat and moisture exchangers (HME-HMEF) collecting the heat and moisture of the expired gases "passively" heat and humidify the inspired gases during the successive inspiration.

Several factors may influence the gas conditioning performance of HME-HMEF during mechanical ventilation: 1) type of HME-HMEF (hydrophobic or hygroscopic-hydrophobic); 2) patient temperature and ambient temperature; 3) ventilatory settings (very high or very low tidal volume, minute ventilation, and inspiratory flow).

Type of HME

The initials models of HME were purely "hydrophobic" filters; being water retention only a physical phenomenon. The expired gases pass through a cool exchanger, which provokes a condensation on the HME surface. This happens because a gradient of temperature is created between the two sides of the device, determined by the temperature of the expired gases and the ambient temperature.

The effectiveness of this type of HME is quite good to heat the inspired gases but not to provide optimal humidification (AH between 22-25 mg/L)22,23.

So a new version of HME "hygroscopic-hydrophobic" was developed in which a hygroscopic unit actively binds the water molecules present in the expired gases, increasing the water content of the inspired gases and ameliorating the humidity added to the inspired gases24.

Many studies evaluated the performance of HME in critically ill patients. The initial studies using hydrophobic HMEs, reported an increase in tracheal tube occlusions using25,26. Subsequently several studies showed no significant differences in tube occlusions between HME and conventional humidifier when hygroscopic-hydrophobic HME was used22,23,27,28.


Since HME works due to a gradient of temperature, any gradient variation (i.e., patient or ambient) will increase or decrease its performance26,29

Ventilatory setting

Many studies clearly showed that the hygroscopic-hydrophobic HME provided better humidification compared to hydrophobic HME22,23,27,28. Moreover several studies demonstrated that the hygroscopic-hydrophobic HMEs maintained adequately humidifing capacities also at high tidal volume and minute ventilation (up to 10 L/m)25,30. On the contrary hydrophobic HMEs markedly decrease their performance at high minute ventilation.

HME and respiratory mechanics

Because the HME is placed between the Y piece of the ventilator circuit and the endotracheal tube or tracheostomy can affect the airflow resistance31 and increase the dead space32. The airflow resistances can increase with the clinical use31 moreover in the presence of copious amount of secretions33 however dynamic hyperinflation does not develop34. The additional dead space (i.e., range 50-100 ml) can increase the minute ventilation32 and the respiratory work35.

The use of an HME by increasing the inspiratory work, could affect the outcome of weaning trials in weak patients36. However it is possible to reduce the added respiratory work by the HME simple increasing the level of mechanical assistance35.


HME and antimicrobiological activity

Besides HME, having a bacterial barrier effect (efficiency > 99.99%) keeps the ventilator circuit clean and free of condensate and reduces the incidence of ventilator circuit colonization37. However the HME does not reduce the tracheal colonization and the ventilator associated pneumonia37-39.

The manufactures recommend that HME be changed every 24h, although many studies showed that the hygroscopic-hydrophobic HME could be changed every 48 h without any adverse mechanical or microbiological effects40,41. Moreover a recently study demonstrated that the same HME could be safely used for 7 continuos days of mechanical ventilation in all ICU patients except for COPD42.

Hot humidifiers

Hot high flow humidifiers (HH) are able to conditioning the inspired gases with RH of 100% at temperatures similar to body temperature by heating a water bath1. The temperature set on the HH falls along the ventilator circuit and the AH reaching the patient's airway will be lower than expected, so a temperature probe at the Y piece is essential. The thermistor has a slow response and reflects the mean temperature of the inspired gases. So during mechanical ventilation the temperature of the inspired gases reaching the patient, fluctuates around the preselected value.

At the present, the commercially available HH are the passover, cascade, wick and vapor phase humidifiers43. The simpler HH is the passover, in which the inspired gases pass over a heated water bath. The cascade is a "bubble humidifier", in which the inspired gases passed beneath the surface of the water reservoir and bubble upward through a grid. The wick is similar to the cascade humidifier, but the inspired gases pass through a cylinder that is lined with a wick of blotter paper. The base of the wick is inserted in the water. The moisture heated wick increases the RH of the inspired gases. With the vapor phase humidifier the water is heated and the water vapor penetrates through a hydrophobic filter to humidify the inspired gases.

With all these devices is very important to have a stable and adequate level of the water in the reservoir, to minimize the compressible gas volume and to avoid the temperature fluctuations. To overcome these problems and also to reduce the risk of contamination the new HH have a closed system that maintain stable the level of water in the reservoir.

It is important to remind that during the continuous flow CPAP, because is used a bias flow rate up to 100 L/m, the time of contact between the inspired dry gases and the humidifying elements is reduced and a good level of conditioning can be difficult to be reached44.

Due to the higher temperature of gases leaving the HH, when passing through the ventilator circuit, condensation will occur. This condensate in the circuit can be a reservoir of nosocomial infection37.




Both HME and HH have advantages and disadvantages (table 4).

Recently new solutions were developed in order to produce new humidifiers able to join the advantages of HME and HH and avoid their disadvantages. In other words the goals were: 1) to limit the water air contact, 2) to avoid condensation in the ventilator circuit, 3) to reduce the cost.

The possible commercially available solutions are: a new cartridge with a new air water interfaces (Dar HC 2000); the heated ventilator circuit; the passive-active humidifier.

Dar HC 2000

This system is an active humidifier, that different from HH humidifier because the inspired gases are conditioned by using the water vaporization principle instead of pass over a heated water bath. A hydrophobic Gore-Tex membrane is the interface between the water and the vapor dividing the cartridge in two spaces. In the inner one the inspired gas flows while in the outlet one there is the water bath. The main advantages with this system are: no direct contact between water and air, likely reducing contamination and the reduction in gas compressible volume which makes this system very reliable in pediatric settings. The main disadvantages are that the efficiency can be reduced especially when high minute volume (greater than 15 L/m) are delivered and the relatively high cost.

Heated-ventilator circuit

To prevent the condensation and to give a more stable conditioning, a heated ventilator circuit (HC) was developed45. There are two available heating ventilator circuits. The first heats by using an internal resistance in direct contact with the inspired gases. The second heats the inspired gases by a wire inserted inside the wall of the ventilator circuit without any direct contact with the inspired gases. Both systems seem not particularly affected by the ventilatory settings. However, if the gases passing through the HC are over heated, i.e., above the temperature of the gases leaving the HH, the RH is reduced and secretions in the endotracheal tube and in the airways can be dried causing dangerous gases obstruction.

New passive-active humidifier

The most effective passive system can maximum deliver to the patients 80-85% of the expired humidity and heat. Recently a new passive-active humidifier has been proposed (Humid-Heat Gibeck, HME-BOOSTER Tomtec). They consist in a conventional HME with an additional heating element and water supply. Thus it is combined the efficiency of a HH with the simplicity of a HME46. The possible advantages are the stability of gases conditioning independently from patient temperature, ambient temperature and ventilatory setting, the absence of condensation and thus the possible decrease of ventilator associated pneumonia. The possible disadvantages are the fixed inspiratory temperature at 37°C and 100 RH (Humid-Heat) or the restricted amount of water supply between 2-5 ml per hour (HME-BOOSTER).

The development of a new passive-active humidifier is in progress (Perfomer StarMed). It consists of a HME containing a heating element and the possibility of an additional water supply. With this system is possible to modify the temperature of the heating element and the water supply to obtain an adequate conditioning of the inspired gases in every conditions (patient, ambient, ventilatory setting).


In our opinion an adequate gas conditioning is mandatory in the clinical management of mechanically ventilated patients. Nowadays, different humidifiers are commercially available with different advantages and disadvantages. A correct knowledge of the basic physiological principles, regulating the heat and moisture exchange, is required to choose the appropriate humidifier system in each individual patient and clinical environment.



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