It is released again by condensation which occurs when expired air flows over the cooled mucosa. After all the effort and energy expended on warming and wetting the inspired gas mixture, it makes some sense that the upper respiratory tract should try to reclaim some of the water and heat.
It is less efficient at this dehumidification than it is at humidification. Cole inserted a thermister up the nose of an undescribed "subject" and described the temperature of the escaping breaths. This exchange occurs because the air, coming in through the airways, cools their mucosal surface; on the way out the air is warmer than the surrounding mucosa, and donates some of its heat back to the walls of the airways. Some heat and moisture are reclaimed thereby. The rest is lost from the organism, which seems wasteful but is in fact a minimal contributor to the total body heat loss.
Davis' Basic Physics and Measurement in Anaesthesia p. The recondensation of heat and water is probably more important for species which are constantly exposed to perverse conditions.
In these animals, reclaimed heat amounts constitute a savings of The respiratory tract is extremely efficient at heating and humidifying inspired gas, even in the face of very low ambient temperatures. Of the dogs who were not sacrificed for histology, the rest were "lively, happy and not significantly disturbed by the experimental procedure" even after breathing the superchilled gas or up to minutes.
Of course, for obvious reasons , the Australian Intensive Care trainee will probably be more interested in the other temperature extreme. For this, we also have some data, again from earlier in the 20th century.
The hot air on the way into the lungs would therefore have warmed the respiratory mucosa, preventing the normal expiratory condensation of dew from taking place. The reclamation of moisture, therefore, suffers in hot conditions. Even though the diagram below was borrowed from Schmidt-Nielsen et al who were discussing the cactus wren , it can still be used to illustrate the change in the amount of water recovered from respiratory gases as the ambient heat increases.
In short, it is halved between 15 and 30 degrees. This makes sense: the hotter it gets the more moisture you lose. The mechanisms responsible for humidification of inspired gas are made all the more remarkable by their extreme resilience against any variation in respiratory function. Instead, the isothermic boundary normally about 5cm from the carina just moved deeper into the lung.
Theoretically, the humidification functions of the conductive airways continue throughout the generations of bronchi all the way up to the respiratory bronchioles. As ventilation increases or the temperature of inspired gas decreases, the isothermic boundary moves further and further into the lung. This factoid is repeated extensively throughout the literature eg. Because we respect your right to privacy, you can choose not to allow some types of cookies.
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Failure to provide humidified inspired gases can lead to complications and patient harm. Systems designed to humidify inspired gases can be described as active or passive and choice of device will depend on clinical scenario.
Devices used for non-invasive ventilation NIV may not incorporate humidification, but the use of humidification during NIV has been shown to improve patient comfort. Adequate humidification is an important consideration in the delivery of anaesthetic gases and supplementary oxygen therapy to those patients requiring additional respiratory support, including mechanical ventilation.
Failure to provide humidified respiratory support can lead to unwanted complications and therefore the anaesthetist needs to be equipped with a fundamental understanding of the principles of humidification and the equipment used to provide it. A liquid, such as water, consists of molecules. Such molecules have variable kinetic energy and the temperature of the liquid is determined by the mean kinetic energy of its molecules. At an interface between a liquid and a gas, molecules with sufficient kinetic energy will be able to overcome the forces of attraction within the liquid and escape into the gas as a vapour.
The molecules escaping from the liquid surface are those with the greatest kinetic energy. As they leave, the mean energy of the molecules within the liquid and therefore its temperature will decrease as evaporation occurs. The molecules in the vapour formed above the liquid will exert a partial pressure within the gas. In a sealed container at constant temperature, equilibrium will develop where equal numbers of molecules escape and re-enter the liquid phase.
At steady state, the gas is saturated with vapour and the partial pressure the vapour molecules exert on the container is known as the saturated vapour pressure. Saturated vapour pressure is dependent on temperature Fig. With the addition of heat energy to a liquid, the temperature of the liquid increases as does the mean kinetic energy of the molecules. Greater numbers of molecules are capable of escaping the liquid phase thereby exerting a greater saturated vapour pressure.
The massic enthalpy of evaporation latent heat of vaporization is the heat required to convert 1 g of substance from the liquid phase to the gaseous phase at a given temperature. For molecules of a liquid to evaporate, they must be moving in the right direction near the surface and have sufficient kinetic energy.
At a molecular level, there is no clear boundary between liquid and vapour states, and this is described as the Knudsen layer, whereby the phase is undetermined.
Once the critical temperature has been reached, the substance will only exist as a gas Table 1. The non-linear relationship between the saturated vapour pressure of water and temperature. Humidification describes the addition of water vapour to a gas and absolute humidity is the mass of water vapour per unit volume of gas.
Relative humidity is the ratio of the actual mass of water vapour in a volume of gas to the mass of water vapour required to saturate that volume of gas at a given temperature. It is expressed as a percentage.
Relative humidity can also be calculated as the water vapour pressure over the saturated water vapour pressure. As temperature increases in a closed system, the relative humidity decreases. If the temperature decreases, the relative humidity increases.
This temperature is known as the dew point. A further decrease in temperature below this leads to some of the moisture condensing, as the maximum water vapour capacity is exceeded. The anatomical point at which the inspired gases become fully saturated in the airways is termed the isothermic saturation boundary ISB.
Under resting conditions, the ISB is thought to be just below the carina. The nose functions as an excellent humidifier. While inhaled air is cold and dry, the highly vascularized nasal mucosa facilitates heat and moisture exchange and this is further enhanced by the presence of nasal turbinates.
These increase available surface area and alter the characteristics of the airflow, thus maximizing the transfer of heat and water. As the nasal mucosa gives up water to the dry inspired air, there is some heat loss through massic enthalpy of evaporation.
Consequently, warm expired air is cooled, with subsequent condensation of the water vapour. Compared with the amount of water added during inspiration, only a proportion will condense during expiration and within a 24 h period, there is approximately a ml loss of water from the respiratory tract. Within the airways, humidification is achieved by evaporation of water from the airway surface liquid contained within the mucus, present on all respiratory surfaces.
Respiratory mucus consists of two interacting layers: a luminal gel layer containing mucin and a deeper aqueous sol layer, which is produced by serous cells. Mucus not only provides a mechanism for humidification, but also entraps inhaled debris. Embedded within the mucus layer are cilia, which beat times per minute in a co-ordinated fashion, allowing the transport of mucus and debris back up to the pharynx.
When humidity of an inspired gas is too low, greater amounts of water evaporate from the mucus. There is initially movement of water from the aqueous layer to the gel layer, but this compensation is limited with eventual increased viscosity of the mucus.
The impaired mucociliary elevator further adds to atelectasis through sputum retention and this compounds intrapulmonary shunting. Pulmonary complications for patients undergoing anaesthesia with dry gases exceed those compared with patients breathing humidified gases. Conversely, if the humidity of inspired gas is too high, the viscosity of the mucus reduces. Table 1 shows humidity requirements for gas delivery at different anatomic sites in the airway [ 12 ].
Heat and moisture exchange is one of the most important functions of the respiratory system. The connective tissue of the nose is characterized by a rich vascular system of numerous and thin walled veins. This system is responsible for warming the inspired air to increase its humidity carrying capacity. The respiratory mucosa is lined by pseudostratified columnar ciliated epithelium and with numerous goblet cells. These cells, as well as submucosal glands underneath the epithelium, are responsible for maintaining the mucous layer that serves as a trap for pathogens and as an interface for humidity exchange.
At the level of the terminal bronchioles, the epithelium turns into a simple cuboidal type with minimal goblet cells and scarce submucosal glands. Hence, the capacity of these airways to carry on the same level of humidification maintained by upper airways is limited [ 14 ].
After endotracheal intubation, as the upper airway loses its capacity to heat and moisture inhaled gas, the ISB is shifted down the respiratory tract. This imposes a burden on the lower respiratory tract, as it is not well prepared for the humidification process.
In fact, inhalation of large volumes of cold air during exercise is thought to be the inciting event of exercise-induced asthma [ 16 ]. During the exhalation process, the expired gas transfers heat back to the upper airway mucosa. As the airway temperature decreases, the capacity to hold water also decreases. Therefore, condensed water is reabsorbed by the mucosa, recovering its hydration.
Importantly, in periods of cold weather, the amount of water condensation may exceed the mucosal capacity to accept water. Therefore, the remaining water accumulates in the upper airway with consequent rhinorrhea.
In order to avoid the aforementioned consequences associated with lack of humidification in mechanically ventilated patients, a variety of devices humidifiers have been introduced in clinical practice. In the following paragraphs, we describe current types of humidifiers utilized in mechanical ventilation. Humidifiers are devices that add molecules of water to gas. Active humidifiers act by allowing air passage inside a heated water reservoir.
These devices are placed in the inspiratory limb of the ventilator circuit, proximal to the ventilator. As condensation of water vapor may accumulate as the surrounding temperature of the inspiratory limb decreases, these systems are used with the addition of water traps, which require frequent evacuation to avoid risk of contamination of the circuit. Due to the aforementioned shortcoming, heated humidifiers are usually supplied with heated wires HWH along the inspiratory limb to minimize this problem.
These humidifiers have sensors at the outlet of the humidifier and at the Y-piece, near the patient. These sensors work in a closed-loop fashion, providing continuous feedback to a central regulator to maintain the desired temperature at the distal level Y-piece.
When the actual temperature exceeds or decreases beyond certain extreme level, the alarm system is triggered. Even though the ideal system should permit autocorrections based on humidity levels, commercially available sensors provide feedback based on changes in temperature [ 18 ].
Figure 2 shows an active humidifier with a heated wire in the inspiratory limb; both temperature sensors, one at the side of the patient and the other at the outlet of the heated reservoir, are shown [ 17 ]. The performance of humidifiers may be affected by room temperature, as well as patient minute ventilation. In the last situation, an increase in minute ventilation preserving the same temperature of the heated reservoir may not be adequate to deliver appropriate AH to the patient.
Therefore, some humidifiers are supplemented with automatic compensation systems, which compute the amount of thermal energy needed to humidify certain volume of gas and change the temperature of the water reservoir accordingly.
Lellouche et al. One of the tested humidifiers had an automatic compensation system for changes in minute ventilation. This model achieved higher AH levels than those that relied only on temperature sensors [ 19 ].
Furthermore, other studies have also reinforced the effect of room temperature, variance in minute ventilation, and ventilator gas temperature on levels of absolute humidity delivered to patients [ 20 — 22 ]. Notably, some studies indicate that heated humidifiers without heated wires achieve higher levels of humidification than HWHs. Nevertheless, it is clear that they are associated with more condensation and respiratory secretions [ 23 ].
Hence, these types of humidifiers are becoming increasingly unpopular among respiratory care providers. As previously mentioned, inspiratory heated wires can minimize condensation. However, exhaled air can form rainout in the expiratory limb.
This has led to the utilization of double heated wire DHW circuits. This practice has replaced the use of single heated wires SHW circuits in some countries [ 24 ]. Another described technique to limit condensate in the expiratory limb is to use porous expiratory circuits [ 25 ]. Heated humidifiers have different designs and different techniques for humidification. Accordingly, these devices are classified as 1 bubble; 2 passover; 3 counter-flow; and 4 inline vaporizer. In bubble humidifiers, gas is forced down a tube into the bottom of a water container Figure 3.
The gas escapes from the distal end of the tube under water surface forming bubbles, which gain humidity as they rise to the water surface. Some of these humidifiers have a diffuser at the distal end of the tube that breaks gas into smaller bubbles.
The smaller the bubbles, the larger the gas-water interface allowing for higher water vapor content. Other factors that influence water vapor content of the produced gas are the amount of water in the container and the flow rate.
Simply, the higher the water column in the container, the more gas-water interface will ensue, so water levels should be checked on a frequent basis. In terms of flow rate, when slow flows are delivered, there is more time for gas humidification. Bubble humidifiers may be unheated or heated. Typically, unheated bubble humidifiers are used with low-flow oral-nasal oxygen delivery systems.
Heated bubble humidifiers provide higher absolute humidity. These humidifiers usually use diffusers to increase the liquid-air interface. A problem with heated bubble humidifiers is that they exhibit high resistance to airflow imposing higher work of breathing than passover ones [ 26 , 27 ]. Furthermore, they may generate microaerosol [ 28 , 29 ].
Nevertheless, the CDC guidelines for prevention of health care associated pneumonia reported that the amount of aerosol produced by these types of humidifiers may not be clinically significant [ 30 ]. Despite this statement, the use of bubble humidifiers during mechanical ventilation has fallen in favor of passover ones. In passover humidifiers Figure 3 , gas passes over a heated water reservoir carrying water vapor to the patient.
These are typically used for the purpose of invasive and noninvasive mechanical ventilation. Another variant of passover humidifiers is the wick one Figure 3. In this type of device, the gas enters a reservoir and passes over a wick that acts as a sponge that has its distal end immersed in water. The wick pores provide more gas-water interface allowing for more humidification compared to simple passover humidifiers. The water reservoir is fed through a closed system. This system can be supplied with water either manually through a port or float feed system that ensures the water level remains constant all the time.
As dry gas enters the chamber and travels through the wick, heat and moisture increase. Due to the fact that gas does not emerge underneath the water surface, no bubbles are generated. A third type of passover humidifier involves a hydrophobic membrane Figure 3. As with the wick device, dry gas passes through a membrane. Nevertheless, its hydrophobic characteristic only allows passage of water vapor, precluding liquid water to travel through it.
Similarly to the wick humidifier, bubbles and aerosols are not generated. As mentioned previously, these humidifiers are more commonly used during mechanical ventilation than bubble ones due to their lower flow resistance and absence of microaerosols. In all cases, a temperature probe is placed near the Y piece of the ventilator circuit to ensure delivery of gas with optimal temperature. As it was stated above, the presence of condensate in the tubing may increase resistance, which can decrease volume delivered in pressure controlled, or increase peak pressure in volume controlled modes.
Despite the need of the aforementioned heated wires to avoid undesirable condensation, it is also worth mentioning that use of these wires does not come without thermal risks [ 31 ]. In terms of humidifier heating systems, currently there are 6 types of devices. The hot plate element, which sits at the bottom of the humidifier, is one of the most commonly used. Other devices include the wraparound element, which surrounds the humidifier chamber; a collar element, which sits between the reservoir and the outlet; the immersion heater, which is placed directly inside the water reservoir; and the heated wire, which is placed in the inspiratory limb of the ventilator.
In the recently described counter-flow humidifier, water is heated outside the vaporizer. After being heated, water is pumped to the top of the humidifier, enters the inside of the humidifier through small diameter pores, and then runs down a large surface area. Gas flows in counter direction. During its passage through the chamber of the humidifier, the air is moisturized and warmed to body temperature.
Schumann et al. The authors demonstrated that the counter-flow device imposed less work of breathing compared with the other ones. In addition, the humidification performance of the counter-flow model was independent of flow and respiratory rate, in contrast to the heated passover humidifier in which humidification performance decreased with increasing ventilator rates [ 32 ].
This technology is promising but more studies are needed before it becomes widely adapted. The novel inline vaporizer uses a small plastic capsule where water vapor is injected into the gas in the inspiratory limb of the ventilator circuit immediately proximal to the patient wye. In addition to the water vapor, gas heating is supplemented by a small disk heater in the capsule. Water is delivered to the capsule by a peristaltic pump housed in a controller. The amount of water sent to the capsule is set by the clinician based on minute volume through the circuit.
Both temperature and humidity are adjustable and displayed constantly. The proximity to the wye connection obviates the requirement for heated wires and external temperature probes. The manufacturer reports very high AH production with this system. However, this system was only studied during high frequency percussive ventilation [ 33 , 34 ]. Heat and moisture exchangers are also called artificial noses because they mimic the action of nasal cavity in gas humidification.
They operate on the same physical principle, as they contain a condenser element, which retains moisture from every exhaled breath and returns it back to the next inspired breath. Unlike heat humidifiers, which are placed in the inspiratory limb of the circuit, these devices are placed between the Y piece and the patient Figure 4. This may increase resistance to airflow not only during inspiration, but also during the expiratory phase.
In situations in which administration of aerosolized medications is needed, HMEs need to be removed from the circuit to avoid aerosol deposition in HME filters. Initial designs of HMEs used condensers made of metallic elements that had high thermal conductivity. These devices were known as simple HMEs. They were not disposal and created a significant resistance during mechanical ventilation [ 35 , 36 ]. In hydrophobic HMEs, the condenser is made of a water repelling element with low thermal conductivity that maintains higher temperature gradients than in the case of simple HMEs.
In combined hydrophobic hygroscopic HMEs, a hygroscopic salt calcium or lithium chloride is added inside the hydrophobic HME. These salts have a chemical affinity to attract water particles and thus increase the humidification capacity of the HME. Pure hygroscopic HMEs have only the hygroscopic compartment. During exhalation, vapor condenses in the element as well as in the hygroscopic salts.
Figure 5 illustrates the basic structure and work principle of HMEs. Therefore, the aforementioned HMEs are not frequently used. These filters operate based on electrostatic or mechanical filtration. Specifically, based on the predominant mechanism applied, these filters may be classified into pleated or electrostatic filters. The pleated filters have more dense fibers and less electrostatic charges, whereas the electrostatic filters have more electrostatic charges and less dense fibers.
Pleated filters function better as barriers to bacterial and viral pathogens than electrostatic filters. However, they confer higher airflow resistance [ 38 ]. The electrostatic filters are subjected to an electric field. Since bacteria and viruses carry electric charges, they get trapped within the electric field of these filters.
These filters usually have larger pores than the pleated membranes, and they rely mainly on the electrostatic mechanism. The previously described filter confers little to the humidification process and increases resistance. Therefore, they are mainly used as barriers to pathogens [ 15 ]. In a recent study, Lellouche and colleagues independently assessed the humidification capacity of 32 HMEs. Nevertheless, whether the presence of tubing condensate represents an important factor for the development of VAP in well-maintained circuits remains controversial.
Furthermore, HMEs also present some shortcomings. Specifically, impaction of secretions or blood within the device may increase airway resistance and work of breathing. In extreme circumstances, complete airway obstruction has been reported [ 40 ]. Therefore, patient selection becomes an essential component in the use of HMEs. Table 2 shows contraindications for the use of HMEs [ 11 ].
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