Humidification, Nebulization, and Respiratory Gas Filtering

Humidification, nebulization, and respiratory gas filtering are therapies or techniques that are frequently applied together with mechanical ventilation therapy. Commonly required items for these therapies are listed.

Items required for humidification, in-line nebulization, and gas filtering

These techniques are frequently used with mechanical ventilation, but they are not required parts, because a ventilator system can function perfectly or even better without them. They are added to the system for clinical reasons.

The presence of these items, especially those for humidification, noticeably changes the setup of a breathing circuit. And their functional status can greatly affect the performance of a ventilator system.

Humidification, Nebulization

Artificial humidification

Air conditioning at nose and intubation

Artificial humidification refers to the technique used to artificially warm and humidify the inspiratory gas.

Why is this necessary? To understand, we need to know that the nose has an important but less mentioned physiological function: air conditioning. This refers to the warming, humidifying, and filtering of the inhaled air.

Depending on the local climate, the temperature and relative humidity (RH) of the ambient air vary widely. To compensate for such differences, the nose efficiently warms, humidifies, and cleans the inhaled air. By the time air reaches the carina or the end of the trachea, its temperature is approximately 37°C or 98.6°F and its relative humidity is 100%. Warm and humid air is critical for the physiological functioning of the airway and lungs.

Why do we need to pay special attention to inspiratory gas warming and humidifying in intubated and mechanically ventilated patients?

Intubation involves placing a tube in a patient’s trachea. Through either an endotracheal tube (ETT) or a tracheostomy tube (TT), the inspiratory gas enters the trachea directly; see Fig. 6.1. In either case, the gas is not conditioned.

Fig. 6.1 Intubation with an endotracheal tube (ETT).

Intubation with an endotracheal tube (ETT).

Now let’s measure the temperature and relative humidity of inspiratory gas at the trachea using three different scenarios. In the first scenario, normal breathing, room air is inhaled and conditioned as described above. The second scenario involves a patient who is intubated but not mechanically ventilated, so they inhale unconditioned room air. The third scenario involves a patient who is intubated and mechanically ventilated. The inhaled gas originates from the ventilator’s gas source. Typically, such gases are at close to room temperature and are extremely dry (<5% RH). These situations are visualized in Fig. Cold and dry gases directly entering the trachea may cause inspissation of airway secretions, destruction of airway epithelium, atelectasis, and hypothermia. To avoid these complications, inspiratory gas must be sufficiently warmed and humidified before entering the airway, as indicated by the light green arrows.

Fig. 6.2 Inspiratory gas temperature (T) and relative humidity (RH) at the trachea in three scenarios. The two light green arrows indicate how much the gas temperature and relative humidity need to be artificially increased during invasive mechanical ventilation.

Inspiratory gas temperature (T) and relative humidity (RH) at the trachea in three scenarios. The two light green arrows indicate how much the gas temperature and relative humidity need to be artificially increased during invasive mechanical ventilation.

There are two common techniques to artificially warm and humidify inspiratory gases: an active humidifier set, and a heat and moisture exchanger (HME). For every intubated and mechanically ventilated patient, one of these techniques should be deployed.

The AARC (American Association of Respiratory Care) recommends that: (a) an active humidifier provide a humidity level between 33 mg/L and 44 mg/L of water and a gas temperature between 34°C and 41°C at the Y-piece, with RH of 100%; and (b) an HME provide a minimum of 30 mg H2O/L. This is the goal to achieve for artificial humidification during mechanical ventilation (Restrepo and Walsh, 2012).

Gas humidity and temperature

Now let’s learn a few special terms that are frequently used in artificial humidification: condensation, evaporation, absolute humidity, relative humidification, and dew point.

Condensation is the process by which water is converted from its vapour form to its liquid form (droplets). Essentially, condensation is the consequence of air or gas oversaturation. Condensation typically occurs when warm air comes in contact with a cold surface.

Evaporation is the process by which water is converted from its liquid form to its vapour form. Evaporation increases when (a) gas temperature rises, (b) vapour partial pressure is low, and/or (c) the air moves (wind).

Air can hold a certain amount of water vapour. It is interesting and important to know that the maximum amount of vapour content that a unit volume of air can hold is temperature dependent. Warm air can hold more water vapour than cool air.

Two terms are commonly used to express the gas humidity: absolute humidity and relative humidity.

Absolute humidity (AH) is the water vapour present in a unit volume of air, typically expressed in milligrams per litre (mg/L) or kilograms per cubic metre (kg/m3). Absolute humidity does not fluctuate with the temperature of the air.

Relative humidity (RH) is the the ratio of the amount of water vapour in air at a given temperature to the maximum amount that the air could hold at that temperature. With a given amount of water vapour, an increase in air temperature leads to a decrease in the RH simply because warm air can hold more water vapour.

Relative humidity can be regarded as the level of vapour saturation. A relative humidity of 50% means that the air holds just half of the maximum vapour content it can hold at the current temperature.

Dew point is the temperature at which the air becomes 100% saturated with vapour. For a given vapour content, there is a fixed dew point (temperature).

Fig. 6.3 shows the relationship between temperature, absolute humidity, and relative humidity.

Fig. 6.3 The relationship between temperature, absolute humidity (AH), and relative humidity (RH).

The relationship between temperature, absolute humidity (AH), and relative humidity (RH).

Active humidifier

An active humidifier, also known as a heated humidifier, is a medical device designed to continuously warm and humidify the gas that a intubated and mechanically ventilated patient inhales. An active humidifier is integrated into the breathing circuit of a ventilator system.

An active humidifier typically includes these items:

  • A humidifier base (heater).
  • A water reservoir.
  • A gas temperature sensor. Initially, this was just an airway gas thermometer positioned at the Y-piece. Modern active humidifiers often have a dual temperature sensor, which monitors the gas temperatures at the reservoir outlet and the Y-piece simultaneously. The results are used to regulate heating intensity.
  • One or two water traps to remove the condensed water from the circuit tube.
  • Heating wire, which is placed in the circuit tube to prevent the air temperature from dropping as the gas travels through the tube.
  • (Optional) An expiratory filter and heater to minimize condensation in the expiratory limb.

Which parts are used depends upon the design of active humidifier.

The relatively cold and dry gas is warmed and humidified when it travels through the water reservoir, which is continuously heated. The passing gas continuously removes heat and water vapour, so that the reservoir must be heated continuously and refilled periodically.

Fig. 6.4 An active humidifier is comparable to a kettle.

An active humidifier is comparable to a kettle.

This kettle principle is a common foundation of most active humidifiers, including those from Fisher & Paykel and Hudson RCI (Teleflex). An exception is the Gründler humidifier, where inspiratory gas passes through a sponge through which warm water circulates. The structures of these humidifiers are shown in Fig.

Fig. 6.5 Three active humidifier structures.

Three active humidifier structures.

Used with permission from Gründler Medical.

A general goal of active humidification is to ensure that the inspiratory gas at the Y-piece is approximately 37°C (or 98.6°F) and 100% in relative humidity (i.e. 44 mg/L in vapour content).

During mechanical ventilation, inspiratory flow is not stable. At a high flow, the passing gas carries away more heat, and vice versa. Therefore, an active humidifier needs to regulate its heating intensity according to the measured gas temperature.

Managing condensation

The water reservoir and the Y-piece are connected with an inspiratory tube, as shown in Fig. This tube is relatively cold, so the warm gas cools as it passes through the tube.

Fig. 6.6 Circuit rainout occurs if saturated, warm gas cools.

Circuit rainout occurs if saturated, warm gas cools.

In order to offset the cooling effect and ensure that the gas temperature at the Y-piece is 37°C, the gas at the reservoir must be warmer (i.e. up to 45°C). This is technically possible. However, if the gas is fully vapour saturated (100%), we have a new problem—condensation.

As we saw earlier, warmer air can hold more vapour than cooler air. At 45°C one litre of air can hold 66 mg/L, while at 37°C it can only hold a maximum of 44 mg/L. As the gas cools, the surplus vapour of 22 mg/litre condenses into liquid form.

Condensation also occurs in the expiratory limb due to this cooling effect. At the Y-piece, the exhaled gas is approximately 37°C, with vapour content of 44 mg/L. At the expiratory valve, it is approximately 30°C, with vapour content of 30 mg/L. The surplus is 12 mg/L. In both cases condensation is inevitable.

Water condensation is a continuous process. Initially, there may be just some drops on the inner surface of the tube. Over time, more and more water accumulates in the circuit tubes. You can easily recognize the water by listening for the sound of water being pushed by the passing gas flow. An accumulation of condensed water in the breathing circuit is called circuit rainout.

Does water inside the circuit really matter?

The answer depends entirely on the amount. Condensed water always stays in the lowest parts of the tubes due to gravity (Fig.). It presents a moving obstacle to the passing gas flow, resulting in local turbulence. If the amount of water is tiny, the influence is negligible. However, a significant amount of water can seriously interfere with gas movement, which is easily visible in a pressure–time waveform (Fig.). Such interference is an important cause of auto-triggering (i.e. the triggering of the ventilator by pneumatic artefact rather than the patient’s inspiratory efforts); we will discuss normal and abnormal triggering in detail in Chapter 7. The clinical consequences of auto-triggering include hyperventilation and high respiratory frequency, along with their corresponding alarms.

Fig. 6.7 Condensed water accumulates in the lowest part of a tube.

Condensed water accumulates in the lowest part of a tube.

Fig. 6.8 Representative pressure–time waveforms with and without accumulated water in the circuit.

Representative pressure–time waveforms with and without accumulated water in the circuit.

If more water accumulates, the tube may become totally occluded. This is an emergency, and requires immediate corrective action.

What can be done to solve the problem of circuit condensation? There are three common solutions: (1) drain off the water, (2) minimize or prevent condensation, and (3) increase evaporation.

Use a water trap to drain off the water. A water trap can be used to drain condensed water from the circuit tube in order to minimize the disturbance to gas movement.

A water trap may be designed for single use or multiple uses (see Fig.). It has a cover and a container. The cover is integrated into the circuit limb. Gas passes through the cover, and the water falls into the container due to gravity. The container can be detached from the cover for emptying. The circuit should remain gas tight when the trap container is detached.

Fig. 6.9 Reusable water trap (left) and single-use water trap (right).

Reusable water trap (left) and single-use water trap (right).

Keep the following in mind when using a water trap:

  • Use a water trap whenever circuit condensation may occur.
  • Make sure the water trap is always positioned at the lowest part of the circuit, and keep the cover upward, as drainage depends on gravity. This is not as easy as it sounds. Drainage cannot occur if the water trap is in any of the positions shown in Fig. 6.10.
  • Periodically inspect water traps and empty them if necessary.
  • After emptying the container, make sure to tightly secure the container to the cover.
  • The collected, condensed water is contaminated.
  • Discard a water trap if it is cracked or if it leaks.
  • Do not reuse water traps that are designed for single use.

Fig. 6.10 Three incorrect water trap positions (a–c) in which condensed water can hardly enter or stay, resulting in drainage failure.

Three incorrect water trap positions (a–c) in which condensed water can hardly enter or stay, resulting in drainage failure.

  1. 2. Use a heated circuit to minimize condensation. We know that water condenses when warm and humid gas cools. Logically, if the travelling gas stays as warm as or even warmer than it was when it left the humidifier reservoir, condensation will not occur. This is the basic principle behind the heated circuit.

Fig. shows a typical breathing circuit with an active humidifier. Note that the inspiratory limb, between the reservoir and the Y-piece, is heated to approximately 39°C.

Fig. 6.11 Breathing circuit with inspiratory limb heated.

Breathing circuit with inspiratory limb heated.

Inspiratory limb heating causes two important changes. First, the gas temperature at the reservoir outlet is 37°C rather than 45°C because there is no cooling effect at the tube. Second, the tube heating also slightly warms the gas so that its relative humidity drops from 100% to around 90%. Under these conditions, condensation is unlikely to occur, at least in theory, so a water trap is unnecessary when the inspiratory limb is heated.

Technically, the heated wire solution involves a loop of heating wire, which is a special isolated metal wire. The wire becomes hot when an electrical current passes through it. A special cable called an electrical adapter connects the heating wire to the humidifier base. The heating wire is powered by the humidifier and the heating intensity is electrically regulated.

Initially, heating wires were designed for multiple use—for use with a reusable circuit with silicone tubes. After use on a patient, such a circuit must be disassembled, reprocessed (i.e. cleaned and disinfected/sterilized), and reassembled.

Today, ready-made, disposable heated circuits dominate the market. They are made of biocompatible polymers and are available in various sizes and designs. The circuits are convenient to use, but not environmentally friendly.

A heating wire that spans a tube’s full length imposes additional resistance to flow. In order to eliminate the imposed resistance and to distribute the heat more evenly, some new circuits have the heating wire embedded in the wall of the tube. Fig. shows three common heating wires.

Fig. 6.12 Heating wire in three forms: (a) A reusable heating wire and an electric adapter; (b) A single-use circuit with the heating wire spiral in the lumen; and (c) A single-use circuit with the heating wire embedded in the wall of tube.

Heating wire in three forms: (a) A reusable heating wire and an electric adapter; (b) A single-use circuit with the heating wire spiral in the lumen; and (c) A single-use circuit with the heating wire embedded in the wall of tube.

  1. 3. Increase evaporation with a heated filter. The third solution is effective but less used. It is intended to minimize condensation at the expiratory limb by adding a large filter enclosed in an electrical heater. The filter temperature is maintained between 55°C and 70°C. This can effectively decrease the RH of the passing gas and intensify the evaporation process.

Such a heated filter can be integrated into a ventilator, as in the Puritan Bennett 840 ventilator (Fig.). Or, it may be added to the expiratory limb, as with the filter heater from VADI Medical (Fig.).

Fig. 6.13 Integrated heated filter in Puritan Bennett 840 ventilator.

Integrated heated filter in Puritan Bennett 840 ventilator.

Fig. 6.14 (a) VADI Heated filter installed at expiratory limb (b) to minimize condensed water.

(a) VADI Heated filter installed at expiratory limb (b) to minimize condensed water.

Reproduced with permission from VADI Medical.

The heated filter protects the moisture-sensitive sensors at the expiratory valve and it filters the expiratory gas for staff protection.

Humidification and circuit composition

The composition of the breathing circuit varies based on the active humidifier used. There are three common circuit compositions.

Type A: Neither circuit limb heated

In this configuration (Fig.), the gas temperature at the reservoir outlet must be sufficiently high to offset the tube cooling effect described earlier. Condensation is inevitable in both the inspiratory and expiratory limbs, so we need to install a water trap in both limbs to drain off the water. A typical example of this circuit includes the Fisher & Paykel MR410 humidifier. This circuit may be either reusable or for single use.

Fig. 6.15 Type A circuit with no heated limb.

Type A circuit with no heated limb.

Type B: Heated inspiratory limb

As shown in Fig, a large portion of the inspiratory limb, between the reservoir and the Y-piece, is heated by a heating wire.

Fig. 6.16 Type B circuit with inspiratory limb heated.

Type B circuit with inspiratory limb heated.

The inspiratory limb prevents the tube cooling effect and the resultant condensation, so the inspiratory water trap is omitted. The gas temperature at the reservoir outlet is around 37°C. However, the expiratory water trap is still required. A typical example includes the Fisher & Paykel MR730 humidifier. Note that the tube between the ventilator outlet and the reservoir is a part of the inspiratory limb but not heated. This circuit may be either reusable or for single use.

Type C: Both circuit limbs heated

As shown in Fig. both inspiratory and expiratory limbs are heated so that condensation is not expected. Therefore, this circuit does not have a water trap. A typical example includes the Fisher & Paykel MR850 humidifier. This circuit is typically for single use.

Fig. 6.17 Type C circuit with two heated limbs.

Type C circuit with two heated limbs.

Common problems associated with active humidification

Although the active humidifier serves important clinical needs, it often introduces problems to the ventilator system. The consequences are not only dysfunction of the humidifier, but also disturbance to the functionality of the ventilator system. Table 6.2 lists some common problems.

Heat and moisture exchangers (HMEs)

heat and moisture exchanger (HME) is the other common way to warm and humidify the inspiratory gas. It is also known as a hygroscopic condenser humidifier or an artificial nose.

Technically, an HME is a special filter. Although it may resemble a non-HME airway filter, do not confuse them.

An active humidifier, described above, actively increases the heat and vapour content of inspired gas. An HME, on the other hand, operates passively by storing the heat and moisture from the patient’s exhaled gas and releasing them back to the inspired gas in the next breath. An HME should be able to provide a minimum of 30 mg/L. An active humidifier and HME are never used together. An HME is positioned between the Y-piece and the ETT, and it becomes a part of the artificial airway.

When an HME is used, circuit condensation should not occur, so there is no need for a water trap or circuit limb heating. Use of an HME greatly simplifies the breathing circuit (Fig).

Fig. 6.18 Type D circuit with a heat and moisture exchanger (HME).

Type D circuit with a heat and moisture exchanger (HME).

An HME is most suitable for a patient on short-term mechanical ventilation (i.e. less than 96 hours). Active humidifiers, by contrast, are for both short-term and long-term mechanical ventilation.

An HME is an effective gas conditioning solution for many patients, but it is not recommended for patients with the following:

a. Thick, copious, or bloody secretions;
b. Expired tidal volume less than 70% of the delivered tidal volume (e.g. those with large bronchopleurocutaneous fistulas or incompetent or absent endotracheal tube cuffs);
c. Body temperature less than 32°C;
d. High spontaneous minute volume (>10 L/min).

An HME must be removed during in-line nebulization.

Use of an HME poses two major risks. First, excessive liquid (secretion or blood clots) from the trachea or medication aerosols can clog an HME partially or even completely, causing dyspnoea or suffocation. Second, an HME contributes to instrumental dead space and decreases alveolar ventilation; see section 5.3.5. This adverse effect is insignificant if the tidal volume is large, but it may be deadly if the tidal volume is tiny, as in neonates.

For further information, refer to the AARC clinical practice guideline, ‘Humidification during invasive and noninvasive mechanical ventilation’. (Restrepo and Walsh, 2012).

In-line nebulization

Aerosol nebulization

Aerosol therapy or nebulization therapy is a common therapy to directly administer drugs, or even sterile water, into the upper and lower airway and lungs. Aerosol therapy produces aerosols, airborne liquid droplets, or solid particles suspended in a gas. The therapy is widely used to treat pulmonary diseases, especially for outpatients or for home care.

The medical devices required for aerosol generation are called nebulizers. They generate medical aerosols with two common techniques.

Pneumatic (jet) nebulizer

A spray or perfume bottle is an everyday example of a pneumatic nebulizer or jet nebulizer. A pneumatic nebulizer (Fig.) is typically composed of the following items: a reservoir that contains the liquid to be aerosolized, a thin tube, a very small hole (nozzle), and a compressed gas source. Liquid aerosols are generated by passing gas at a high velocity through the small hole. The resulting low pressure at the jet sucks the liquid from the reservoir up through the tube, and the liquid is shattered into liquid particles. The generated aerosols spurt out, carried in an invisible gas stream.

Fig. 6.19 A handheld jet nebulizer set (left) and an aerosol spray can (right).

A handheld jet nebulizer set (left) and an aerosol spray can (right).

Pneumatic nebulizers for medical use may have more sophisticated designs, but the basic principle remains the same.

Pneumatic nebulizers are simple, effective, compact, and inexpensive.

Ultrasonic nebulizer

Ultrasonic (or piezoelectricnebulizers use sound waves at ultrasound frequencies to produce fine medical aerosols. An ultrasonic nebulizer is typically composed of a radio-frequency generator, a shielded cable, a piezoelectric crystal transducer, and a reservoir chamber with an inlet and an outlet. Electric current is converted into sound waves at the crystal transducer. The sound energy is transferred either indirectly through a water-filled reservoir (as shown in Fig) or directly to a solution chamber. The gas passing through the chamber carries with it the generated aerosols.

Fig. 6.20 Schematic of ultrasonic nebulizer: 1) radiofrequency generator; 2) shielded cable; 3) piezoelectric crystal transducer; 4) water-filled couplant reservoir; 5) solution chamber; 6) chamber inlet; 7) chamber outlet.

Schematic of ultrasonic nebulizer: 1) radiofrequency generator; 2) shielded cable; 3) piezoelectric crystal transducer; 4) water-filled couplant reservoir; 5) solution chamber; 6) chamber inlet; 7) chamber outlet.

Reprinted with permission from Egan’s Fundamentals of Respiratory Care, 8th edition, Wilkins R.L., Stroller J.K., and Scanlan C.L., p753. Copyright (2003) with permission from Mosby, Elsevier Inc.

An ultrasonic nebulizer is typically larger, more complicated, and more expensive than a pneumatic nebulizer.

A relatively new type of ultrasonic nebulizer is the vibrating mesh nebulizer. This device has a mesh installed at the bottom of a liquid container. When the nebulizer is powered, this mesh, made of a piezoelectric material, vibrates at a high frequency, causing a pumping action. Aerosols are produced when the liquid passes through the mesh. The size and flow of the aerosol particles are determined by the diameter of the exit aperture holes. A typical example is the Aeroneb Solo from Aerogen.

Fig. 6.21 (a) Aeroneb Pro and (b) Aeroneb Solo nebulizers.

(a) Aeroneb Pro and (b) Aeroneb Solo nebulizers.

Reprinted with permission from Aerogen.

An in-line nebulizer is typically composed of three items, listed in Table

Open and in-line nebulization

Aerosol therapy is an effective treatment commonly used for a wide range of respiratory diseases. Integrated into mechanical ventilation, aerosol therapy serves the same purpose and employs the same techniques. However, there is a fundamental difference between the conventional use of aerosol therapy and that used with mechanically ventilated patients.

During conventional aerosol therapy, a patient’s airway is open to the atmosphere, so the patient inhales and exhales the air and the medication aerosols. Conventional aerosol therapy is known as open nebulization.

During mechanical ventilation, a patient’s airway is connected to a ventilator system, as shown in. The patient must inhale and exhale through the connected ventilator system. The nebulizer jar is typically installed in the inspiratory limb. The exhaled gas and aerosols exit the ventilator system exclusively through the expiratory route. Such an arrangement is known as in-line nebulization.

Fig. 6.22 (a) Open and (b) in-line nebulization.

(a) Open and (b) in-line nebulization.

It is important to understand the differences between open and in-line nebulization. A failure to understand these differences is the root cause of several complications of mechanical ventilation.

Complications of in-line nebulization

Occlusion of the gas passageway

Occlusion of the gas passageway may occur with both pneumatic and ultrasonic nebulization.

With in-line nebulization using the pneumatic or ultrasonic technique, the nebulizer jar is typically installed in the inspiratory limb of the breathing circuit. The inspiratory gas carries the medication aerosols to the patient’s respiratory tract and lungs during inspiration.

Nebulization is intended to deposit therapeutic aerosols at certain locations within the respiratory tract. The depth of penetration of an aerosol particle is inversely proportional to the particle size. The deposition is influenced by a number of factors such as inertial impaction; gravity; the kinetic activity of the particles; the physical nature of the particles; the temperature and humidity of the carrier gas; the patient’s ventilatory pattern; and physical characteristics of the patient’s airways.

During expiration, the exhaled gas carries out moisture, secretion, and residual aerosols: not all aerosols enter the patient’s respiratory tract or are deposited. As a result, aerosols are invariably deposited on the inner surface of the expiratory path (Fig. 6.23). Over time, these accumulated aerosols can occlude the expiratory valve, making expiration increasingly difficult. This is an emergency, requiring immediate corrective action.

Fig. 6.23 The areas where aerosols are present.

The areas where aerosols are present.

A typical example of this scenario involves the administration of Mucomyst (acetylcysteine), a medication used to treat various bronchopulmonary disorders. Its solution is very sticky.

Obviously, such an event can potentially occur in any intermittent positive pressure ventilation (IPPV) ventilator, regardless of brand and model, but the following recommendations can minimize that likelihood:

 Make sure your intensive care unit (ICU) staff are well informed about the potential risks of in-line nebulization, how they manifest, and what to do if they occur.
 Make sure any medications intended for in-line nebulization are approved.
 Pay attention to ventilator alarms indicating increased expiratory resistance.
 Check the expiratory valve performance after every episode of in-line nebulization.
 Replace the expiratory valve (block) if the expiratory resistance exceeds the threshold permitted.

Installation of a filter just before the expiratory valve may be an effective measure, but it also has its drawbacks. The filter can prevent medication aerosols from being deposited at the expiratory valve, but the aerosols can gradually block the filter instead. An expiratory filter should be used if it is difficult to inspect and replace the expiratory valve. If you do use an expiratory filter, be sure to periodically check the performance of the filter and replace it if resistance increases noticeably.

Nebulization volume

The use of a pneumatic nebulizer in a volume mode may increase the tidal volume delivered to the patient beyond the set level.

You may recall that during open nebulization, the carrier gas dissipates into the atmosphere, while in-line nebulization occurs in a closed system. In order to generate the aerosols, a pneumatic nebulizer needs driving gas flow, which enters the circuit too. This means that the gas delivered into the breathing circuit comes from two sources. The primary source is the ventilator, and the secondary one is the nebulizer. The total delivered gas volume is the sum of the two.

This additional nebulization volume is determined by:

 The flow rate of the driving gas. It is usually constant at 4–8 L/min, or 67–134 ml/second.
 The time pattern of in-line nebulization: nebulization is activated continuously or intermittently during inspiration only.

This additional nebulization volume has an effect during inspiration, but not expiration, because it is vented when the expiratory valve opens.

Let’s assume that a ventilator is set to a volume mode, with a Ti of 1 second. The nebulizer flow is 6 L/min or 100 ml/second. The pneumatic nebulization adds 100 ml gas to the current tidal volume.

Does the nebulization volume really matter? The impact of the nebulization volume depends on the set tidal volume. The smaller the tidal volume, the greater the impact is. Let’s say you ventilate a baby in a volume mode (unusual) with a tidal volume of 20 ml, a rate of 30 b/min, and a Ti of 0.5 second. The nebulizer flow is constant at 6 L/min or 100 ml/second. The nebulization volume becomes 50 ml per breath, which is much larger than the tidal volume. For this reason, it is not recommended to use pneumatic nebulization in neonates and small children. Ultrasonic nebulization is the recommended alternative, as it introduces no additional volume.

Pneumatic nebulization and FiO2

Pneumatic nebulization can impact the FiO2 of the delivered gas. As we have seen, a pneumatic nebulizer requires driving gas. Depending on the design of ventilator, the driving gas can be either pure oxygen, pure air, or a mixture.

With either pure oxygen or air, pneumatic nebulization can cause the FiO2 to deviate from its initial setting. This deviation depends on the set tidal volume, the difference between the set FiO2 and the O2 concentration of the driving gas, and the nebulization gas flow. FiO2 monitoring cannot detect this deviation.

This deviation can be entirely eliminated if the gas from the nebulizer and the gas from the ventilator have the same FiO2. This is the case with the GALILEO ventilator.

Moving from open nebulization to in-line nebulization is a natural process. However, we need to understand the difference between these applications and the possible consequences. The clinical efficacy of in-line nebulization depends on such factors as humidification, nebulizer type, position of the nebulizer jar, ventilator settings, continuous versus synchronized nebulization, and the properties of the medications to be administered. To answer these questions, extensive studies are necessary.

Respiratory gas filtering

In mechanical ventilation, one or more gas filters, also known as breathing system filters, may be installed in the breathing circuit or artificial airway. We need to understand some basics about use of these filters.

 Why do we want to filter the gas?
 When should we use a filter?
 Where should we install a filter?
 What are the expected benefits from using a filter?
 What are the potential risks?
 How can we minimize the potential risks?

Let’s take a look at some answers.


filter is a device that removes something from whatever passes through it. In our context, a gas filter is used to remove particles, including droplets, secretions, microgerms, or medication aerosols, from respiratory gas.

A filter is composed of three essential components: the housing, the filtering medium, and adapting ports. The housing may be enlarged in the middle to increase the filtering surface.

Fig. 6.24 Structure of a gas filter.

Structure of a gas filter.

Breathing system filters may be classified into two types (electrostatic and mechanical), based on the filtering medium used.

Electrostatic filters have a loosely woven medium whose fibres are permanently bipolar charged. The static electric charge causes this type of filter to retain the particles in the passing gas.
Mechanical (pleated paper) filters, whose medium is constructed of tightly packed layers of mixed strands of glass fibre filter paper. The medium acts as a sieve, retaining unwanted particles while allowing gases and humidified air to travel through with minimal obstruction.

Regardless of the type, all breathing system filters have four key characteristics to consider:

 Filtration efficiency: how efficiently the filter captures intended particles under normal conditions of use;
 Pressure drop (resistance): the resistance to gas flow imposed by the filter;
 Internal volume: the dead space that the filter adds when installed at the artificial airway;
 Filter size and weight.

Filter manufacturers should provide this information.

Fig. 6.25 Specifications of filters from Intersurgical.

Specifications of filters from Intersurgical.

Reprinted with permission from InterSurgical.

A filter may be mounted in one of three positions in a breathing circuit for different purposes

Fig. 6.26 Three possible positions for a gas filter.

Three possible positions for a gas filter.

The first position is the artificial airway. This is the position of an HME filter for short-term airway humidification.

The second position is the beginning of the inspiratory limb. The inspiratory filter is intended to separate the ventilator and the ventilated patient. On one hand, use of an inspiratory filter can prevent the ventilator from being contaminated. Bear in mind that disinfection or sterilization of the gas passageway inside the ventilator is almost impossible. In addition, it can prevent particles from the source gases or ventilator from reaching the patient.

The third position is the end of the expiratory limb. An expiratory filter can safeguard the medical staff by filtering the expired gas. This is particularly valuable if the patient has infectious diseases, especially airborne transmitted diseases such as SARS (severe acute respiratory syndrome) or H1N1 flu. It also protects delicate sensors. On many ventilators, the primary flow sensor is located at the expiratory valve. This flow sensor may be very sensitive to droplets, secretions, microgerms, or medication aerosols. In this case, an expiratory filter is required.

Safety concerns when using a filter

For all their benefits, breathing circuit filters also introduce risks. An occluded filter may be the most common root cause of circuit occlusion (Williams and Stacey, 2002Peady, 2002Walton et al., 1999). As we mentioned earlier, an invasively ventilated patient has no choice but to breathe through the ventilator system, so an occlusion can become a life-threatening emergency to the patient. A ventilated patient cannot inhale with a completely occluded inspiratory limb. The patient cannot exhale with a completely occluded expiratory limb. And they can neither inhale nor exhale with a completed occluded airway.

The purpose for placing a filter in a gas passageway is to remove particles from the gas that passes through it. Where do the particles go? Clearly, the particles accumulate at the surface of the filter material.

A vacuum cleaner is a perfect example. To assure the normal functioning of a vacuum cleaner, we must periodically replace or empty its dust bag, which is comparable to a filter. When the bag is new, empty, and clean, the resistance is negligible. Over time, the resistance increases progressively when the dust bag fills up.

In a ventilator system, the gas at the airway and the expiratory limb contains particles, including moisture, secretion, microgerms, blood clots, and medication aerosols. Filters installed at these two locations will gradually occlude. So, the question is not whether the occlusion will occur, but when or how fast the occlusion will become clinically relevant. An inspiratory filter, by contrast, is seldom blocked because inspiratory gas is typically clean and dry.

Be aware too that an HME at the artificial airway is also a filter, although its purpose is to warm and humidify the inspiratory gas. Note that the use of an HME is contraindicated under some conditions. One of the conditions is that the ventilated patient has bloody, thick, or copious sections.

In-line nebulization dramatically increases the probability of the filter occlusion, evidenced by the case report in Box.

How to recognize filter occlusion

The risk of occlusion exists any time a filter is used at the airway or in the expiratory limb. Therefore, whenever the ventilated patient shows signs of respiratory distress syndrome (RDS), we should check or replace the filter or HME in order to eliminate that possibility.

Preventive recommendations

Follow these recommendations to prevent possible filter occlusion:

 Do not use an HME on patients with thick, copious, or bloody secretions.
 Use a filter only when the potential benefits outweigh the potential risks.
 Ensure that all clinicians involved are sufficiently informed about the potential risks of using filters at the airway or in expiratory limb.
 Inspect the filter periodically, and document the inspection. Remove or replace the filter immediately if you suspect occlusion.
 Remove any HME and expiratory filter from the circuit during in-line nebulization.
 Immediately disconnect the ventilated patient at the airway opening whenever severe respiratory distress syndrome appears, and continue ventilation manually.
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