Adapted from Hinds WC. Aerosol Technology. Properties, behaviour, and measurement of airborne particles. 2nd ed. New York: John Wiley and Sons; 1999.

Most penetrating particle size

The relative efficiencies of the five mechanisms of filtration vary with the size of the particle (Fig. 11.2). In particular, particles of a certain size, typically in the range 0.05–0.5 µm, pass through the filter more easily than others. This size is known as the most penetrating particle size. Particles of this size are too small to be directly intercepted by fibres and too large to undergo substantial Brownian motion.

Figure 11.2 Filtration efficiency as a function of particle aerodynamic diameter due to the different filtration mechanisms. Note the minimum efficiency (maximum penetration) at a diameter of about 0.3 µm.

Adapted from Hinds WC. Aerosol Technology. Properties, behaviour, and measurement of airborne particles. 2nd ed. New York: John Wiley and Sons; 1999.

Types of filter

There are two main types of filter material used in breathing system filters:

Glass fibre filters

This filter material consists of a sheet of resin-bonded glass fibres. The fibres are packed densely (Fig. 11.3A) and hence the sheet has a high resistance to gas flow per unit area. A sheet with a large surface area is used to reduce the resistance to gas flow to an acceptable level. The sheet is then pleated to minimize the required volume, and hence dead space, for the housing. This type of filter material is hydrophobic and under normal clinical conditions, does not absorb water.

Figure 11.3 Scanning electron microscope photographs of the surface of different types of filter material. A. Glass fibre; B. tribocharged electrostatic; C. fibrillated electrostatic. Note the 100 µm scale marker in the bottom right corner of the photographs. Bacteria are about 1 µm in diameter.

Electrostatic filters

There are two main types of electrostatic filter material. In both types, the fibre density is lower than in glass fibre filters and hence the resistance to gas flow is lower per unit area. The filtration performance is enhanced by using electrostatically charged material, which attracts and binds with any particles passing through the filter material. Therefore, this type of filter material does not need to be pleated, and a flat layer is generally used in breathing system filters.

1 Tribocharged filters

An electrostatic charge can be induced on two dissimilar fibres by rubbing them together during the manufacturing process, so that one type becomes positively charged and the other type negatively charged (tribocharging). One such filter material is made from fibres of polypropylene and modacrylic which can then be converted into a non-woven felt (Fig. 11.3B).

2 Fibrillated coronal-charged filters

An electrostatic charge can be applied to a sheet of polypropylene by using a point electrode emitting ions (corona charging). An opposite charge can be induced on the rear of the sheet. This type of material is often called an electret. If the sheet of polypropylene is now stretched, the strength of the molecular bonds is enhanced in the direction of the stretching, but reduced in a direction perpendicular to it. The sheet can then be split into fibres – a process called fibrillation – and made into a non-woven filter wad (Fig. 11.3C).

Measuring the performance of breathing system filters

The filtration efficiency of a filter is determined by measuring the number of particles passing through the filter as a percentage of the number of particles in an aerosol challenge to the filter. This percentage is the penetration value for the filter. Although challenges of microbes can be used,⁶ the standard for breathing system filters specifies that the challenge should consist of a particular quantity of an aerosol containing sodium chloride particles having diameters close to the most penetrating particle size.⁷ The filter is challenged at a flow of 15 or 30 L min⁻¹ for filters intended for use with paediatric or adult patients, respectively. Typical penetration values for filters are shown in Fig. 11.4.

Figure 11.4 Penetration through 104 different filters when challenged with an aerosol of sodium chloride particles at the most penetrating particle size against pressure drop measured across the filter (100 Pa ≈ 1 cm H₂O). Filters were tested at flows of 15 l min⁻¹ (paediatric) or 30 l min⁻¹ (adult), respectively. The size of each bubble is proportional to the internal volume of the filter. An ideal filter would have low values for penetration, pressure drop and internal volume (a small ‘bubble’ in the lower left hand side of the figure). However, filters with low penetration (high filtration efficiency) and low-pressure drop tend to be large filters; smaller filters tend to have higher pressure drops and higher penetration values (low filtration performance). N95, N99 and N100 refer to filtration efficiencies: namely <5%, <1% and <0.03% penetration respectively.

Adapted from Wilkes AR. Heat and moisture exchangers and breathing system filters: their use in anaesthesia and intensive care. Part 1 – history, principles and efficiency. Anaesthesia 2011;66:31-39 with permission from Wiley-Blackwell.

Humidifiers

Humidity

Humidity is used to describe the amount of water vapour in air or gas. The mass of water vapour in the gas is the absolute humidity (g m⁻³). The maximum amount of humidity that gas can contain is limited by temperature (Fig. 11.5). At the maximum humidity for a particular temperature, the gas is said to be saturated with water vapour, and the level of humidity is the humidity at saturation. The relative humidity (RH; %) is the absolute humidity of the gas at a particular temperature as a percentage of the humidity at saturation at the same temperature.

Figure 11.5 Humidity at saturation against temperature. Humidity deficit indicates the humidity that must be added by the airways to increase the humidity to (a) BTPS conditions (37°C, 44 g m⁻³, 100% relative humidity). (b) Typical room air (22°C, 10 g m⁻³, 50% RH); (c) room air saturated with water vapour (22°C, 20 g m⁻³, 100% RH); (d) room air cooled so that condensation occurs (11°C, 10 g m⁻³, 100% RH); (e) room air warmed to body temperature (37°C, 10 g m⁻³, 23% RH); (f) expired gas (34°C, 38 g m⁻³, 100% RH); (g) minimum moisture output for humidifiers intended for use with patients whose airways have been bypassed (33 g m⁻³).

Room air at 22°C typically has an absolute humidity level of approximately 10 g m⁻³. The humidity at saturation of air at 22°C is approximately 20 g m⁻³, so that the room air has a relative humidity of approximately 50% RH. If the air is cooled, a point is reached at about 11°C when the absolute humidity level equals the humidity at saturation, and hence the relative humidity is 100%. If the air cools to an even lower temperature, condensation will occur. If room air is inspired, the air is warmed to 37°C by the upper air passages by the time it reaches the lungs, and the humidity is increased from 10 to 44 g m⁻³ (BTPS conditions: body temperature and pressure, saturated). The difference between the two (−34 g m⁻³) is the humidity deficit: humidity must be added by the airways to reduce this deficit to 0 g m⁻³. If the room air is warmed from 22 to 37°C without any humidification, the relative humidity will fall to 100 × (10 ÷ 44) = 23%. The massic enthalpy (latent heat) of evaporation of water is 2.4 kJ g⁻¹. To saturate inspired gasses, which have a low level of humidity, a considerable proportion of the body’s heat production must be used (up to one-third for a neonate). This can then lead to a fall in the patient’s core temperature of more than 1°C.

Humidification requirements

The level of humidity acceptable in gasses delivered to patients whose upper airways have been bypassed depends on the length of time of the bypass. For short-term use the level of humidity may only need to be 20 g m⁻³ (45% of BTPS conditions).⁸ For longer use, for example, for patients in intensive care units, the level of humidity should be at least 33 g m⁻³ (75% of BTPS conditions).⁹ One group has proposed that the level of humidity should be close to BTPS conditions (44 g m⁻³).³

Humidification equipment

Gas can be humidified using either passive or active systems. Passive systems, such as heat and moisture exchangers (HMEs), rely on the patient’s ability to add moisture to the inspiratory gas. Active systems, such as heated humidifiers, add water vapour to a flow of gas independently of the patient. Combined passive and active devices are also now available.

Passive humidification systems

1 Heat and moisture exchangers (HMEs)

HMEs return a portion of the exhaled heat and moisture to the next inspiration. If the exhaled gas is at 34°C and is saturated with water vapour (38 g m⁻³) then, even if the HME is 80% efficient, only 30 g m⁻³ is returned to the patient.

These devices generally consist of a transparent plastic housing so that any obstructions and secretions in the device can be seen readily. The housing contains a layer of either foam or paper that is commonly coated with a hygroscopic salt such as calcium chloride. The expired gas cools as it passes through this layer and condensation occurs, releasing the massic enthalpy of vaporization, which is partly retained by the HME layer. The hygroscopic salt absorbs water vapour, hence reducing the relative humidity of the gas to below saturation level, although some moisture is always lost into the breathing system. During inspiration, the humidity level of the gas entering the HME is usually low, so that the condensate evaporates using the absorbed heat, which also warms the gas. The hygroscopic salt, to which the water molecules are loosely bound, releases the water molecules when the water vapour pressure is low. The inspired gas is, therefore, warmed and humidified to an extent that depends on the moisture content of the expired gas, and, hence, on the patient’s core temperature.

A layer of filter material may also be added to the device (heat and moisture exchange filters, HMEFs). Devices are also available that consist only of a layer of filter material. An electrostatic filter layer used on its own has a very low moisture-conserving performance, but a pleated hydrophobic filter layer used on its own can return some moisture as a temperature gradient builds up within the pleats, allowing condensation during expiration and evaporation during inspiration.

Classification of filters and heat and moisture exchangers

There are, therefore, five broad types of filter and/or heat and moisture exchange devices (Fig. 11.6):

Figure 11.6 Cross-section through HMEs, HMEFs and filters. A. HME only; B. pleated hydrophobic (glass fibre) filter only; C. electrostatic filter only; D. pleated hydrophobic (glass fibre) filter and HME; E. electrostatic filter and HME. Note the gas sampling port (g) and the storage port (s) for the cap from the port when the port is in use.

• A heat and moisture exchange-only device, without a filter layer

• A filter-only device, without a heat and moisture exchange layer, which can be either:

electrostatic, or

pleated hydrophobic (glass fibre) type

• Combined devices with both a heat and moisture exchange layer and a filter layer (HMEF), which can be either:

electrostatic, or

pleated hydrophobic (glass fibre) type.

The range of typical levels of moisture output available is shown in Fig. 11.7. Devices available from some manufacturers are colour-coded to indicate particular types of device (Fig. 11.8).

Figure 11.7 Range of humidification performance of different devices. The moisture output of bottle humidifiers depends on the ambient temperature and the degree of cooling. The moisture output of HMEs, HMEFs and filters varies for different devices and also depends on the tidal volume. In theory, a heated humidifier can be set to deliver any level of humidity if it has the appropriate controls. The values shown illustrate those that can be obtained when following the manufacturer’s recommendations.

Figure 11.8 Examples of HMEs, filters and HMEFs from one manufacturer. Note the colour-coding: blue, HME-only; yellow, filter-only; green, HMEF. Other manufacturers use different colour-coding. The different sizes of devices are intended for use with paediatric and adult patients.

Filters and HMEs are available with different connectors. These include angle-pieces, catheter mounts and gas sampling ports. The gas sampling port is a female Luer-lock connector. When the cap on the port is removed, it can be placed on a storage port (if one is available) or, preferably a tether prevents the cap from becoming mislaid. HMEs are also available that are intended to be connected to a tracheostomy tube (Fig. 11.9).

Figure 11.9 Heat and moisture exchangers intended to be attached to a tracheostomy tube. An oxygen tube can be connected to a port on some devices; some devices also have a port through which the patient’s airways can be suctioned. One device has a valve which, when pressed closed, occludes, allowing the patient to speak.

2 Circle breathing systems

In these breathing systems, a portion of the exhaled gas is usually returned to the inspiratory limb of the breathing system in order to recycle the anaesthetic agents. This gas is first passed through a container of absorbent in order to remove the exhaled carbon dioxide. This reaction generates both heat and water (see Chapter 5).

The removal of one mole of carbon dioxide from exhaled gas generates one mole of water. If all this water vaporizes, and assuming ideal gas conditions exist, an identical volume of water vapour will replace the volume of carbon dioxide. If the exhaled gas contains 5% carbon dioxide, sufficient water vapour could be produced to saturate gas at 33°C, as saturated gas at 33°C contains 5% water vapour by volume (36 g m⁻³ water vapour). This is much more than the minimum of 20 g m⁻³ recommended when the upper airways are bypassed during short-term procedures.⁸ Hence, provided low fresh-gas flows are used with the circle breathing system (so that the humidity in the gas is not diluted excessively by the dry fresh-gas), adequate levels of humidity may be produced by the circle system alone (Fig. 11.10). During anaesthesia, the humidity of the gas in the circle system therefore increases, although it may take up to 1 hour or more to reach a maximum level. The humidity in the breathing system will augment the moisture content of the gas delivered to the patient from a filter or HME sited at the patient connection port.¹⁰

Figure 11.10 Breathing systems with different methods of humidification. A. Open breathing system; B. breathing system with HME; C. breathing system with heated humidifier; D. circle breathing system; E. circle breathing system with HME. The width of the arrows indicates the level of humidity at various points in the breathing system. High levels of humidity can cause condensation if an appropriate temperature is not maintained.

Active humidification systems

In these devices, water vapour is added to the inspired gasses independently of the patient.

1 Bottle humidifier

The simplest type of active humidifier is the bottle humidifier (Fig. 11.11). In its simplest form, gas is directed over the surface of the water (Fig. 11.11A). The water evaporates and the water vapour mixes with the gas, increasing its humidity. The process is inefficient in that, unless the gas flow is very low, there is not sufficient time for the gas to become saturated with water vapour before it leaves the humidifier. The efficiency of humidification can be improved by increasing the surface area for evaporation. This can be achieved by bubbling the gas through the water, either through a tube (Fig. 11.11B) or through a sintered filter (Fig. 11.11C), or by placing a wick into the water: water is drawn up the wick again increasing the surface area for evaporation (Fig. 11.11D). Bubbling gas through the water increases the pressure drop across the device, and the resistance to gas flow may, therefore, be unacceptably high for a spontaneously breathing patient who is required to draw gas through the humidifier.

Figure 11.11 Ambient temperature or bottle humidifier. A. Simple bottle humidifier; B. bubble-through humidifier; C. bubble-through humidifier with sintered filter to reduce the size of bubbles and hence increase surface area for evaporation; D. wick humidifier.

However, in all these systems, the ambient temperature limits the maximum level of humidity that can be generated. For typical room air, the maximum level of humidity (humidity at saturation) that can be achieved is about 20 g m⁻³. However, because the water will cool as evaporation occurs, the level is likely to be less than this. To achieve higher levels of humidity, either the gas or the water (or both) must be heated. Alternatively, a nebulizer may be used (see below).

2 Active heat and moisture exchanger

In this device, there is a heat and moisture exchange layer that retains and returns a portion of the exhaled heat and moisture as with passive versions. However, there is also a source of water that is heated within the device so that additional water vapour can be added to the inspiratory gasses. One version of the device can be used with any HME or HMEF, and, therefore, if a filter is used, the patient can be protected from inhaling any infective droplets (Fig. 11.12). This device adds about 5 g m⁻³ to the humidity provided by the HME or filter with which it is used.¹¹

Figure 11.12 The Medisize Hygrovent Gold. Water is gravity-fed into a space between a heater plate and a Goretex membrane. The water is heated and evaporates: the water vapour then passes through the Goretex membrane into the breathing system.

3 Heated humidifiers

Heated humidifiers typically consist of the following (Fig. 11.13). Water is heated in a chamber that includes a wick, which absorbs the water and increases the surface area for evaporation. The water vapour produced mixes with the supplied gas as it flows through the humidifier. The water lost by evaporation is either manually or automatically replenished from a reservoir. The humidified gas then flows to the patient through a delivery tube, which contains a sensor that monitors the temperature of the gas at the patient end. The desired temperature of the delivered humidified gas is set using a control knob on the humidifier. If this gas is allowed to cool as it flows through the delivery tube, condensation may occur. To reduce this cooling, a heater wire in the delivery tube maintains the temperature of the gas. The temperature of the gas at the outlet of the humidification chamber is also monitored. The difference in temperature between the humidification chamber and the patient connection port is set on a second control knob. If the heater wire increases the temperature of the gas as it flows through the delivery tube, the risk of condensation forming is reduced, but the relative humidity of the gas also decreases. Alternatively, if the temperature of the gas falls, because of the settings on the two knobs, some condensation will occur, but the gas remains fully saturated with water vapour (Fig. 11.14).

Figure 11.13 Hudson RCI Conchatherm IV heated humidifier. There are separate controls for ‘TEMPERATURE’ (temperature of the gas delivered to the patient) and ‘TEMP GRADIENT’ (difference between the temperature at the patient end of the delivery tube and the humidifier outlet).

Figure 11.14 A. Heated humidifier with heated delivery tube. B. Humidifier set so that the temperature of the gas at the patient end of the delivery tube (T1) is higher than the humidification chamber outlet (T2). C. Humidifier set so that the temperature of the gas at the patient end of the delivery tube (T1) is lower than the humidification chamber outlet (T2). In this case, condensation will occur in the delivery tube. The humidity of the gas delivered to the patient is the same in both cases.

Unless a second heater wire is used, condensation will also occur in the expiratory limb. Alternatively, a water trap can be fitted to collect any condensation that forms.

In one type of humidifier, the user has only to select whether the patient is intubated or receiving humidified gasses via a facemask. The temperature and humidity of the gasses are then controlled by the humidifier to provide the optimum level for the patient. The humidifier controls the humidity and temperature and also takes into account the flow of gas (Fig. 11.15).

Figure 11.15 Fisher and Paykel MR850 heated humidifier. To use the humidifier, the user has merely to choose whether the patient is intubated or receiving humidified gas via a facemask. A heated wire in the delivery tube maintains the temperature of the delivered gas and reduces condensation.

Nebulizers

Nebulizers are used to humidify respirable gasses by generating aerosols containing droplets of water from a reservoir of water (Fig. 11.16). The water may additionally contain medication. Some of these droplets evaporate as the gas flows to the patient so that the gas is likely to be fully saturated with water vapour when it reaches the patient’s airways. However, as heat is required for evaporation, the temperature of the gas will fall. The temperature can be increased by warming the water in the reservoir.

Figure 11.16 Aerosol generated by a nebulizer.

There are two types of nebulizer: gas-driven or ultrasonic. Both types can generate large numbers of water droplets, equivalent to a moisture output of several hundred grams per cubic metre, compared to 44 g m⁻³ for humidity at saturation at 37°C.

1 Gas-driven nebulizers

These devices use the Bernoulli principle (see Fig. 7.13) to force driving gas under pressure to entrain water into the system from a reservoir. The water is sucked into the gas stream and is broken up into droplets by the high flow of gas. An anvil, placed in the path of the flow of gas, breaks up the larger droplets into smaller ones, suitable for delivery to the patient (Fig. 11.17A).