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Humidification and filtration
Inhaling dry gases can cause damage to the cells lining the respiratory tract, impairing ciliary function. Within a short period of just 10 minutes of ventilation with dry gases, cilia function will be disrupted. This increases the patienťs susceptibility to respiratory tract infection. Inhaling dry gases has been shown to contribute to the cause of ventilator-associated pneumonia (VAP). A decrease in body temperature (due to the loss of the latent heat of vaporization) occurs as the respiratory tract humidifies the dry gases. It is thought that humidifying and warming inspired dry gas accounts for about 15% of the body’s total basal heat expenditure.
Air, fully saturated with water vapour, has an absolute humidity of about 44 mg/L at 37°C. During nasal breathing at rest, inspired gases become heated to 36°C with a relative humidity of about 80%–90% by the time they reach the carina largely because of heat transfer in the nose. Mouth breathing reduces this to 60%–70% relative humidity. The humidifying property of soda lime can achieve an absolute humidity of 29 mg/L when used with the circle breathing system.
The isothermic boundary point is where 37°C and 100% humidity have been achieved. Normally it is a few centimetres distal to the carina. Insertion of a tracheal or tracheostomy tube bypasses the upper airway and moves the isothermic boundary distally. A similar effect happens when cold and dry gases are inhaled, as a greater proportion of the airways have to participate in heat and moisture exchange to achieve full saturation. Relative humidity in the operating theatre is usually measured with a hair hygrometer.
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Absolute humidity is the mass of water vapour per unit volume of gas measured in mg/L (at a specific temperature).
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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.
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Relative humidity can also be calculated as the water vapour pressure over the saturated water vapour pressure.
Characteristics of the ideal humidifier
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Capable of providing adequate levels of humidification.
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Has low resistance to flow and low dead space.
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Provides microbiological protection to the patient.
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Maintenance of body temperature.
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Safe and convenient to use.
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Economical.
Heat and moisture exchanger humidifiers
Also known as ‘artificial noses’! Heat and moist exchanger (HME) humidifiers are compact, inexpensive, passive and effective humidifiers for most clinical situations ( Figs. 9.1 and 9.2 ). The British Standard describes them as ‘devices intended to retain a portion of the patienťs expired moisture and heat and return it to the respiratory tract during inspiration’.
The efficiency of an HME is gauged by the proportion of heat and moisture it returns to the patient. Adequate humidification is achieved with a relative humidity of 60%–70%. Inspired gases are warmed to temperatures of between 29°C and 34°C. HMEs should be able to deliver an absolute humidity of a minimum of 30 g/m 3 water vapour at 30°C. HMEs are easy and convenient to use with no need for an external power source.
They are positioned between the breathing system and the catheter mount, which in turn is connected to the patient’s mask, tracheal tube or supraglottic airway device (SGA).
Components
- 1.
Two ports that are designed to accept 15- and 22-mm size tubings and connections. Some designs have provision for connection of a sampling tube for gas and vapour concentration monitoring.
- 2.
The head/housing contains a medium ( Fig. 9.3 ), which can be:
- a.
Hydrophobic (water repelling), such as aluminium or coated glass fibres to provide simple and cheap but less efficient HME. This medium has low thermal conductivity maintaining high temperature gradients.
- b.
Hygroscopic (water retaining), such as paper or foam impregnated with calcium chloride or lithium chloride to provide better efficiency.
- c.
Combined hygroscopic–hydrophobic to provide best efficiency.
- a.
Mechanism of action
- 1.
In the hydrophobic HME, warm humidified exhaled gases pass through the humidifier, causing water vapour to condense on the cooler HME medium. The condensed water is evaporated and returned to the patient with the next inspiration of dry and cold gases, humidifying them. There is no addition of water over and above that previously exhaled.
- 2.
In the hygroscopic version, the low thermal medium preserves the moisture by a chemical reaction with the salts, resulting in the chemical affinity to attract water particles, making it more efficient.
- 3.
The greater the temperature difference between each side of the HME, the greater the potential for heat and moisture to be transferred during exhalation and inspiration.
- 4.
The HME humidifier requires about 5–20 minutes before it reaches its optimal ability to humidify dry gases.
- 5.
Some designs with a pore size of about 0.2 μm can filter out bacteria, viruses and particles from the gas flow in either direction, as discussed later. They are called heat and moisture exchanging filters (HMEFs).
- 6.
Their volumes range from 7.8 mL (paediatric practice) to 100 mL. This increases the apparatus dead space.
- 7.
The performance of the HME is affected by:
- a.
water vapour content and temperature of the inspired and exhaled gases
- b.
inspiratory and expiratory flow rates affecting the time the gas is in contact with the HME medium, hence the heat and moisture exchange
- c.
the volume and efficiency of the HME medium: the larger the medium, the greater the performance. Low thermal conductivity, i.e. poor heat conduction, helps to maintain a greater temperature difference across the HME, increasing the potential performance.
- a.
Problems in practice and safety features
- 1.
The estimated increase in resistance to flow due to these humidifiers ranges from 0.1 to 2.0 cm H 2 O depending on the flow rate and the device used. Obstruction of the HME with mucus or because of the expansion of saturated heat exchanging material may occur and can result in dangerous increases in resistance.
- 2.
It is recommended that they are used for a maximum of 24 hours and for single patient use only. There is a risk of increased airway resistance because of the accumulation of water in the filter housing if used for longer periods.
- 3.
The humidifying efficiency decreases when large tidal and/or minute volumes are used.
- 4.
For the HME to function adequately, a two-way gas flow is required.
- 5.
For optimal function, HME must be placed in the breathing system close to the patient.
- 6.
Generally, when aerosolized medications are administered, HMEs need to be removed from the breathing system to avoid aerosol deposition in the medium. However, some HMEs are designed to accept aerosolization.
Heat and moisture exchanger humidifiers
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Water vapour present in the exhaled gases is condensed or preserved on the medium. It is evaporated and returned to the patient with the following inspiration.
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A relative humidity of 60%–70% can be achieved.
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Some designs incorporate a filter.
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Apparatus dead space and airway resistance is increased.
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The water vapour content and temperature of gases, flow rate of gases and the volume of the medium affect performance of HMEs.
Exam tip:
The HME is a very popular topic in the exams. Know how they function and their advantages and limitations.
Bubble humidifier ( Fig. 9.4 )
This is an active humidifier in which the fresh gas flow (FGF) is simply bubbled through a sterile water container/bottle. The small bubbles gain humidity as they rise to the surface of the water. Such humidifiers are used with low-flow oxygen delivery devices, e.g. nasal cannulae. They are not very efficient due to the water losing latent heat of vaporization, so cooling it and making it less volatile, so reducing the amount of vapour produced.
Hot water bath humidifier
This provides active humidification by adding water vapour to a flow of gas in addition to the humidification efforts of the patient. It is used to deliver relative humidities higher than the heat moisture exchange humidifier. It is usually used in intensive care units ( Fig. 9.5 ).
Components ( Fig. 9.6 )
- 1.
It contains a disposable reservoir of sterile water with an inlet and outlet for inspired gases. Heated sterile water partly fills the container.
- 2.
A thermostatically controlled heating element with temperature sensors, both in the reservoir and in the breathing system. The feedback temperature sensor is located close to the patient.
- 3.
Tubing is used to deliver the humidified and warm gases to the patient. It should be as short as possible. A water trap is positioned between the patient and the humidifier along the tubing. The trap is positioned lower than the level of the patient.
Mechanism of action
- 1.
Powered by electricity, the water is heated to between 45°C and 60°C ( Fig. 9.7 ).
- 2.
Dry cold FGF enters the container. This allows FGF to pass over the water surface, bubble through the water or come into contact with wicks dipped in the water, thereby dramatically increasing the surface area available for evaporation. Some gas passes far from the water surface, gaining minimal saturation and heat.
- 3.
The container has a large surface area for vaporization. This is to ensure that the gas is fully saturated at the temperature of the water bath. The amount of gas effectively bypassing the water surface should be minimal.
- 4.
The tubing has poor thermal insulation properties, causing a decrease in the temperature of inspired gases. This is partly compensated for by the release of the heat of condensation.
- 5.
By raising the temperature in the humidifier above body temperature, it is possible to deliver gases at 37°C and fully saturated. The temperature of gases at the patienťs end is measured by a thermistor. Via a feedback mechanism, the thermistor controls the temperature of water in the container.
- 6.
The temperature of gases at the patienťs end depends on the surface area available for vaporization, the flow rate and the amount of cooling and condensation taking place in the inspiratory tubing.
- 7.
Some designs have heated elements placed in the inspiratory and expiratory limb of the breathing system to maintain the temperature and prevent rainout (condensation) within the tube.
- 8.
Some designs have an expiratory limb made from material that is permeable to water vapour.
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