Humidification and filtration
Heat and moisture exchanger (HME) humidifiers
These 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 patient’s expired moisture and heat, and return it to the respiratory tract during inspiration’.
Components
1. Two ports, 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 which contains a medium with hygrophobic properties in the form of a mesh with a large surface area (Fig. 9.3). It can be made of ceramic fibre, corrugated aluminium or paper, cellulose, metalized polyurethane foam or stainless-steel fibres.
Fig. 9.3 Heat and moisture exchanger.
Mechanism of action
1. 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. 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.
3. The HME humidifier requires about 5–20 min before it reaches its optimal ability to humidify dry gases.
4. 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 (HMEF).
5. Their volumes range from 7.8 mL (paediatric practice) to 100 mL. This increases the apparatus dead space.
6. 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.
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 H2O 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 h 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 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.
Hot water bath humidifier
This humidifier is used to deliver relative humidities higher than the heat moisture exchange humidifier. It is usually used in intensive care units (Fig. 9.4).
Components
1. A disposable reservoir of 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 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. 9.5).
2. Dry cold gas enters the container where some passes close to the water surface, gaining maximum saturation. 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 patient’s 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 patient’s 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 rain out (condensation) within the tube.
Problems in practice and safety features
1. The humidifier, which is electrically powered, should be safe to use with no risk of scalding, overhydration and electric shock. A second backup thermostat cuts in should there be malfunction of the first thermostat.
2. The humidifier and water trap(s) should be positioned below the level of the tracheal tube to prevent flooding of the airway by condensed water.
3. Colonization of the water by bacteria can be prevented by increasing the temperature to 60°C. This poses greater risk of scalding.
4. The humidifier is large, expensive and can be awkward to use.
5. There are more connections in a ventilator set up and so the risk of disconnections or leaks increases.
