Humidification and filtration

Published on 07/02/2015 by admin

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Last modified 07/02/2015

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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 min of ventilation with dry gases, cilia function will be disrupted. This increases the patient’s susceptibility to respiratory tract infection. A decrease in body temperature (due to the loss of the latent heat of vaporization) occurs as the respiratory tract humidifies the dry gases.

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.

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’.

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° and 34°C. HMEs should be able to deliver an absolute humidity of a minimum of 30 g/m3 water vapour at 30°C. HMEs are easy and convenient to use with no need for an external power source.

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:

Problems in practice and safety features

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).

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

Hot water bath humidifier

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