The simplest method of humidifying the inspired gases is the instillation of water droplets directly into the trachea, but this is the least satisfactory technique and cannot be recommended for routine use. It carries the danger that, if the drip is run too rapidly, the water or saline instilled into the trachea may lead to a condition similar to the acid-aspiration syndrome.
The heat and moisture exchanger (HME) provides another simple but more effective technique of humidification. Figure 12.5 shows one type of HME to illustrate its principle. It has an inlet and an outlet which can be screwed together to form an enclosed unit. Inside this unit is a disposable element of paper, sponge, foam or fibre material impregnated with a’ hygroscopic substance such as calcium chloride, lithium chloride or silica gel. When the warm moist expired gas passes through this element the gas is cooled and water condenses while the element is simultaneously warmed by the latent heat of the water condensing on it and by the exhaled gas. During inspiration the reverse process occurs; the moisture evaporates humidifying the inspired gas and since the element has been warmed above room temperature, the inspired gas is also warmed.
As this technique of conservation of moisture occurs in the nose this humidifier has been called the ‘artificial nose’. The name ‘condenser humidifier’ has also been used. The efficiency of the humidifier will depend on the ambient temperature; thus in a hot climate there may be little temperature change within the hygroscopic element and negligible condensation. Under optimum conditions, however, an inspired air humidity of over 25 g m−3 may be achieved with some types of HME.
At high minute volumes the HME is less efficient and a general problem with HMEs is that secretions may be deposited on the mesh, particularly if the patient coughs increasing the resistance to breathing and the risk of infection with organisms such as pseudomonas. Frequent changes of the humidifier are therefore recommended by the manufacturer.
A variant of the HME is available which includes a bacterial filter; it is known as the Heat and Moisture Exchanging Filter (HMEF) and is used to isolate infected patients from breathing systems and to protect patients who are particularly vulnerable to infection.
Another system of humidification uses a water bath; in its simplest form this system consists of dry gases bubbling through water at room temperature, but there are then two main disadvantages. Firstly, such a system lacks efficiency because of the large bubbles, although by passing the gas through sintered glass to reduce the bubble size, the efficiency can be increased. The second problem is the loss of heat from the water by latent heat of vaporization resulting in cooling which reduces the humidity obtainable. This problem of cooling is overcome by adding an electric heater, which can also obviate the need to bubble the gas stream through the water (Fig. 12.6), and adjustment of the water bath temperature gives flexibility in the amount of water vapour which can be added to the gas stream.
The temperature at which the humidifier is operated depends on the humidity required at the patient and the temperature drop along the tubing. The operating temperature is normally about 40 to 45°C, but higher temperatures up to 60°C may sometimes be employed to prevent the growth of bacteria within the humidifier. There is a danger of scalding the patient if too high a temperature is present. This risk may be further increased in the case of patients on ventilators, especially if a ventilator fault occurs that results in a sudden increase in flow through the humidifier.
If the patient continuously inspires fully saturated gas at body temperature or higher, the heat normally lost via the respiratory tract in the warming and humidification of the inspired air is prevented. In fact, heat may even be added to the patient and, if other physiological methods of heat loss are depressed and inadequate, hyperthermia may occur especially in young children.
To avoid the dangers of overheating of the humidifier, a thermostat, usually operated by a bimetallic strip, is present (Fig. 12.6). If the water overheats, the thermostat automatically switches off power to the heater. In modern humidifiers there is usually also a second thermostat in reserve in case the first fails as well as a thermometer to indicate the temperature of the humidifier.
A potential problem with this humidifier is illustrated in Fig. 12.7. The water vaporized can condense in a redundant loop of the delivery tubing to obstruct the gas flow to the patient; such a pool of hot water could even be blown into a patient’s tracheostomy. It is important, therefore, to ensure that the water condensing out can run back to the humidifier or into a water trap.
Figure 12.8 illustrates further refinements to the heated water bath principle. This is the cascade humidifier, in which gas bubbles through a perforated screen at the bottom of a wide tube causing a foam of water and gas to pass up the tube. The large surface area of gas exposed to the water ensures that it is fully humidified. The thermistor positioned at the patient connection on the right of the figure monitors the temperature of the gases delivered and controls the heater element in the water bath to maintain an optimal delivery temperature and avoid any risk of burns to the patient. This gives safer and more accurate control than is possible when only the water bath temperature is monitored. As the gases are fully saturated, the temperature at the patient also indicates absolute humidity at this point.
An alternative technique for adding water as vapour is the heated element humidifier illustrated in Fig. 12.9. Water is vaporized by dripping it on to an electric element heated to 100°C, the high temperature ensuring sterility. Also shown is a water trap which collects excess water. In an alternative form of this device, small pulsed volumes of water are delivered to the heated element and an element temperature of 250°C is used. However, humidifiers with such hot elements are less suited to use with anaesthetic vapours or volatile drugs as the heat may cause chemical changes. The amount of water vapour delivered from these humidifiers must be controlled according to the minute volume and humidity required, and a thermistor at the patient connector may be present to assist this control and avoid any risk of burns.
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