PHYSICAL PRINCIPLES

Humidity, the amount of water vapour in a gas, may be expressed as:

Table 28.1 Relationship of temperature and saturated vapour pressure

PHYSIOLOGY

Clearance of surface liquids and particles from the lung depends on beating cilia, airway mucus and transepithelial water flux. Airway mucus is derived from secretions from goblet cells, submucosal glands and Clara cells, and from capillary transudate. Conducting airways are lined with pseudostratified, ciliated columnar epithelium and numerous fluid-secreting glands. As the airway descends, the epithelium becomes stratified, and then cuboidal and partially ciliated, with very few secretory glands at the terminal airways. The cilia beat in a watery (sol) layer over which is a viscous mucous layer (gel), and move a superficial layer of mucus from deep within the lung toward the glottis (at a rate of 10 mm/min at 37°C and 100% RH). Both cilia function and mucus composition are influenced by temperature and adequate humidification.

The nasal mucosa has a large surface area with an extensive vascular network which humidifies and warms inhaled gas more effectively than during mouth-breathing. Heating and humidification of dry gas are progressive down the airway, with an isothermic saturation boundary (i.e. 100% RH at 37°C or AH of 43 g/m³) just below the carina.¹ Under resting conditions, approximately 250 ml of water and 1.5 kJ (350 kcal) of energy is lost from the respiratory tract in a day. A proportion (10–25%) is returned to the mucosa during expiration due to condensation.

The minimal moisture level to maintain ciliary function and mucus clearance is uncertain. Although reproducing an isothermic saturation boundary at the carina may be ideal, it does not seem essential in all situations. Mucus flow is markedly reduced when RH at 37°C falls below 75% (AH of 32 g/m³), and ceases when RH is 50% (AH of 22 g/m³).² This suggests that an AH exceeding 33 g/m³ is needed to maintain normal function. At this AH level, inspired gas temperature does not appear to be important unless excessive.³ Mucociliary function is also impaired by upper respiratory tract infection, chronic bronchitis, cystic fibrosis, bronchiectasis, immotile cilia syndrome (including Kartagener’s syndrome), dehydration, hyperventilation, general anaesthetics, opioids, atropine and exposure to noxious gases. High fractional inspired oxygen concentrations (FiO₂) may lead to acute tracheobronchitis, with depressed tracheal mucus velocity within 3 h.⁴ Inhaled β₂-adrenergic agonists increase mucociliary clearance by augmenting ciliary beat frequency, and mucus and water secretion.⁵

CLINICAL APPLICATIONS OF HUMIDIFICATION

METHODS AND DEVICES

WATER BATH HUMIDIFIERS

Inspired gas is passed over or through a water reservoir to achieve humidification. Their efficiency is dependent on ambient temperature and the surface area available for gas vaporisation.

COLD-WATER HUMIDIFIERS

These units are simple and inexpensive, but are inefficient, with a water content of around 9 g/m³ (i.e. about 50% RH at ambient temperatures). They are also a potential source of microbiological contamination. Routine use of cold-water humidifiers to deliver oxygen with simple facemasks is unnecessary.

HOT-WATER HUMIDIFIERS (FigureS 28.1 and 28.2)

Inspired gas is passed over (i.e. blow-by humidifier, e.g. Fisher–Paykel, Fisher and Paykel Medical, New Zealand) or through (i.e. bubble or cascade humidifier, e.g. Bennett Cascade, Bennett Medical Equipment, USA) a heated-water reservoir. Gas leaving the reservoir theoretically contains high water content. The water bath temperature is thermostatically controlled (e.g. at 45–60°C) to compensate for cooling along the inspiratory tubing targeting an inspired RH of 100% at 37°C. A heated wire may be sited in the inspiratory tubing to maintain preset gastemperature and humidity (e.g. Fisher–Paykel humidifier). It is commonly believed that hot-water humidifiers do not produce aerosols, but microdroplets (mostly less than 5 μm diameter) have been reported with bubble humidifiers,¹¹ and this may be a potential source of infection.

Figure 28.1 Hot-water ‘blow-by’ humidifier.

Figure 28.2 Hot-water ‘cascade’ or ‘bubble’ humidifier.

Fisher–Paykel humidifier

This is a commonly used blow-by humidifier. The delivery hose is heated by an insulated heating wire to achieve a manual preset inspired temperature. An additional servo-control unit is available, while more recent models have a dual servo unit which combines this function in the heater base. Audible alarms indicate disconnection and variations over 2°C from the set delivery temperature. The heater base is protected from overheating by a thermostat set at 47°C. If this fails, another safety thermostat operates at 70°C. A disposable humidification chamber is filled manually with water or by a gravity-feed set. To minimise rainout, the chamber outlet can be set at a temperature below the delivery hose outlet temperature – 2–3°C is usually adequate and does not compromise RH. Temperature alarms are fixed at 41°C and 29.5°C, with a back-up safety set at 66°C for the delivery chamber.

Although these humidifiers are often regarded as providing optimal humidification in ventilated patients, their performance is lower than expected (36 mg/l with MR 850 with the outlet temperature set to 37°C and the delivery hose set at 40°C, and optimal conditions in bench studies, and up to 41.9 mg/l in practice).¹² However, under conditions of high ambient temperature and/or high ventilator output temperature the inlet temperature to the humidification chamber can be high enough that the heater base operates at a low enough temperature that humidification is impaired,with reduction in inspired water content as low as 19 mg/l in bench studies, and 23.4 mg/l in practice.¹² The automatic compensation now available on this humidifier helps compensate for poor performance, which can also be prevented by setting the chamber outlet and hose to 40°C. High minute ventilation is well tolerated, but a small decrease in water content output has also been found with higher respiratory rates.¹³

Counter flow-heated humidifiers

Using the countercurrent principle for gas and heated water, newer designs may further improve performance so that performance is independent of gas flow and respiratory rate, with lower imposed work of breathing.¹³

HEAT AND MOISTURE EXCHANGERS

Modern HMEs are popular intensive care unit (ICU) humidifiers due to their simplicity and increased efficiency. They all work on the basic principle of heat and moisture conservation during expiration, allowing inspired gas to be heated and humidified. HMEs may be hydrophobic or hygroscopic, and may also act as a microbial filter (HMEF). However, since nosocomial pneumonia is primarily due to aspiration of oropharyngeal secretions followed by secondary ventilator tubing colonisation, HMEFs have not been shown to reduce the frequency of nosocomial pneumonia, and the incidence is similar when HMEs and hot-water humidifiers are compared during prolonged ventilation.^14,15

Modern HMEs are light with a small dead space (30–95 ml), but are varied in their level of humidification. Hygroscopic HMEs adsorb moisture on to a foam or paper-like material that is chemically coated (often calcium chloride or lithium chloride), and this tends to increase their efficiency (i.e. AH around 30 g/m³) compared to hydrophobic HMEs (i.e. AH 20–25 g/m³).^16–21 The efficiency of older HMEs decreases with time;¹³ however, modern HMEs may retain their ability to humidify for at least 4 days with minimal change in resistance.²² Consequently, HMEs that achieve relatively high AH may be suitable for long-term mechanical ventilation in selected patients, particularly as the majority of reported HME complications (e.g. thick secretions and endotracheal tube occlusion) occurred with units of lower humidification levels.^23–25 Nevertheless, HMEs increase dead space and imposed work of breathing, and cannot match the humidification offered by hot-water humidifiers, which remain the ‘gold standard’, particularly if secretions are thick or bloody, minute ventilation is high,^15,19 humidification is necessary for more than 4 days²⁶ or if used in children and neonates.

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