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Definitions
Absolute humidity = mass of water held in a given volume of gas
Relative humidity = amount of water vapour expressed as a percentage of
the amount which that volume of gas could hold at the same temperature if it was
fully saturated
Consequences of inadequate humidification
- destruction of cilia and damage to mucus glands
- disorganization and flattening of pseudostratified columnar epithelium and
cuboidal epithelium
- disorganization of basement membrane
- cytoplasmic and nuclear degeneration
- desquamation of cells
- mucosal ulceration
- reactive hyperaemia following damage
Net result is impaired function of the mucociliary elevator. Damage to the
basement membrane and the cells of the airway also leads to bronchiolar collapse
and ultimately atelectasis
Degree of damage is directly proportional to the duration of ventilation with
dry gases
Consequences of excessive humidification
- mucosal heating and burns leading to pulmonary oedema and airway stricture
formation (heated humidifiers)
- f inspired gases are not heated but large amounts of water are administered
to the respiratory tract may be a fall in body temperature due to loss of latent
heat of evaporation
- water overload
- impaired function of mucociliary elevator. May result from an increased volume
of mucus requiring clearance so that the capacity of the mucociliary elevator
may be exceeded
Tolerated range of humidification
- difficult to determine
- under normal conditions isothermic saturation boundary (ie point at which
inspired air reaches 37° C and 100% saturation) is
just below carina
- reproducing an isothermic boundary at the same level in ill patient may be
ideal but not essential in all situations
- absolute humidity >33 g/m3 may be needed to maintain normal
mucociliary function
Ideal humidifier
Should include following features:
- inspired gas delivered into trachea at 32-36° C
with water content of 33-43 g/cubic metre (43 = 100% humidity at 37°
)
- temperature constant at set temperature
- humidification and temperature unaffected by large range of FGF, especially
high flows
- simple to use and service
- humidification can be provided for any mixture of inspired gas
- can be used with spontaneous or controlled ventilation
- safety mechanisms and alarms against overheating, overhydration, and
electrocution
- resistance, compliance and dead space do not adversely affect spontaneous
breathing
- inspired gas remains sterile
Cold water humidifier
- simple and cheap but inefficient
- newer models can achieve 100% relative humidity but gas is unheated or
slightly cooled
- cannot provide adequate humidification for a patient whose airway is bypassed
Hot water humidifiers
- inspired gas passed over (blow-by) or through ("bubble" or
"cascade") heated water reservoir
- gas leaving the resevoir contains high water content, often >43 g/m3
- water bath temperature is thermostatically controlled to allow cooling along
the tubing for an inspired relative humidity of 100% at 37°
- heated wire may be sited in delivery tubing to maintain preset gas
termperature and humidity
- micro-droplets have been reported with bubble humidifiers: may be a potential
source of infection
- note that a drop in temperature of 1°C occurs for every 10 cm of tubing
beyond the end of the delivery hose
- should be used if secretions are thick or bloody, humidification is necessary
for more than 4 days or for children or neonates
- disadvantages:
- overheating malfunction may cause a rise in core temperature and excessive
humidification
- increased work of breathing: greater for Bennett cascade (bubble-through)
than Fisher-Paykel (blow-by) humidifiers
- infection due to colonization of reservoir by bacteria. Current evidence
suggests that humidifiers are not an important source of nosocomial
pneumonia
- excessive condensate in circuit due to cooling of gases ("rain
out") (older models)
Heat and moisture exchanger
- eg Pall filter
- passive humidifiers
- not capable of producing same degree of humidification as hot water
humidifiers
- consists of chamber containing a screen through which respiratory gases pass
in both directions
- screen may consist of layers of wire mesh, block of hygroscopic foam or spiral
or corrugated aluminium foil or of chemically coated (often calcium chloride or
lithium chloride) paper
- during expiration warm moist expired gases pass through cooler drier screen
and water vapour condenses. Specific heat of expired gases and latent heat of
water warm screen. During inspiration relatively cooler and drier inspired gas
is warmed and humidified as it passes through the screen
- large pore size hygroscopic filters are less efficient bacterial filters but
more efficient humidifiers than hydrophobic HMEs (eg Pall, Filtra-therm). Some
filters screen out as many as 99.9977% of bacterial organisms
- hygroscopic HMEs may be suitable for long term ventilation in selected adult
patients as the majority of complications associated with HMEs (thick secretions
and endotracheal tube occlusion) occurred with less efficient HMEs
- disadvantages:
- humidification may be inadequate
- may impose additional work of breathing, especially at high flows.
Pressure differential of up to 5 cmH2O not uncommon at flows of
60 l/min
Other humidifiers
- Venturi/Bernoulli type, water jet type and ultrasonic nebulizers are
actually nebulizers rather than humidifiers
- "Hot-rod" type: water fed onto a heated surface. Volume of water can
be metered to ensure production of a certain degree of humidity at a given
temperature
Further reading
Bersten AD, Oh TE. Humidification and inhalation therapy. In Oh TE. Intensive
Care Manual. 4th ed. 1997
© Charles Gomersall July 1999
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