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Humidification devices
Humidity describes the amount of water vapour in a gas
humidification is the addition of water vapour to a volume of gas
Humidification is particularly important in prolonged anaesthesia, patients at extremes of age, those with respiratory morbidity and during inhalational anaesthesia to reduce coughing and breath holding.
Absolute humidity
= the mass of water vapour in a volume of gas (g/m3 or mg/litre)
Maximum achievable absolute humidity ∝ saturated vapour pressure (SVP) of that liquid at a given temperature
↑ temperature → ↑ SVP → ↑ ability of that gas sample to hold water in the vapour form
The importance of this relationship can be demonstrated when comparing cool and heated inspired gases
| Temp | relative humidity | absolute humidity |
|---|---|---|
| room temp 20°C | 100% | 34 g/m3 |
| body temp 37°C | 100% | 44 g/m3 |
Relative humidity
= the mass of water vapour in a volume compared with the maximum achievable mass,
→ thus can be expressed as a ratio or percentage
depends on ambient temperature
Physiology of airway humidification
In normal breathing, inspired air is heated and humidified within the nasopharynx, delivering gas with a temperature of 36°C and a relative humidity of greater than 80% to the carina. Key to this process is the large surface areas and turbulence of flow generated by the nasal turbinates
~10% of total body heat loss occurs from the respiratory tract
- 6% the result of natural humidification
- latent heat of vaporization → energy transfer from the patient = heat loss
Delivery of highly saturated and warmed air to the alveoli results in more efficient gas exchange, optimal conditions for mucociliary clearance and avoidance of mucosal injury from drying of secretions
The opposite process occurs during expiration
- water condensing onto the airway surface
- → rewarming the airway fluid lining
- This bi-directional process is a counter-current exchange system which efficiently allows heat and water conservation.
Fresh gases from cylinders, manifolds and pipeline supplies are provided at room temperature and with a relative humidity of 0%, therefore energy losses from the body in the humidification and warming of these gases are higher
use of a supraglottic or endotracheal airway device further exacerbates the above problems as the nasopharynx (the normal site of maximum humidification) is entirely bypassed. Thus, the potential for secretion thickening, ciliary paralysis, airway keratinization/ulceration and hypothermia is increased without the use of a humidification device
damage through delivery of dry gases can occur quickly, with mucociliary clearance stopping after 10 minutes, and epithelial damage apparent within an hour
Humidification devices
active vs passive
Energy input: driving gas vs heat vs electromechanical power
Passive humidification
Heat and moisture exchange devices (HME)
contain a particle and bacterial filter along with a hygroscopic material in the inner core
Counter-current exchanger
- expiration
- water condenses on the inner material
- filter is warmed by the latent heat of condensation
- to a temperature >20°C
- inspiration
- inspired gas flow passing through HME humidified + warmed by the previous energy expenditure
HME require a short period of use to reach their peak efficiency of 50–70%
Efficient function depends on ambient temperature:
- ↑ ambient temperature ↓ efficiency ∵ ↓ temperature gradient
- → poorer condensation
Limitation
- ↑ respiratory dead space
- esp in paed
- ↑ resistance to gas flow
- obstruction of the filter due to secretions
- inefficient at ↑MV
- risk of bacterial colonization
- combined w/ microbiological filter
- pore size <0.2µm trap 99.9% bacteria
- yet still need to replace regularly
Soda lime
used in circle system
usually composed of NaOH & Ca(OH)2
Unlike HME devices, soda lime takes a much longer period of time to provide a suitable level of humidification (approximately 1 hour to provide 20 g/m3).
most efficient at ↓FGF ∴ works best with low flow anaesthesia
Cold water bath humidifier
FGF bubbled through cold water
relatively inefficient: up to 10g/m3
energy loss as latent heat of vapourisation → ↓system entropy → ↓efficiency
water source should be kept below the patient for safety reasons
Active humidification
Hot water bath humidifier
incorporated into the breathing system
water heated to 60°C (to prevent bacterial growth) with FGF passed over within an enclosed unit.
allows for ~80% humidification
efficiency can ↑ with ↑ surface area e.g. wicks
while relative humidity is ↓ at the temperature within device, gas cooling in transit to the patient ↑ humidity to near saturation ∵ SVP falls
A thermistor at the patient end of system for feedback control of temperature
Limitations
- equipment being complex
- heavy & lacking portability
- oversaturation of air
- → condensation/drowning at alveolar level
- scalding risk
- electrocution risk
- bacterial colonization
- maintenance costs
- water condensation in the tubing
- ↑ resistance of the system
- potentially being transferred to patient or impairing ventilation.
Nebulisers
addition of water droplets into a gas flow
→ not "true" humidification device
Gas-driven nebulizers
- utilize a Venturi system and the Bernoulli principle for jet entrainment
- Liquid drops entrained into the gas flow
- in some systems split by an anvil into smaller particles
- partial evaporation & loss of energy from latent heat of vaporization
Spinning disc nebulizers utilize an electrical motor to apply centrifugal forces to liquid in order to produce water droplets.
Ultrasound nebulizers
- a plate vibrating at an ultrasonic frequency (3 MHz)
- break water into droplets of between 1-2 micrometres
- Smaller particles will deposit in the alveoli
- ∵ high efficiency → significant risk of oversaturation + drowning
- ↑ density of gases
- ↑ resistance
- ↑ turbulent gas flows.