HUMIDIFIERS

Heat and moisture exchange filter (HMEF)

• HMEF is inexpensive, disposable, passive and efficient enough to provide adequate humidification of dry gases for up to 24 hours.
• It creates a sealed unit, near the patient end of the breathing system, containing a hygroscopic material such as calcium chloride or silica gel.
• As the warm and moist gas from the patient reaches the HMEF, the moisture from the gas condenses onto the hygroscopic surface, simultaneously heating the element via the latent heat of condensation
• With the next inspiration of dry cold gas over the moist element this process is reversed, warming and humidifying the gas the patient receives.
• This process is about 80% efficient
• The addition of a 0.2 mm filter renders the interface impermeable to bacteria and viruses
• The disadvantages are that because it is a passive device, the HMEF is not 100% efficient and the patient will therefore lose heat and moisture over time, although this is negligible. Also, it adds dead space ranging from 8 mL in a paediatric HMEF to 100 mL in an adult, while the additional resistance can be up to 2.0 cm H2O ( may not create issues if respiratory function is not compromised significantly).
• The hygroscopic material and filter can act as a dam to secretions, greatly increasing the work of breathing. This is easily remedied by vigilance and replacement.

Water bath humidifiers

• Active water baths can achieve 100% efficiency and can also be used to heat the patient
• But they are bulky and complex and better suited for patients requiring longer-term ventilation or oxygen therapy.
• Passive water baths simply consist of a chamber of water through which the inspired gas is bubbled to achieve full saturation.
• The disadvantage is that the temperature of the water limits the maximum achievable humidity.
• Cooling of the water bath happens secondary to the latent heat of vaporization as the water is vaporized. This is remedied by an active system incorporating a heating element and thermostat.
• The system is designed to keep the water bath at a specific temperature (40–60°C). This increases the temperature of the gas mixture and therefore the achievable humidity. This system is capable of delivering fully saturated gas at 37°C at high flow rates which represents a significant advantage over the HMEF.
• All water baths need to include a water trap in their design, because the cooling of the gas as it moves away from the hot bath to the patient will result in condensation which can accumulate and could result in wet drowning. This risk may be minimized by heating the tubing and preventing condensation forming.
• Water baths at around 40°C minimize the risk of scalding the patient’s airways with overly heated gas, but run the risk of creating an ideal environment for microbial growth.
• By heating the water to 60°C the risk of bacterial contamination is reduced but the gas temperature must now be very carefully monitored.
• A thermistor on a feedback loop to the water bath’s thermostat can adjust the temperature of the water, and therefore inspired gas, to ensure that the patient does not suffer from airway scalding.
• The ideal size of water droplets for humidification is 5–10 microns. Smaller droplets will descend to the alveoli and larger ones will condense in the trachea.
• Scalding is a risk associated with water bath types when the temperature within exceeds 37C.
• Nebulisers are more efficient than water bath types of humidifiers. The Bernoulli effect describes the drop in pressure occurring at a jet, where velocity is greatest, which is employed to draw up water from a reservoir. This effect is used in spinning disc and gas driven humidifiers among others.