Heat and Moisture Exchange (HME) Filters in Respiratory Support
Heat and Moisture Exchange (HME) filters function by collecting heat and moisture from a patient's expired air and returning it to the inspired air, providing passive humidification while potentially filtering viral particles, making them preferable to heated humidification systems which can aerosolize pathogens. 1
Basic Principles of HME Filters
HMEs act as "artificial noses" by capturing the heat and moisture from expired breath and adding it to the subsequent inspired breath, maintaining physiological air conditioning for patients with bypassed upper airways 2
The primary function is passive humidification of inhaled air, which helps prevent damage to the respiratory tract from dry and cold inspired gases 2, 3
HMEs can provide equivalent moisture retention (approximately 33-34 mgH₂O/L) compared to heated humidifiers, even in long-term ventilated patients 2
When functioning optimally, HMEs can maintain a minimum target moisture output of 30 g/m³ for long-duration use in intensive care and 20 g/m³ for short-duration use in anesthesia 3
Types and Construction of HME Filters
HMEs vary in design and filtration properties:
Simple hygroscopic devices provide basic moisture exchange but show poor airborne and liquid-borne filtration efficiency 4
Composite devices with relatively large pores perform well in dry airborne filtration but show increased air flow resistance and poor filtration in the presence of liquid 4
Pleated membrane filters with small pores demonstrate good airborne filtration efficiency and prevent passage of liquid, maintaining low resistance to air flow even in wet conditions 4
HMEs with viral filters (filtration efficiency >99.9%) and bidirectional design are specifically recommended for patients with infectious respiratory conditions 1
Clinical Applications and Benefits
HMEs are strongly preferred over heated humidification (HH) systems in patients with infectious respiratory conditions, as HH involves open flow of humidified air which can aerosolize viral particles 1
A Cochrane review of 33 trials (n=2833) showed equivalence between HH and HMEs in adequacy of humidification and prevalence of occlusion 1
HMEs should be placed immediately when disconnecting from closed-circuit ventilation systems to minimize aerosolization of potentially infectious particles 1
HMEs can be more cost-effective than heated humidifiers, with significantly lower labor and daily costs 2
Limitations and Considerations
HME performance is affected by tidal volume and breathing frequency - as tidal volume increases, absolute humidity (AH) decreases; at higher breathing frequencies, AH tends to increase 5
When supplemental oxygen is delivered through HMEs, absolute humidity can decrease to below 30 mg/L, which may provide inadequate humidification 5
Not all HMEs can filter viral particles, and some lack oxygen ports 1
HMEs can be difficult to breathe through and may require frequent changes due to accumulation of secretions 1
HMEs create a small increase in airway resistance (approximately 3.1 ± 2.5 mbar/L·s), which should be considered during difficult weaning procedures 2
Best Practices for HME Use
For patients with infectious respiratory conditions, use HMEs with viral filters that have filtration efficiency >99.9% and bidirectional design 1
In mechanically-ventilated patients, suspend positive-pressure ventilation before disconnecting ventilator circuits from tracheostomy tubes and place an HME device with viral filter on the tube in the interim 1
Change HMEs when they malfunction mechanically or become visibly soiled, but not routinely more frequently than every 48 hours 1
If HME is unavailable and there is high risk of mucus plugging in a patient with a single cannula tube, consider instilling saline drops through the inline suction port with scheduled suctioning to reduce mucus buildup 1
All discarded HMEs should be considered infectious and disposed of quickly and properly 1