Prevention of cilial damage and reduced drying of secretions
Cilial paralysis and reduced rates of mucus flow occur below 50% relative humidity at 37 °C, but how long it takes for irreversible and/or significant changes is not known. During brief general anaesthetics, this is not a problem. Prolonged severe dehydration of the bronchial tree leads to encrustation of mucus and bronchial or endotracheal obstruction, particularly in neonates and patients with respiratory infection.
Some heat-moisture exchangers incorporate viral/bacterial filters.
All humidifiers increase the component count in the breathing circuit and increase the risk of disconnection. Heated humidifiers commonly require actively heated delivery tubes, and these may be heavy and bulky or use non-standard connectors. This is much less of a problem with heat-moisture exchangers, although some have non-standard connectors or are quite bulky.
An uncommon event with modern servo-controlled active humidifiers, although burns from the delivery hose have been recently reported. Overheating is impossible with heat-moisture exchangers.
Usually only a problem with nebuliser-type devices. It is impossible with heat-moisture exchangers.
Circuit resistance, deadspace and circuit compliance changes
Most modern heat-moisture exchangers cause a small increase in resistance to gas flow, typically 2 cm H2O at 40 L/min (a typical inspiratory flow rate during anaesthesia). Obstruction of heat-moisture exchangers with mucus, or as a result of expansion of saturated heat-exchanging material, may occur and can result in dangerous increases in resistance. Heater humidifiers also increase circuit resistance but usually to a lesser extent (provided that tubing of adequate diameter is used). Bubble-through humidifiers cause obvious increases in resistance. Rain-out may cause obstruction of breathing tubes.
Deadspace considerations in heat-moisture exchangers limit their performance but not their clinical utility. Greater mass of heat-exchanging material improves performance (especially with larger tidal volumes) but the deadspace increases as well, so it is usually necessary to choose the right size heat-moisture exchanger to suit the patient.
Increased circuit compliance is important to consider when ventilating neonates.
This is not a problem with disposable heat-moisture exchangers, but can occur in the ICU with the water bath of heated humidifiers.
Possible on tilting the water bath of some heater humidifiers, particularly for neonates on continuous flow circuits. Cannot occur with heat-moisture exchangers.
Interference with other devices
Excessive humidity in the proximal breathing circuit from heated humidifiers may interfere with sampling (side-stream)-type CO2 analysers, and condensation may affect the reading on some tidal volume meters. Rain-out from active humidifiers may be a considerable problem, particularly in ICUs.
Heat-moisture exchangers are probably inadequate for:
- Prolonged ICU use, i.e. more than 2 to 3 days
- More than 6 hours or so where respiratory secretions are a problem
- Warming cold patients
Active heater humidifiers can provide 100% relative humidity at 37 °C or more for prolonged periods and are preferable in the above situations.
I] Heated air humidification versus cold air nebulization in newly tracheostomized patients. Head Neck 2017 Dec;39(12):2481-2487. doi: 10.1002/hed.24917. Epub 2017 Oct 9.ii] Further reading: Physiology Humidification: Basic Concepts