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Do not use heat moisture exchangers (HMEs) for patients who have thick, copious, or bloody secretions, expired tidal volumes of less than 70% of the delivered tidal volume, a body temperature less than 32°C (89.6°F), or high spontaneous minute volumes greater than 10 L/min.1 Do not use HMEs for infants.
When the upper airway is bypassed during invasive mechanical ventilation, humidification is necessary to prevent hypothermia, disruption of the airway epithelium, bronchospasm, atelectasis, and airway obstruction. In severe cases, thickened airway secretions may occlude the endotracheal (ET) tube. The American Association for Respiratory Care states that it is mandatory to humidify inspired gas during mechanical ventilation when an ET or tracheostomy tube is present.1
Two systems are available for warming and humidifying gases delivered to patients who are mechanically ventilated: (1) active humidification through a heated humidifier, and (2) passive humidification through an HME. There are three types of HMEs or artificial noses: hydrophobic, hygroscopic, and filtered. HMEs are better than heated humidifiers for short-term use (96 hours or less) and for use during transport.1 A heated humidifier should be used for patients who exhibit contraindications to HME use.
Heated humidifiers operate actively to increase the heat and water vapor content of inspired gas. HMEs operate passively by storing heat and moisture from the patient’s exhaled gas and releasing it to the inhaled gas.
The respiratory tract heats and humidifies inspired gas so the gas entering the alveoli is warmed to body temperature and fully saturated with water vapor. The upper airway provides 75% of the heat and moisture supplied to the alveoli.1 When the upper airway is bypassed, the humidifier supplies this missing heat and moisture. Because the total required moisture input is 44 mg H2O/L, the portion that is supplied by the humidifier is 0.75 × 44 mg H2O/L = 33 mg H2O/L.1 During normal respiration, the humidity in the trachea ranges from 36 to 40 mg H2O/L, and the optimal required moisture below the carina is 44 mg H2O/L (100% relative humidity at 37°C [98.6°F]).1
For active humidification for patients who are invasively ventilated, the device should provide a humidity level between 33 and 44 mg H2O/L and a gas temperature between 34°C and 41°C (93.2°F and 105.8°F) at the circuit Y-piece, with a relative humidity of 100%, to prevent the drying out of secretions in the artificial airway.1 Although modern active heated humidifiers are capable of delivering gas at temperatures of 41°C (105.8°F) at the Y-piece, a maximum delivered gas temperature of 37°C (98.6°F) and 100% relative humidity (44 mg H2O/L) at the circuit Y-piece is recommended, which is evidenced by the presence of condensate in the ET tube connector.1
When providing passive humidification to patients undergoing invasive mechanical ventilation, the HME should provide a minimum humidity level of 30 mg H2O/L.1 HMEs are not recommended for providing humidification to patients with low tidal volumes, such as when lung-protective ventilation strategies are used, because they contribute additional dead space, increasing the ventilation requirement and arterial partial pressure of carbon dioxide (PaCO2). HMEs are contraindicated for patients who have frank bloody or thick, copious secretions, an expired tidal volume of less than 70% of the delivered tidal volume, body temperature below 32°C (89.6°F), or high spontaneous minute volumes (greater than 10 L/min).1
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Fink, J., Arzu, A. (2021). Chapter 39: Humidity and bland aerosol therapy. In R.M. Kacmarek, J.K. Stroller, A.J. Heuer (Eds.). Egan’s fundamentals of respiratory care (12th ed., pp. 817-8431). St. Louis: Elsevier.
*In these skills, a “classic” reference is a widely cited, standard work of established excellence that significantly affects practice and may also represent the foundational research for practice.
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