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Do not use a heat moisture exchanger (HME) for a patient who has 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.undefined#ref2">2 Do not use an HME for an infant.
The respiratory tract heats and humidifies gas entering the lungs, so it 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 by an artificial airway during invasive mechanical ventilation, humidification must be added to the dry inhaled gas.2 Humidity is necessary to prevent hypothermia, disruption of the airway epithelium, bronchospasm, and atelectasis, and it keeps airway secretions thinned. In severe cases, thick airway secretions may occlude the artificial airway or obstruct the lower airways. Two types of humidification systems are available for use with mechanical ventilation: (1) active heated humidifier (HH), and (2) passive HME.
During normal respiration, the humidity in the trachea ranges from 36 to 40 mg H2O/L,2 and the optimal required moisture below the carina is 44 mg H2O/L (100% relative humidity at 37°C [98.6°F]).2
Active HHs increase the heat and water vapor content of inspired gas. The humidifier should provide a humidity level between 33 and 44 mg H2O/L2 and a gas temperature between 34°C and 41°C (93.2°F and 105.8°F)1 at the circuit Y-piece, with a relative humidity of 100% to prevent the drying out of secretions in the artificial airway.2 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.2 A relative humidity of 100% may be confirmed by the presence of condensate in the circuit Y-piece connector.2
HMEs operate passively by storing heat and moisture from the patient’s exhaled gas and releasing it to the inhaled gas. There are three types of HMEs: hydrophobic, hygroscopic, and filtered. The HME should provide a minimum humidity level of 30 mg H2O/L.2 An HME is better than HH for short-term use (96 hours or less)2 and for use during transport.1 An HH should be used for patients who exhibit contraindications to HME use or who are on mechanical ventilation for an extended time.
HMEs should not be used on patients with low tidal volumes or increased dead space, ventilation requirement, or arterial partial pressure of carbon dioxide (PaCO2) levels. HMEs are contraindicated if the patient has:
*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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