Heat and Moisture Exchangers
An HME is most often a passive humidifier, also described as an “artificial nose.” Similar to the nose, an HME captures exhaled heat and moisture and uses it to heat and humidify the next inspiration. In contrast to the nose, with its rich vasculature and endothelium, most HMEs do not actively add heat or water to the system. A typical HME is a passive humidifier, capturing both heat and moisture from expired gas and returning up to 70% of both to the patient during the next inspiration.
Traditionally, use of HMEs has been limited to providing humidification to patients receiving invasive ventilatory support via endotracheal or tracheostomy tubes. More recently, HMEs have been used successfully in meeting the short-term humidification needs of spontaneously breathing patients with tracheostomy tubes.19 Kapadia20 reviewed airway accidents in the intensive care unit for a 4-year period and noted an increasing trend in the incidence of blocked tracheal tubes, which was associated with an increased duration of HME filter use. More recent evidence supports long-term use of HMEs for spontaneously breathing patients.21
The three basic types of HMEs are (1) simple condenser humidifiers, (2) hygroscopic condenser humidifiers, and (3) hydrophobic condenser humidifiers. Simple condenser humidifiers contain a condenser element with high thermal conductivity, usually consisting of metallic gauze, corrugated metal, or parallel metal tubes. On inspiration, inspired air cools the condenser element. On exhalation, expired water vapor condenses directly on its surface and rewarms it. On the next inspiration, cool, dry air is warmed and humidified as its passes over the condenser element. Simple condenser humidifiers are able to recapture only approximately 50% of a patient’s exhaled moisture (50% efficiency).
Hygroscopic condenser humidifiers provide higher efficiency by (1) using a condensing element of low thermal conductivity (e.g., paper, wool, foam) and (2) impregnating this material with a hygroscopic salt (calcium or lithium chloride). By using an element with low thermal conductivity, hygroscopic condenser humidifiers can retain more heat than simple condenser systems. In addition, the hygroscopic salt helps capture extra moisture from the exhaled gas. During exhalation, some water vapor condenses on the cool condenser element, whereas other water molecules bind directly to the hygroscopic salt. During inspiration, the lower water vapor pressure in the inspired gas liberates water molecules directly from the hygroscopic salt, without cooling. Figure 35-5 depicts the overall process of humidification with a hygroscopic condenser humidifier, showing the changes in temperature and the relative and absolute humidity occurring during the cycle of breathing. As shown, these devices typically achieve approximately 70% efficiency (40 mg/L exhaled, 27 mg/L returned).
FIGURE 35-5 Process of humidification with a hygroscopic condenser humidifier. AH, Absolute humidity; RH, relative humidity; T, temperature.
Hydrophobic condenser humidifiers use a water-repellent element with a large surface area and low thermal conductivity (Figure 35-6). During exhalation, the condenser temperature increases to approximately 25° C because of conduction and latent heat of condensation. On inspiration, cool gas and evaporation cools the condenser down to 10° C. This large temperature change results in the conservation of more water to be used in humidifying the next breath. The efficiency of these devices is comparable to hygroscopic condenser humidifiers (approximately 70%). However, some hydrophobic humidifiers that provide bacterial filtration may reduce the risk of pneumonia but be unsuitable for patients with limited respiratory reserve or who are prone to airway blockage because they may increase artificial airway occlusion.22,23
FIGURE 35-6 Process of humidification with a hydrophobic condenser humidifier. AH, Absolute humidity; RH, relative humidity; T, temperature.
Design and performance standards for HMEs are set by the International Organization for Standardization (ISO).24 The ideal HME should operate at 70% efficiency or better (providing at least 30 mg/L water vapor); use standard connections; have a low compliance; and add minimal weight, dead space, and flow resistance to a breathing circuit.25 According to Lellouche and colleagues,26 HME performance varies from brand to brand, and only 37.5% of 32 HMEs tested in the study performed well. Table 35-3 compares performance of several commercially available HMEs according to their moisture output, flow resistance, and dead space.26
Comparison of 25 Heat and Moisture Exchangers
AH, Absolute humidity; NA, not available.
Modified from Lellouche F, Taille S, Lefrancois F, et al: Humidification performance of 48 passive airway humidifiers: comparison with manufacturer data. Chest 135:276, 2009.
As shown in Table 35-3, the moisture output of HMEs tends to decrease at high volumes and rates of breathing. In addition, high inspiratory flows and high FiO2 levels can decrease HME efficiency.25
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