Heat and moisture exchangers

The heat and moisture exchanger (HME) is located between the tracheal tube and the Y-piece of the ventilator ( Fig 1). It must be positioned above the tracheal tube so that the condensed water and the patient's secretions that are deposited in the filter do not increase the pressure drop ( Table. . . 2). Currently, HMEs are changed every 24 h in order to limit bacterial colonization, reduce the pressure drop, and restore the optimal conditioning capacities of the inspired gases.

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Fig. 1 Position of the HME in the ventilatory circuit. (Reproduced with permission from T.h£m,§cb°L§La/ (.199.5.)..)

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Table 2 International standard ISO 9360/1992 (E) for heat and moisture exchangers

Hydrophobic heat and moisture exchangers

The hydrophobic HME consists of a membrane punctured with pores of diameter approximately 0.2 ^m. Gases and water vapor can pass through the membrane, but water in liquid form at the pressures normally used during mechanical ventilation cannot. The additional inspiratory resistances are very low. The hydrophobic HME functions like the human nose, capturing water vapor and energy from the expired gases and restoring them during the following inspiration. Air is exhaled from the lungs at a temperature of 37 °C and 100 per cent RH (AH = 44 mg H2O/l). If the exhaled air is cooled, both AH and the saturation point decrease, whereas RH remains at 100 per cent. Water vapor condenses on neighboring surfaces or forms very small droplets suspended in the gas. The expired air reaches the end of the tracheal tube at a temperature of approximately 33 °C. RH is still 100 per cent, but AH is only 36 mg H 2O/l. Thus 8 mg H2O/l has condensed on the natural and artificial airways. When the expired air reaches the HME, water condenses on its surfaces, liberating the latent heat of the water vapor which heats the HME. The temperature of the gas leaving the HME is lower. If the expired gas leaves the HME at 20 °C, the AH is 17 mg H 2O/l. This leaves 19 mg of water vapor per liter of gas in the HME associated with the latent heat of vaporization of water. The higher the temperature difference between the two sides of the HME, the greater is the quantity of heat and moisture retained by the HME.

On the next inspiration, the HME transmits the heat and moisture stored during expiration, i.e. 19 mg H 2O/l. The tracheal tube or the tracheostomy provides 4 to 5 mg H2O/l. In all, the HME and the tracheal tube provide approximately 25 mg H2O/l. The patient should be able to provide the remaining moisture and heat from his bronchial tree.

Hygroscopic heat and moisture exchangers

Hygroscopic HMEs add the chemical conservation of water in a hygroscopic layer to the simple physical phenomenon of the hydrophobic HME ( Fig 2). The hygroscopic layer is composed of wool or foam coated with calcium, magnesium, or lithium chloride which retains moisture. On expiration, some of the water is condensed in the condenser element and some is retained in the hygroscopic layer and is released on inspiration without any energy requirement. The heat accumulated by the HME serves only to reheat the air and is no longer required to vaporize the water. The resistance of hygroscopic HMEs is low but can be increased in cases of excess water retention.

Fig. 2 Hygroscopic condenser humidifier. (Reproduced with permission from Thomachot etai (1995).)

Advantages and disadvantages of heat and moisture exchangers

HMEs are also antibacterial filters. They have the advantage of being positioned at the end of the tracheal tube and therefore can filter inspired and expired gases.

Pall® hydrophobic HMEs have an antibacterial efficacy of 99.9999 per cent. This means that only one out of 1 000 000 bacteria above the filter ( Serratia marcescens at 0.5 ^m in monodispersed form) will pass through it. Their cost is very competitive compared with other methods of conditioning inspired gases. Bronchopleural fistula and significant leaking around the tracheal tube are absolute contraindications for the use of this device. Hyperproduction of mucus and hypothermia are relative contraindications.

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