Evaluating humidity recovery efficiency of currently available heat and moisture exchangers: a respiratory system model study

Abstract

Objectives: To evaluate and compare the efficiency of humidification in available heat and moisture exchanger models under conditions of varying tidal volume, respiratory rate, and flow rate.

Introduction: Inspired gases are routinely preconditioned by heat and moisture exchangers to provide a heat and water content similar to that provided normally by the nose and upper airways. The absolute humidity of air retrieved from and returned to the ventilated patient is an important measurable outcome of the heat and moisture exchangers' humidifying performance.

Methods: Eight different heat and moisture exchangers were studied using a respiratory system analog. The system included a heated chamber (acrylic glass, maintained at 37 degrees C), a preserved swine lung, a hygrometer, circuitry and a ventilator. Humidity and temperature levels were measured using eight distinct interposed heat and moisture exchangers given different tidal volumes, respiratory frequencies and flow-rate conditions. Recovery of absolute humidity (%RAH) was calculated for each setting.

Results: Increasing tidal volumes led to a reduction in %RAH for all heat and moisture exchangers while no significant effect was demonstrated in the context of varying respiratory rate or inspiratory flow.

Conclusions: Our data indicate that heat and moisture exchangers are more efficient when used with low tidal volume ventilation. The roles of flow and respiratory rate were of lesser importance, suggesting that their adjustment has a less significant effect on the performance of heat and moisture exchangers.

Keywords: Heat and moisture exchangers; Humidity; Mechanical ventilation; Temperature.

Figure 1 -

Mechanical model. Components include: a large plastic box maintained at 37°C (1), preserved swine lungs (2, 3) and a cascade-type humidifier (7) inside the box. Three unidirectional valves (4, 5 and 6) were inserted within the tubing section external to the box (representing the airway) to direct flow through the humidifier prior so that it could ventilate the lungs. In this same simulated airway segment, a sensor (Vaisala, HMI 32, Woburn, MA, USA) (8) was positioned to detect AH and temperature. The HME (9) under test was individually installed using a random sequence for each testing period. A mechanical ventilator (10) was used to simulate different ventilatory conditions.

Figure 2 -

Effect of tidal volume (200, 500 and 1000 mL) in the context of %RAH. The dead-space volume for each HME is shown in parentheses and the HME units are listed in order of increasing dead-space volume. Overall, the HME models differed slightly in terms of their ability to maintain absolute humidity levels, except when a high tidal volume was delivered. There was an inverse relationship between the efficiency of humidification and tidal volume. Hygrobac S was the HME model with the greatest %RAH.

Figure 3 -

Effect of respiratory frequency (5, 10 and 20/min) with changing %RAH. The %RAH did not depend on frequency. Again, differences in %RAH between HME were minimal.

Figure 4 -

Effect of flow rates (30, 60 and 90 L/min) with changing %RAH. %RAH did not depend heavily on flow rates, and we failed to identify any differences between the HME models in terms of humidification efficiency.