Comparison of conventional heated humidification to a new active heat and moisture exchanger in the ICU

Background

Heated humidification (HH) is commonly used with or without a heated wire circuit (HWC) to humidify inspired gases during mechanical ventilation (MV). We compared HH and HH with a HWC to a new active heat and moisture exchanger (AHME). The AHME (Humid Heat, Gibeck, Sweden) consists of a typical HME and a heat and water source delivered between the patient and the HME. The volume of water delivered and heat output are based on a set minute ventilation. A pre-set airway temperature of 37°C is used.

Methods

Thirty patients requiring MV for >72 h were studied. Pts received humidification via a HH, HH + HWC (Fisher & Paykel), and AHME in random sequence for 24 h each. All devices were set to deliver 37°C at the proximal airway. During each period of ventilation, the following were measured; airway temperature, min and max body temperature, number of suctioning attempts, volume of secretions, consistency of secretions, number and volume of saline instilled, water usage, condensate, ventilator settings, minute volume, number of circuit disconnections. Water usage was measured by weighing the water bag before and after 24 h use. Consistency of secretions were judged as thin, moderate, or thick as previously described (Suzukawa: Respir Care 1989, 34:976). Condensate was measured by emptying fluid into a graduated container and sputum volume measured by collecting secretions in a Luken's trap. Airway temperature was measured at the ET tube using a rapid response thermistor. Resistance of the AHME was measured before and after use.

Results

There were no differences in any of the variables related to humidification efficiency (secretion volume and consistency, number of suctioning attempts, or volume of saline used). Water usage and volume of condensate were significantly different between devices, but delivered airway temperatures were not. Statistical analysis was done with ANOVA. *P < 0.05, see Table.

Minute volume was similar between groups (11.6 ± 3.3 vs 11.9 ± 3.4 vs 11.8 ± 2.7 l/min) as was bias flow during flow triggering (5.8 ± 2.5 vs 5.4 ± 2.6 vs 5.9 ± 2.3). AHME resistance before and after use was unchanged (1.66 ± 0.11 vs 2.28 ± 0.82 cm H₂O/l/s).

Conclusion

In this early study, the AHME provided equivalent humidification as HH and HH + HWC with a lower water usage. This occurs because the HME portion of the AHME returns –32 mgH₂O/l, which only requires the active portion to add ~ 12 mgH₂O/l to reach 44 mgH₂/O/l. Additionally, by placing the AHME between the patient and ventilator circuit, continuous flow from flow triggering systems is not humidified. No other differences were noted. Disadvantages of the AHME include deadspace (-70 ml), weight on the ET tube and the heat source near the patient. Measured external temperature of the AHME did not exceed 38°C. Further long term studies are required to define the role of the AHME.

Table 1