Comparison of the effects of heat and moisture exchangers and heated humidifiers on ventilation and gas exchange during non-invasive ventilation

Objective: To compare the short-term effects of a heat and moisture exchanger (HME) and a heated humidifier (HH) during non-invasive ventilation (NIV). Design: Prospective, clinical investigation. Setting: Intensive care unit of a university hospital. Patients: Twenty-four patients with acute respiratory failure (ARF). Intervention: Each patient was studied with a HME and a HH in a random order during two consecutive 20min periods of NIV Measurements and results: Respiratory rate (RR), expiratory tidal volume (VTe) and expiratory minute ventilation (VE) were measured during the last 5 min of each period and blood gases were measured. Mean pressure support and positive end-expiratory pressure levels were, respectively, 15 +/- 4 and 6 +/- 2 cmH(2)O. VE was significantly greater with HME than with HH (14.8 +/- 4.8 vs 13.2 +/- 4.3 l/min; p<0.001). This increase in VE was the result of a greater RR for HME than for HH (26.5 +/- 10.6 vs 24.1 +/- 9.8 breaths/min; p=0.002), whereas the VT for HME was similar to that for HH (674+/-156 vs 643 +/- 148 ml; p=0.09). Arterial partial pressure of carbon dioxide (PaCO2) was significantly higher with a HME than with a HH (43.4 +/- 8.9 vs 40.8 +/- 8.2 mmHg; p<0.005), without significantly changing oxygenation. Conclusion: During NIV the increased dead space of a HME can negatively affect ventilatory function and gas exchange. The effect of HME dead space may decrease efficiency of NIV in patients with ARF.