Effect of the humidification device on the work of breathing during noninvasive ventilation

Objective: Heat and moisture exchangers (HME) increase circuitry deadspace compared to heated humidifiers (HH). This study compared the effect of HH and HME during noninvasive ventilation (NIV) on arterial blood gases and patient's effort assessed by respiratory muscles pressure-time product and by work of breathing (WOB). Design and setting: Randomized cross-over study in a medical intensive care unit. Patients: Nine patients receiving NIV for moderate to severe acute hypercapnic respiratory failure. Measurements: HME was randomly compared to HH during periods of 20 min. Each device was studied without (ZEEP) and with a PEEP of 5 cmH(2)O. At the end of each period arterial blood gases, ventilatory parameters, oesophageal and gastric pressures were recorded and indexes of patient's effort calculated. Results: Minute ventilation was significantly higher with HME than with HH (ZEEP: 15.8 +/- 3.7 vs. 12.8 +/- 3.6 l/min) despite a similar PaCO2 (60 +/- 16 vs. 57 +/- 16 mmHg). HME was associated with a greater increase in WOB (ZEEP: 15.5 +/- 7.7 vs. 8.4 +/- 4.5 J/min and PEEP: 11.3 +/- 5.7 vs. 7.3 +/- 3.8 J/min) and indexes of patient effort. NIV with HME failed to decrease WOB compared to baseline. Addition of PEEP reduced the level of effort, but similar differences between HME and HH were observed. Conclusions: In patients receiving NIV for moderate to severe acute hypercapnic respiratory failure, the use of HME lessens the efficacy of NIV in reducing effort compared to HH.