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OBJECTIVETo evaluate the influence of airway humidification devices on the efficacy of ventilation in difficult to wean patients.DESIGNA prospective, randomized, controlled physiologic study.SETTINGA 22-bed medical intensive care unit in a university hospital.PATIENTSChronic respiratory failure patients.INTERVENTIONSPerformances of a heated humidifier and a heat and moisture exchanger were evaluated on diaphragmatic muscle activity, breathing pattern, gas exchange, and respiratory comfort during weaning from mechanical ventilation by using pressure support ventilation. Eleven patients with chronic respiratory failure were submitted to four pressure support ventilation sequences by using the heated humidifier and the heat and moisture exchanger at two different levels of pressure support ventilation (7 and 15 cm H2O).MEASUREMENT AND MAIN RESULTSCompared with the heated humidifier and regardless of the pressure support ventilation level used, the heat and moisture exchanger significantly increased all of the inspiratory effort variables (inspiratory work of breathing expressed in J/L and J/min, pressure time product, changes in esophageal pressure, and transdiaphragmatic pressure; p < .05) and dynamic intrinsic positive end-expiratory pressure (p < .05). Similarly, the heat and moisture exchanger produced a significant increase in Paco2 (p < .01) responsible for severe respiratory acidosis (p < .05), which was insufficiently compensated for despite a significant increase in minute ventilation (p < .05). This resulted in respiratory discomfort for all patients with the heat and moisture exchanger (p < .01). Adverse effects were partially counterbalanced by increasing the pressure support ventilation level with the heat and moisture exchanger by ≥8 cm H2O.CONCLUSIONSThe type of airway humidification device used may negatively influence the mechanical efficacy of ventilation and, unless the pressure support ventilation level is considerably increased, the use of a heat and moisture exchanger should not be recommended in difficult or potentially difficult to wean patients with chronic respiratory failure. [ABSTRACT FROM AUTHOR]