CO2 Clearance During Noninvasive Ventilation (NIV)

In this randomized cross over study the investigators will evaluate the effects on gas exchange and patients' tolerance of a new CO2 non rebreathing devices incorporated in the mask vs the traditional mask COPD patients undergoing NIV for an episode of Acute Hypercapnia Respiratory Failure, will be randomized after at least 12 hour of ventilation to 1 h.trial with the usual full face mask (FM) or the same mask with the addition of a new CO2 clearance device (FM+d) In the interval between NIV sessions, arterial blood gases will be monitored and if PaCO2 will return to a baseline value with a tolerance of 10%, the patients will undergo another 1 h trial using the other mask. The subjects will be studied in the sitting position and asked to relax throughout the period of the study. The patients will be ventilated using pressure support ventilation (PSV) The order of use of the two interfaces will be determined for each patient by a previously generated randomized sequence. FIO2 will be adjusted to obtain oxygen saturation around 92% at baseline and never modified throughout the study. End-expiratory pressure will always fixed at 2 cm H2O, and trigger sensitivity will set at -0.5 cm H2O. Inspiratory assistance during either PSV will be titrated to achieve a tidal volume between 6-8 mL/kg while avoiding occurrence of discomfort or visually assessed "wasted efforts". The ventilatory settings will determined at the beginning of the first trial, and maintained unmodified throughout all the 3 runs. Patient tolerance to NIMV, defined as the level of comfort using the different mask, will be assessed by means of a scale used and validated in previous studies that is defined as follows: 1, bad; 2, poor; 3, sufficient; 4, good; 5, very good. The patients were asked by the respiratory therapist to answer the following question: "How do the patient feel your breathing is using this mask?" The patient will give the score to the therapist at the end of each run, just before ABG measurements.