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We thank Drs Fiorentino and Esquinas for their interest in our paper. They suggest that background breathing pattern and neural drive influence the tidal volume (Vt) obtained during non-invasive ventilation (NIV). While this may be true with pressure support modes, we used pressure control in our study, this being our mode of choice for the majority of patients on long-term NIV. Pressure control does not require the patient to trigger the ventilator; when set up with an adequate inspiratory pressure (IPAP), the patient’s own respiratory muscles do not contribute to Vt, thus maximizing respiratory muscle rest. We feel that this mode allowed us to assess the passive mechanics of the respiratory system, which are the major determinants of Vt during pressure control NIV.
Modern ventilators do indeed estimate Vt but maintain the target volume only by increasing IPAP. Higher IPAP levels increase leakage and decrease tolerance of NIV, with progressively smaller increases in Vt with each increase in IPAP, as dictated by the decreasing slope of the pressure–volume curve. Our observations (and indeed our clinical experience) suggest that lowering the expiratory positive airway pressure (EPAP) setting, or using an exhalation valve with zero EPAP, will increase ventilation without the need to increase IPAP. Our patients were not in respiratory failure, and we point out that further studies involving measurements of gas exchange would be necessary to ascertain that lowering EPAP does not lead to atelectasis and end-expiratory airway collapse, particularly in the acute situation.
For the study, we used nasal masks (Fisher and Paykel) with exhalation valves (B and D Medical), with the subjects in their habitual sleeping position. All of them used NIV every night, including that prior to the study, so we feel that any additional effect of lung inflation during the study on compliance is likely to be small.