We designed a prospective, physiological study in 12 patients affected by chronic respiratory disorders. The study was aimed at assessing the diaphragm energy expenditure (PTPdi), lung resistance (R L ) and elastance (E L ), arterial blood gases (ABG), breathing pattern, and dyspnea measured by a visual analog scale during invasive pressure support ventilation (i-PSV) and noninvasive PSV (n-PSV). The ventilator settings were kept the same. Both i-PSV and n-PSV significantly reduced the PTPdi

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... minute, compared with that during a T-piece trial ( 204.4 Ϯ 93.8 cm H 2 O ϫ s/min [i-PSV]; 197.5 Ϯ 119.8 [n-PSV]; 393.8 Ϯ 129.0 [T-piece]). Expired tidal volume (V T e) was significantly higher (p Ͻ 0.05) during n-PSV (615 Ϯ 166 ml) than during i-PSV (519 Ϯ 140 ml). The respiratory pump (PTPdi/V T e) was more effective (p Ͻ 0.05) with noninvasive ventilation (22.3 Ϯ 2 .3 cm H 2 O ϫ s/L for i-PSV versus 17.2 Ϯ 3.3 for n-PSV). R L and E L were similar with the two modes of ventilation. Overall dyspnea was significantly (p Ͻ 0.05) better during n-PSV than i-PSV, whereas ABG were similar. We have shown, in patients affected by stable chronic respiratory disorders not ready to sustain totally spontaneous breathing, that i-PSV and n-PSV are equally effective in reducing the PTPdi and in improving ABG, but that n-PSV seems to be better tolerated.