Abstract: The helmet has been shown to be effective in ventilating patients with hypoxemic ARF (1).Its use has been limited by several issues related to its intrinsic characteristics.Few physiologic data in healthy subjects have been published.Aim of this study was to investigate the helmet efficacy in unloading respiratory muscles, reducing the inspiratory effort and the WOB by the PTPPes analysis.CO2 rebreathing by analyzing PiCO2 was evaluated.METHODS. 8 volunteers were enrolled.After the positioning of an oesophageal balloon and a 20' spontaneous breathing trial, subjects were ventilated through the helmet with 4 different ventilator settings (10/5, 15/5, 20/5 and 15/10 of PS and PEEP).The helmet cushion was inflated only at 15/5 and the subject was ventilated with same ventilator setting.V',Pao,Pes,PiCO2 traces were recorded and the data were analyzed by specific softwares.ANOVA for repeated measures and Tukey test were performed.P<0.05 was significant. RESULTS.During pressure support delivered through the helmet there was a significant reduction in swingPes (p=0.04),PTP per min (p<0.0001),PTPPes/L (p=0.0002) and RR (p<0.0001) and a significant increase in Vt (p<0.0001) and Vmin (p=0.005)compare to the spontaneous breathing.By increasing the pressure support delivered we also observed a significant decrease in the CO2 inspiratory pressure. CONCLUSION.The results of this study show as NIMV delivered by helmet with all the tested levels of PS and PEEP is efficient in unloading the respiratory muscles and reducing the PTP.In healthy subjects with normal respiratory drive we rarely noticed subject-ventilator asynchrony.Analysis of CO2 rebreathing showed a significant reduction of PiCO2 by increasing PS level.