Objective: To assess the behavior of different helmets after discontinuation of fresh gas flow by disconnection at the helmet inlet, flow generator, or gas source. Design and setting: Randomized physiological study in a university research laboratory. Patients: Five healthy volunteers. Intervention: CPAP (FIO250%, PEEP 5 cmH2O) delivered in random sequence with three different helmets: 4Vent (R\uc3\ubcsch), PN500 (Harol), CaStar (StarMed) with antisuffocation valve open or locked. For each helmet all three disconnections were randomly employed up to 4 min. Measurements and results: During flow disconnection we measured: respiratory rate and tidal volume by respitrace; inspiratory and expiratory CO2concentration, and FIO2from a nostril; SpO2by pulse oxymetry. Independently of the site of disconnection we observed a fast increase in CO2rebreathing and minute ventilation, associated with a decrease in inspired O2concentration. In the absence of an operational safety valve, larger helmet size and lower resistance of the inlet hose resulted in slower increase in CO2rebreathing. The presence of the safety valve limited the rebreathing of CO2, and the increase in minute ventilation but did not protect from a decrease in FIO2and loss of PEEP. Conclusions: While the use of a safety valve proved effective in limiting CO2rebreathing, it did not protect from the risk of hypoxia related to decrease in FIO2and loss of PEEP. In addition to a safety antisuffocation valve, a dedicated monitoring and alarming systems are needed to employ helmet CPAP safely. \uc2\ua9 2006 Springer-Verlag
