Danger of helmet continuous positive airway pressure during failure of fresh gas source supply

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 (FIO₂ 50%, PEEP 5 cmH₂O) delivered in random sequence with three different helmets: 4Vent (Rüsch), 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 CO₂ concentration, and FIO₂ from a nostril; SpO₂ by pulse oxymetry. Independently of the site of disconnection we observed a fast increase in CO₂ rebreathing and minute ventilation, associated with a decrease in inspired O₂ concentration. In the absence of an operational safety valve, larger helmet size and lower resistance of the inlet hose resulted in slower increase in CO₂ rebreathing. 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 FIO₂ and loss of PEEP. Conclusions: While the use of a safety valve proved effective in limiting CO₂ rebreathing, it did not protect from the risk of hypoxia related to decrease in FIO₂ and loss of PEEP. In addition to a safety antisuffocation valve, a dedicated monitoring and alarming systems are needed to employ helmet CPAP safely. © 2006 Springer-Verlag.