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The “helmet” has been used as a novel interface to deliver noninvasive ventilation without applying direct pressure on the face. However, due to its large volume, the helmet may predispose to Co2 rebreathing. We hypothesized that breathing with the helmet is similar to breathing in a semiclosed environment, and therefore the Pco2 inside the helmet is primarily a function of the subject’s Co2 production and the flow of fresh gas through the helmet.Human volunteer study.Laboratory in a university teaching hospital.Eight healthy volunteers.We delivered continuous positive airway pressure (CPAP) with the helmet under a variety of ventilatory conditions in a lung model and in volunteers.Gas flow and Co2 concentration at the airway were measured continuously. End-tidal Pco2, Co2 production, and ventilatory variables were subsequently computed. We found that a) when CPAP was delivered with a ventilator, the inspired Co2 of the volunteers was high (12.4 ± 3.2 torr [1.7 ± 0.4 kPa]); b) when CPAP was delivered with a continuous high flow system, inspired Co2 of the volunteers was low (2.5 ± 1.2 torr [0.3 ± 0.2 kPa]); and c) the inspired Co2 calculated mathematically for a semiclosed system model of Co2 rebreathing was highly correlated with the values measured in a lung model (r2 = .97, slope = 0.92, intercept = −1.17, p < .001) and in the volunteers (r2 = .94, slope = 0.96, intercept = 0.90, p < .001).a) The helmet predisposes to Co2 rebreathing and should not be used to deliver CPAP with a ventilator; b) continuous high flow minimizes Co2 rebreathing during CPAP with the helmet; and c) minute ventilation and Pco2 should be monitored during CPAP with the helmet.