Continuous positive airway pressure delivered with a helmet: Effects on carbon dioxide rebreathing

Objective. 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 Co-2 rebreathing. We hypothesized that breathing with the helmet is similar to breathing in a semiclosed environment, and therefore the Pco(2) inside the helmet is primarily a function of the subject's Co-2 production and the flow of fresh gas through the helmet. Design: Human volunteer study. Setting: Laboratory in a university teaching hospital. Subjects: Eight healthy volunteers. Interventions: We delivered continuous positive airway pressure (CPAP) with the helmet under a variety of ventilatory conditions in a lung model and in volunteers. Measurements and Main Results: Gas flow and Co-2 concentration at the airway were measured continuously. End-tidal Pco(2), Co-2 production, and ventilatory variables were subsequently computed. We found that a) When CPAP was delivered with a ventilator, the inspired Co-2 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 Co-2 of the volunteers was low (2.5 +/- 1.2 torr [0.3 +/- 0.2 kPa]); and c) the inspired Co-2 calculated mathematically for a semiclosed system model of Co-2 rebreathing was highly correlated with the values measured in a lung model (r(2) = .97, slope = 0.92, intercept = -1.17, p < .001) and in the volunteers (r(2) = .94, slope = 0.96, intercept = 0.90, p < .001). Conclusions: a) The helmet predisposes to Co-2 rebreathing and should not be used to deliver CPAP with a ventilator; b) continuous high flow minimizes Co-2 rebreathing during CPAP with the helmet; and c) minute ventilation and Pco(2) should be monitored during CPAP with the helmet.