Helmet ventilation and carbon dioxide rebreathing: effects of adding a leak at the helmet ports

Objective We examined whether additional helmet flow obtained by a single-circuit and a modified plateau valve applied at the helmet expiratory port (open-circuit ventilators) improves CO 2 wash-out by increasing helmet airflow. Design and setting Randomized physiological study in a university research laboratory. Participants Ten healthy volunteers. Interventions Helmet continuous positive airway pressure and pressure support ventilation delivered by an ICU ventilator (closed-circuit ventilator) and two open-circuit ventilators equipped with a plateau valve placed either at the inspiratory or at the helmet expiratory port. Measurements and results We measured helmet air leaks, breathing pattern, helmet minute ventilation ( V̇ Eh ), minute ventilation washing the helmet ( V̇ Ewh ), CO 2 wash-out, and ventilator inspiratory assistance. Air leaks were small and similar in all conditions. Breathing pattern was similar among the different ventilators. Inspiratory and end-tidal CO 2 were lower, while V̇ Eh and V̇ Ewh were higher only using open-circuit ventilators with the plateau valve placed at the helmet expiratory port. This occurred notwithstanding these ventilators delivered a lower inspiratory assistance. Conclusions Additional helmet flow provided by open-circuit ventilators can lower helmet CO 2 rebreathing. However, inspiratory pressure assistance significantly decreases using open-circuit ventilators, still casting doubts on the choice of the optimal helmet ventilation setup.