Continuous Positive Airway Pressure (CPAP) is a non-invasive ventilation therapy that supports respiratory function by improving functional residual capacity and maintaining open airways through positive pressure. Closed-circuit CPAP configurations are emerging as a promising alternative to conventional open circuits, offering several advantages. However, their effectiveness can be compromised by CO₂ rebreathing. This study aimed to quantify inhaled CO₂ levels during closed-circuit CPAP therapy with different interfaces and explore the effects of interface volume, inlet and outlet port position and airflow rates on CO₂ accumulation.

Four helmets, differing in port positioning, and One-port and Two-ports total-face Masks were tested under three flow conditions (0, 60, and 80 l/min) using an ad hoc test bench to measure CO₂ accumulation inside the interface.

Results demonstrated that interface design strongly influenced CO₂ retention. Lateral Inlet/Lateral Outlet Helmet (current commercial helmet) showed the highest inhaled CO₂ levels (about 2 %), while the Up Inlet/Front Outlet Helmet achieved lower inhaled CO₂ (0.6 % at 80 l/min). Masks, characterized by smaller volumes, consistently exhibited lower CO₂ retention. Notably, the Two-ports Mask maintained inhaled CO₂ levels below 1 % (patient safety threshold) even without additional recirculation airflow. Increasing flow rates effectively reduced CO₂ rebreathing, with the most pronounced reduction occurring between 0 and 60 l/min.

These findings highlight the critical role of interface design, particularly port positioning, in minimizing CO₂ rebreathing. The results enabled selection of safe interfaces for closed-circuit CPAP. Furthermore, these findings can be extended to conventional open-circuit CPAP therapy, enhancing patient safety in non-invasive ventilation.