Experimental assessment of CO2 rebreathing in closed-circuit CPAP therapy with different non-invasive interfaces

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, of- fering 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.