Influence of leaks and mask design on CO 2 clearance during non-invasive ventilation: a combined bench and physiological study
Nattapat Wongtirawit, Rosie Butterworth, Fernando Vieira, Annia Schreiber, Matthew Ko, Laurent BrochardBackground
Non-invasive ventilation (NIV) interface can generate carbon dioxide (CO 2 ) rebreathing, potentially contributing to NIV failure. The impact of leaks and mask design remains unclear. This study evaluates how these factors influence CO 2 rebreathing in oronasal face masks.
Design, settings, participants
A bench study was followed by a physiological study. The bench study used a mannequin head connected to a breathing simulator exhaling CO 2 . Six oronasal face masks designed for single-limb circuits with varying exhalation port positions were tested with three levels of fit. A CO 2 rebreathing index was calculated, along with intentional and unintentional leaks. The best-performing mask was then compared with two clinically used masks in healthy volunteers. Ventilatory ratio (VR, reflecting dead space) was derived from transcutaneous CO 2 (PtcCO 2 ) and ventilation was assessed by electrical impedance tomography. Analyses used Friedman test and linear mixed-effects model.
Results
The bench study showed (1) mask design as a whole, rather than intentional leak flow rate, was the main determinant of CO 2 rebreathing: exhalation ports situated on the mask body had lower CO 2 rebreathing; (2) unintentional leaks decreased CO 2 rebreathing only with poor mask design and (3) a new under-nose mask with nasal nozzles had the lowest CO 2 rebreathing index. The latter was compared with two other masks in ten volunteers, median aged 32 (IQR 29–34): PtcCO 2 decreased from 37.3 (33.7–39.5) mm Hg at baseline to 32.5 (28.9–35.4) with the new mask (p=0.023). VR with the new mask (0.96 (0.93–1.12)) was lower than with the classical masks (1.40 (1.20–1.50) and 1.38 (1.19–1.45); p<0.001 for both).
Conclusions
Mask design significantly influences CO 2 rebreathing during NIV, with leaks’ effects dependent on exhalation port position. A novel under-nose hybrid mask with embedded nozzles reduces CO 2 rebreathing and dead space when compared with classical masks.
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