A passive thin-film mechanical device for visual respiratory monitoring during oxygen mask use: development and bench validation

Objective Continuous respiratory monitoring during transfer from the operating theatre to the post-anaesthesia care unit (PACU) is frequently unavailable despite this being a period of recognised patient vulnerability. Existing electronic respiratory monitors introduce cost, complexity and set-up that may limit use in fast-paced clinical environments. We developed Flow-Bag, a fully passive mechanical breath-indicating device consisting of a thin heat-sealed high-density polyethylene (HDPE) film bag that attaches externally to an oxygen mask. Methods Preliminary film-thickness screening was undertaken using a programmable breathing simulator (PEMDx Pty Ltd, Australia). A 6 µm (per ply) prototype was assessed across respiratory rates of 5-60 breaths min⁻¹ and tidal volumes of 50-600 mL. Three blinded observers positioned 5 m from the device counted visible inflation-deflation cycles over 60-s intervals, with counts compared against simulator-delivered breaths. Performance was additionally assessed across four commercially available oxygen mask models. Results In screening, the lowest detected tidal volume fell from 400 mL with 20 µm film to 150 mL with 6 µm film, the thinnest film that could be reproducibly manufactured using the processes available to us. The final prototype produced observable breath indications in 42 of 56 tested conditions (all tidal volumes ≥150 mL) but not at 50 or 100 mL. Within these detectable conditions, observer counts agreed 100% with simulator-delivered breaths. Performance was maintained across four oxygen mask models. However, sensitivity was reduced by incomplete sealing at either the mask-to-face or device-to-mask interface. Conclusion Flow-Bag is a fully passive mechanical breath indicator. Under controlled bench conditions using a mechanical breathing simulator, it provided reliable visual indication of simulated breathing across respiratory rates of 5-60 breaths min⁻¹ and tidal volumes of 150-600 mL. Subject to clinical validation and safety testing, this device may offer a potentially low-cost adjunct alongside standard monitoring. .