Laboratory performance of oronasal CPAP and adapted snorkel masks to entrain oxygen and CPAP.

body mass index

breaths/min

continuous PAP

coronavirus disease 2019

inspired O2 fraction

L/min

positive airway pressure

positive end‐expiratory pressure

The coronavirus disease 2019 (COVID‐19) pandemic has exhausted supplies of ventilators and patient mask interfaces in many countries around the world. As such, innovative responses from healthcare workers and engineers are combining medical (i.e. continuous positive airway pressure (CPAP)/bi‐level devices) and non‐medical (e.g. adapted Decathlon snorkel mask; Fig. 1) equipment to provide non‐invasive ventilatory support (i.e. oxygen and positive pressure delivery) for patients. In some instances, oxygen (O2) is being entrained to standard CPAP circuits at high flow rates above the manufacturer specifications (e.g. 4 L/min for ResMed S10) to provide both pressure support and supplemental O2 to the patient. Two key limitations of such experimental mask and CPAP setups include: (i) the leak profile and rebreathing characteristics of non‐medical grade masks when pressure and/or O2 are applied are unknown and (ii) quantitative details about the relationship between above specification O2 flow rate and desired inspired O2 fraction (FIO2) levels in closed CPAP circuits are unknown.

Figure 1

Decathlon Easybreath Adult Surface Snorkeling mask with additively manufactured modified valve.

In situations where clinicians have been in short supply of CPAP equipment, they have adapted the Decathlon snorkel mask to provide a ventilatory support interface to those in need. In addition to its role as a salvage option in equipment‐depleted hospital settings, this prototype interface confers potential advantages over conventional medical‐grade designs. It requires no training to fit and non‐critically ill patients can secure the interface without assistance. The mask is versatile in its ability to deliver air/O2 blends with or without positive airway pressure (PAP). Finally, because the Decathlon snorkel mask is sealed, it potentially harnesses infectious aerosols more effectively than conventional O2 delivery systems.

Our aim was to compare the performance/safety characteristics of the adapted Decathlon Easybreath snorkel mask (Decathlon, France) and oronasal CPAP masks across a series of potential clinical applications. Specifically, these include: (i) delivery of high concentrations of O2 and (ii) simultaneous delivery of high‐flow oxygen and positive pressure support.

We conducted a series of experiments with controlled O2 and positive pressure delivery to assess the impact on: (i) FIO2; (ii) inspired carbon dioxide (CO2) and, where appropriate, (iii) mask leak and (iv) mask pressure. All experiments were performed with a sealed oronasal CPAP mask (Airfit F10; Resmed, Sydney, Australia), and repeated with an adapted Decathlon snorkel mask (shown in Fig. 1). Both masks were modified to add sampling ports for measuring O2/CO2 and mask pressure, and to seal their expiratory ports. For the Decathlon mask, the direction of the existing one‐way expiratory/purge valve was swapped (making a one‐way inspiratory valve) and the existing snorkel connection was swapped with a three‐dimensional (3‐D) printed ‘Charlotte valve’ connector which allowed standard medical tubing to be connected to the mask for oxygen/CPAP delivery (Appendix S1 in Supplementary Information). For both masks, expiratory gas was vented through a bio‐filter (SureGard [RJVKB6], Australia). The key aims of the experiments were to assess the performance of both mask interfaces to entrain various concentrations of O2 (30%, 50%, 60%, 80% and 100%), and to determine maximal achievable FiO2 at varied CPAP levels (Resmed S9) with entrained O2, using flow rates above manufacturer specifications.

Two healthy male volunteers were recruited. Participant one (55 years, BMI = 25 kg/m2) completed experiment 1 and participant two (37 years, BMI = 24 kg/m2) completed experiment 2. The participants lay supine with their torso elevated 25° to the horizontal. The participants wore each mask interface attached to each of the experimental respiratory circuits described above. CPAP level, as well as the O2 flow rate and mix were manipulated from an adjoining room (see Appendix S1 (Supplementary Information) for details). Human research ethics approval for this project was obtained by the Monash Health HREC (RES‐20‐0000‐227A‐63 509). Integration across the engineering and clinical science teams was facilitated by the Monash Institute of Medical Engineering and Monash Partners Academic Health Science Centre.

Figure 2 shows the performance of the CPAP mask and Decathlon snorkel mask in delivering various O2 concentrations. For the CPAP mask, FIO2 values were near consistent with the O2 mix delivered. In contrast, the snorkel mask achieved lower FIO2 at every level of O2 mix. The highest FIO2 values achieved (100% O2, 15 breaths/min (bpm)) were 94% for the CPAP mask and 80% for the snorkel mask.

Figure 2

Mask performance of blended O2 circuits. Inspired O2 fraction (FIO2) (left y‐axis) versus the provided O2 (x‐axis) for both continuous positive airway pressure (CPAP) and snorkel masks are shown for both 15 () and 30 () breaths/min (bpm) respiratory rates. Various FIO2 levels were achieved by blending 100% O2 and medical air. The degree of inspired CO2 due to rebreathing is shown for both masks on the lower panels (right y‐axis). Grey line represents the line of identity.

Higher levels of rebreathing (assessed by inspired CO2) were also noted for the snorkel mask (inspired CO2: 13.8–16.7 mmHg, 15 bpm) which was further elevated at the higher respiratory rate of 30 bpm. Rebreathing was minimal with the CPAP mask regardless of respiratory rate (0.1–4.7 mm Hg, 15 bpm).

Similar patterns in FIO2 and inspired CO2 were observed when O2 mixtures were entrained using the venturi valve (Fig. S5 in Supplementary Information). Moreover, the application of a positive end‐expiratory pressure (PEEP) valve to the expiratory line did not alter the performance of either mask, nor did the level of PEEP (range: 5–20 cmH2O) systematically affect the FIO2 or degree of rebreathing (Fig. S6 in Supplementary Information). However, high levels of PEEP (15–20 cmH2O) delivered to the Snorkel mask caused significant leak resulting from the mask ‘lifting’ off the face.

Figure 3 demonstrates the effectiveness of both the CPAP and Decathlon snorkel mask to simultaneously deliver CPAP and high‐flow O2. As expected, the application of CPAP reduced (i.e. diluted) the O2 delivered to the mask. Specifically, CPAP of 10 cmH2O delivered with 15 L/min O2 (100%) produced FIO2 values of 73% (sealed CPAP mask at 15 bpm) and 66% (snorkel mask at 15 bpm). For both masks, the magnitude of O2 dilution increased with higher levels of CPAP. Entrainment of 30 L/min of O2 largely offset this dilution effect and produced higher FIO2 values at all CPAP levels for both masks. These FIO2 values were comparable to delivering 100% O2 alone (without CPAP) at 15 L/min. For the CPAP mask, increasing pressure resulted in a small linear decrease in FIO2 at a rate of 0.8%/cmH2O (15 bpm). In contrast, the snorkel mask demonstrated a steeper, non‐linear decline in FIO2, which accelerated at CPAP >12 cmH2O. This was coincident with an increasing leak, which resulted in the delivered mask pressure plateauing at 14 cmH2O, despite further increases in the CPAP setting. Data from the oronasal mask (and snorkel mask ≤12 cmH2O) show CPAP level was stable and minimally affected by O2 entrainment (Fig. 2).

Figure 3

Mask performance for continuous positive airway pressure (CPAP) and O2 entrainment. (A, B) Inspired O2 fraction (FIO2) (left y‐axis) and inspired CO2 (right y‐axis) across a range of CPAP settings (x‐axis) for both the oronasal CPAP mask and the Decathlon snorkel mask. (C, D) Mask pressure (left y‐axis) and mask leak (right y‐axis) characteristics for the same CPAP settings. All data are shown at both 15 and 30 breaths/min (bpm) respiratory rates, and 15 and 30 L/min (lpm) O2 entrainment (, CPAP mask, O2 30 lpm, 15 bpm; , CPAP mask, O2 30 lpm, 30 bpm; , CPAP mask, O2 15 lpm, 15 bpm; , CPAP mask, O2 15 lpm, 30 bpm; , snorkel, O2 30 lpm, 15 bpm; , snorkel, O2 30 lpm, 30 bpm; , snorkel, O2 15 lpm, 15 bpm; , snorkel, O2 15 lpm, 30 bpm).

Both participants indicated that the oronasal CPAP mask was more comfortable compared to the snorkel mask (further details regarding subjective assessments of mask tolerability are provided in Appendices S2, S3 in Supplementary Information).

Our study demonstrates that the Decathlon Easybreath snorkel mask (i) delivers consistently lower FIO2, (ii) demonstrates higher CO2 rebreathing and (iii) leaks to a greater degree at high CPAP levels, when compared to a sealed oronasal CPAP mask. However, its ease of fit and one‐way valve to entrain room air make it a versatile alternative.

A possible reason for the consistently lower FIO2 with the snorkel mask compared to a CPAP mask is the one‐way valve at the front of the snorkel mask that allows entrainment of room air. An advantage of this valve is that it creates a one‐way open circuit such that delivered gases do not need to be precisely matched to minute ventilation, and allows for dilution of delivered O2 with room air within the mask. Experiments applying positive pressure partly prevent opening of the valve and thus improve FIO2 delivery. The snorkel mask can effectively deliver CPAP levels up to 14 cmH2O. However, above this level, significant leak develops across the forehead seal of the mask prohibiting the ability to deliver higher pressures (despite increases in set pressure). In a COVID‐19 setting, any excessive leak could be problematic as virus aerosolization is a risk for nosocomial spread. Our data show that there was a sharp increase in leak from the snorkel mask beginning at 12 cmH2O. In contrast, the oronasal CPAP mask had fewer difficulties with higher CPAP levels and must be preferred in the setting of delivering higher pressures to COVID‐19 patients.

Importantly, during the CPAP and O2 pressure experiments, we were able to demonstrate the ability of standard CPAP with 30 L/min O2 entrained to deliver high mask FIO2 at consistent CPAP levels. This is a critical demonstration of the equipment capabilities above manufacturer specifications that will give clinicians confidence in delivering treatment in this way.

The snorkel mask demonstrated a higher level of CO2 accumulation at the mask (maximum recorded: 25 mmHg, ~3%) compared to the CPAP mask (5 mmHg, ~0.6%). We feel these levels are tolerable for several hours for most patients receiving care. Caution is advised for hypercapnic patients.

In summary, we have compared the performance characteristics of an adapted Decathlon Easybreath snorkel mask to a standard oronasal CPAP mask for a range of blended O2 and CPAP delivery levels. The Decathlon snorkel mask presents a viable and simpler alternative for selected patients; however, use should be limited to CPAP levels below 12 cmH2O. Standard CPAP with entrained O2 and oronasal mask performs above manufacturer specifications and delivers stable CPAP at high FIO2. In patients suspected of having COVID‐19, mask leak levels should be closely monitored regardless of PAP level or interface type.

Disclosure statement

G.S.H. and S.A.J. have received equipment to support research from ResMed, Philips Respironics and Air Liquide Healthcare. Dr B.A.E. has received funding from Apnimed.

Author contributions

Conceptualization: L.M., D.M., B.A.E., S.A.J. Data curation: D.L.M., D.M. Funding acquisition: D.M. Formal analysis: D.L.M., P.I.T. Visualization: D.L.M., P.I.T. Investigation: S.A.L., L.D.J.T., C.J.B., D.M., B.A.E., S.A.J. Methodology: S.A.L., L.D.J.T., C.J.B., D.M., B.A.E., S.A.J. Project administration: S.A.L., C.J.B., D.M., B.A.E., S.A.J. Resources: L.D., B.A.E., S.A.J. Supervision: B.A.E. Writing—original draft: S.A.L., D.L.M., L.D., L.M., P.I.T., L.D.J.T., C.J.B., G.S.H., D.M., B.A.E., S.A.J. Writing—review and editing: S.A.L., D.L.M., L.D., L.M., P.I.T., L.D.J.T., C.J.B., G.S.H., D.M., B.A.E., S.A.J.

Appendix S1 Methods.

Appendix S2 Results.

Appendix S3 The Landry Intolerability Scale.

Figure S1 Decathlon Easybreath Adult Surface Snorkeling Mask with additively manufactured modified valve attached (left). Close‐up photographs of the 3D printed valve (right).

Figure S2 Respiratory circuit for experiment 1 for both sealed CPAP mask (top two panels) and snorkel mask (bottom two panels).

Figure S3 Respiratory circuit for experiment 1b attached to sealed CPAP mask (left) and snorkel mask (right).

Figure S4 Respiratory circuit for experiment 2 attached to sealed CPAP mask (left) and snorkel mask (right).

Figure S5 Results from the venturi valve when set at 30% and 50% for sealed CPAP mask (left) and snorkel mask (right).

Figure S6 Results from the PEEP experiment for sealed CPAP mask (left) and snorkel mask (right).

Figure S7 General intolerability of each mask (CPAP mask and snorkel mask) for (A) venturi versus wall, (B) PEEP level and (C) CPAP level.

Table S1 Directions for mixing O2 concentrations using O2 and medical air.

Click here for additional data file.