Incomplete Optiflow™ switching and the potential for confusion

Abstract

As the evidence base supporting the use of peri-intubation high-flow nasal oxygen (HFNO) continues to expand [1], there have been advances in the design of the latest generation of systems for use in operating theatres. We would like to highlight several practice point considerations relating to use of such systems.

A limitation of previous designs is that applying a tight-fitting anaesthetic facemask over the incompressible nasal prongs is contraindicated due to the risks of gastric insufflation and barotrauma. The Fisher and Paykel Optiflow™ Switch (Fisher and Paykel, Auckland, New Zealand) modification [2] incorporates a flow-regulated pressure relief valve and a compressible inflow to the nasal prongs so that there is interruption of the 100% oxygen nasal prong gas flow when a tight-fitting facemask is applied. This allows safe and seamless transitions from anaesthetic mask pre-oxygenation and facemask ventilation to HFNO and vice versa.

The user should be aware that, even when a firmly applied facemask is applied over the nasal prongs, the nasal oxygen flow is not completely ‘switched off’. It is likely to be variable but with Optiflow Switch flows above 10 l.min^-1 it is apparent that there is ongoing oxygen ingress via the nasal prongs into the facemask during pre-oxygenation and facemask ventilation. The product literature details that the pressure delivered to the patient will be limited to 30 cmH₂O between flows of 30–70 l.min^-1 [3].

Figure 1 shows the gas analyser data observed when a firmly applied facemask connected to a circle anaesthetic circuit with flow rates 10 l.min^-1 of room air (F_IO₂ 0.21) is applied over the Optiflow Switch nasal prongs and the HFNO flow rate is increased. The analyser measures increasing levels of inspired and end-tidal oxygen consistent with increasing oxygen ingress via the nasal prongs. Additionally, the capnography trace is attenuated, the extent of which is related to the HFNO flow rate. It does not appear to make a significant difference if the mask is a cushion design (e.g. Ambu® King; Ambu A/S, Ballerup, Denmark) or anatomical (e.g. Meditech Anatomical Eco; Meditech Systems Limited, Shaftesbury, UK).

Figure 1

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Photos stitched together show representative gas analyser readings and end-tidal carbon dioxide traces when a firmly applied facemask connected to a circle anaesthetic circuit with flow rates 10 l.min^-1 of room air (F_IO₂ 0.21) is applied over the Optiflow Switch nasal prongs and the high-flow nasal oxygen (HFNO) flow rate is increased.

The incomplete cessation of flow does not detract from the utility, but there is the potential for an occasional user to be confused by the attenuated capnography trace that occurs during pre-oxygenation or facemask ventilation. It is also possible that the end-tidal oxygen reading may not be an accurate indicator of the extent of pre-oxygenation. High-flow nasal oxygen is often reserved for challenging clinical situations and an unexpected capnography trace has the potential to complicate an already difficult situation. As with any advanced airway equipment, it is preferable to develop familiarity with the Optiflow Switch in low-stress situations before using it in high-risk cases.