In the present in vitro study, we developed an apparatus that allowed us to set a specific maximal pressure at the proximal end of the flexible optical scope when insufflating oxygen through either the suction channel or the working channel of the scope (Table 1). Subsequently, we measured the resulting pressures and flows of the oxygen that were delivered at the distal end of the flexible optical scopes (Tables 2 and 3). We aimed to find the adjustable pressure-limiting (APL) valve and fresh gas flow setting on the anesthesia machine that would deliver oxygen at a flow > 3 L·min−1 and pressures ≤ 40 cm H2O at the distal end of the scope.

Table 1 Technical specifications of tested flexible optical scopesTable 2 Pressure and flow measurements of oxygen delivered through the suction channel of flexible optical scopesTable 3 Pressure and flow measurements of oxygen delivered through the working channel of flexible optical scopesDescription of the setup

To control the pressure delivered by the setup, oxygen is delivered to the flexible optical scope by connecting it to the circle system of an anesthesia machine. This allows users to preset a desired maximum pressure that can be delivered to the flexible scope by manually turning the wheel of the APL valve to the targeted pressure. The user then interconnects the circle system and the flexible optical scope either via the suction channel or the working channel. The Electronic Supplementary Material (ESM) eAppendix contains a detailed description of all material.

Insufflation via the suction channel

See Fig. 1 and ESM eVideo 1. The proximal end of the connection piece of an endotracheal tube with an inner diameter of 6.0 mm is connected to the circle system filter (Fig. 1B). The distal end is connected to a 6.0-mm standard oxygen connecting tube, which is then connected to the suction port on the handle of the scope (Fig. 1C–D).

Fig. 1

Stepwise assembly of the setup for insufflating oxygen through the suction channel of a flexible optical scope. (A) All equipment gathered, from top to bottom: circle system and filter, 6.0-mm endotracheal tube connector, 6.0-mm oxygen tube, and flexible optical scope. (B) Proximal end of endotracheal tube connector connected to filter and circle system. (C) Oxygen tube connected to distal end of endotracheal tube connector. (D) Oxygen tube connected to suction port. (E) Filter strapped to handle. (F) Setup fully assembled.

Published with written permission from the copyright holder to use/reuse previously published material (airwaymanagement.dk). See also ESM eVideo 1: https://airwaymanagement.dk/preparing_for_insufflation_of_oxygen_via_the_flexible_scope (accessed September 2025).11

Insufflation via the working channel

See Fig. 2. An extendible catheter mount is connected to the circle system filter (Fig. 2B). The proximal end of a universal 15-mm connector of an endotracheal tube with an inner diameter of 4.0 mm is connected to the extendible catheter mount, and the distal end is pushed through the working channel membrane located on the handle of the scope (Fig. 2C–E). At last, to ease handling and prevent displacement, the plastic tubes of the circle system and the handle of the scope are taped together with adhesive surgical tape (Fig. 1E; Fig. 2G; ESM eVideo 1).11

Fig. 2

Stepwise assembly of the set up for insufflating oxygen through the working channel of the flexible optical scope. (A) All equipment gathered, from left to right: flexible optical scope, 4.0-mm endotracheal tube connector, tape, extendible catheter mount, circle system, and filter. (B) Extendible catheter mount connected to filter and circuit system. (C) Proximal end of 4.0-mm endotracheal tube connector connected to extendible catheter mount. (D) Extendible catheter mount arched. (E) Distal end of 4.0-mm endotracheal tube connector pushed through the working channel membrane. (F) Both pipes of the circle system gathered along the handle of the flexible optical scope. (G) Both pipes strapped to the handle. (H) Setup fully assembled.

Published with written permission from the copyright holder to use/reuse previously published material (airwaymanagement.dk)

Figure 3 and ESM eVideo 2 show the adjustment of flow to obtain the targeted safe pressures at the distal end of the flexible optical scopes. In preliminary tests, a high-flow setting from the anesthesia machine resulted in pressures at the distal end of the scope that were slightly above the maximum pressure set on the APL valve. We therefore aimed to determine which flow settings on the anesthesia machine would deliver the same pressure at the distal end of the scopes as the pressure set on APL valve. We targeted pressures of 40, 30, 15, and 10 cm H2O. The APL valve was set to the targeted pressure, and the oxygen flow from the anesthetic machine was initially set to maximum (18 L·min−1). The manual spontaneous breathing mode on the anesthesia machine was activated, and thus, oxygen flowed through the scope. The distal end was gradually submerged into a water-filled transparent tube with centimeter markings indicating the depth from the water surface. Oxygen flow was visualized from the bubbles exiting the distal end of the scope until the flow of oxygen ceased (ESM eVideo 2).12 The depth at which oxygen flow stopped in the water recipient thus corresponded to the pressure delivered by the setup at its distal end in cm H2O. The flow delivered from the anesthetic machine was then reduced until the pressure delivered at the distal end was identical to the pressure setting of the APL valve. We thus identified the maximal flow setting from the anesthesia machine at which the obtained pressure at the distal end of the scope was equal to the APL valve settings at 10, 15, 30, and 40 cm H2O, respectively (Tables 2 and 3).

Fig. 3

Illustration of the automatic flow stop when adequate pressure is reached.

The adjustable pressure-limiting valve was set to 30 cm H2O. (A)–(B) Distal end of the flexible optical scope submerged towards 30 cm H2O. Oxygen flow visualized from bubbles. (C)–(D) Distal end of scope beyond 30 cm H2O. Flow ceased, as visualized by the absence of bubbles.

Published with written permission from the copyright holder to use/reuse previously published material (airwaymanagement.dk). See also ESM eVideo 2: https://airwaymanagement.dk/adjusting_flow_to_obtain_desired_pressure (accessed September 2025).12

Figure 4 and ESM eVideo 3 show the measurement of flow at the distal end of the flexible optical scopes when applying the predetermined pressure levels. Using the same APL and flow setting at which the targeted pressure was obtained, as described in Tables 2 and 3, an upside-down 1-L water-filled container was pulled up to the surface of a water-filled basin without breaching the surface. The distal end of the scope was placed under the 1-L container. Oxygen flowed from the distal end of the scope into the container and filled it up, and the time taken to fill the 1 L container was recorded.13 The delivered flow in L·min−1 was then calculated; Tables 2 and 3 list the resulting flows.

Fig. 4

Illustration of the technique used for flow measurements. (A) Upside-down 1-L container immersed into water. (B) Container pulled to surface. Flexible optical scope ready. (C) Tip of the flexible optical scope immersed under container. Oxygen flows into container. Time recorded. (D) Container filled with 1 L of oxygen and time recording stopped.

Published with written permission from the copyright holder to use/reuse previously published material (airwaymanagement.dk). See also ESM eVideo 3: https://airwaymanagement.dk/measuring_resulting_flow (accessed September 2025).13

We studied the following five flexible optical scopes:

Ambu® aScope 4 Broncho (Ambu A/S, Ballerup, Denmark); sizes: slim, regular, and large

Olympus® BF-P180 EVIS EXERA II (Olympus, Tokyo, Japan)

Olympus LF-V VISERA (Olympus)

Table 1 presents the specifications of the scopes.

The disposable scopes from Ambu were interchanged with identical models between the first and second test to assess any interindividual variability. Each measurement was repeated three times to assess repeatability.