Laryngeal oxygen insufflation via oxygenation laryngoscope may provide a simple and rapid further oxygenation strategy in an unexpected cannot-intubate-cannot-ventilate scenario. This could provide some extra time for alternative airway management strategies, or to bridge time until a more skilled anaesthetist or a device like a video laryngoscope arrive on scene. It could also provide extra time for suctioning in a soiled airway. The oxygenating laryngoscope is easy to use, and oxygen insufflation can be established within seconds.
In this study, oxygen insufflation via the oxygenating laryngoscope was more effective than conventional nasal oxygen insufflation with 10 l·min− 1, as described previously . Not surprisingly, the decrease in oxygen concentration in our simulation lung was delayed by standard nasal oxygen insufflation compared to the control group using air too.
In this model, oxygen insufflation via the oxygenation laryngoscope was more effective than nasal oxygen insufflation at both standard and high gas flows. We have previously attributed a comparable phenomenon to the kinked human airway. Oxygen given via nasal cannula pours into the upper pharynx, where it most likely mixes with nitrogen entering through the manikin’s mouth as in a mixing chamber . In contrast, during deep laryngeal insufflation, oxygen pours out retrogradely through the manikin’s mouth, which is obviously more effective in continuous denitrogenisation of the airway . Also, oxygen insufflation via the oxygenation laryngoscope is simple and does not interfere with intubation efforts.
We were surprised to find that using a high flow system at 20 l·min− 1 with 90% oxygen concentration (limited due to technical limitations) resulted in a lower oxygen concentration in the simulation lung than when using the oxygenation laryngoscope or even nasal oxygen insufflation at standard flows with 10 l·min− 1 pure oxygen. This is a phenomenon that we cannot completely explain. At higher flow speed oxygen was pouring faster in the upper airway than when using conventional oxygen insufflation at 10 l·min− 1, which may have resulted in a more turbulent airflow. Airflows can be described by Reynolds Number (RE). Increasing flow speed can result in a higher RE and thus a more turbulent gas flow. This more turbulent gas flow may increase the inflow of air in the mouth and thus increase mixing with insufflated oxygen in the upper airway. Another possible explanation might be the suctioning of ambient air in the airway at the nostrils where the high flow cannula was placed. At higher gas flows there may be local sub-atmospheric pressures due to the Bernoulli effect, resulting in ambient air being carried along with the insufflated oxygen into the airway. Regardless of the mechanism, higher gas flows must result in higher grades of mixing air in the upper airway with the applied 90% oxygen, since the oxygen content in the simulation lung decreased faster than during standard nasal oxygen insufflation.
This may also be one possible explanation as to why the application of high flow oxygen does not show improved results during the intubation of critically ill patients , whereas standard oxygen insufflation for apnoeic oxygenation shows benefits in regard to time until oxygen desaturation [10, 11]. We recommend this should be the subject of specific studies. Further, it may have adverse effects, for example, deep nasal insertion of a cannula poses the risk of nasal bleeding, which could compromise an already difficult airway further . Another disadvantage of high flow nasal oxygenation is the increased technical effort compared to standard oxygen application, since it needs special devices, whereas additional oxygen supply lines can be attached to even standard or video laryngoscopes. This may be even an additional device for preparedness in regard of cannot-ventilate-cannot-intubate scenarios until a surgical airway can be implemented . Further, applying oxygen at lower flows could be even an advantage in regards of aerosol production during the intubation process in times of COVID 19.
We were not able to determine whether further increased gas flows improve results, since due to technical reasons the maximum oxygen concentration in the insufflated gas is 45% at the maximum possible 60 l·min− 1. This concentration may be perfectly sufficient in spontaneously breathing patients, but it is not suitable to maintain apnoeic oxygenation. Thus, we did not test this. In contrast to this situation where we suctioned all gases from the simulation lung, in a living organism, nitrogen that has entered the lungs is accumulating there quickly as it does not dissolve in the blood stream and impairs oxygen uptake due to disturbing apnoeic oxygenation. In this regard we aimed to see about 70% oxygen concentration as an acceptable result after 20 min. Due to the accumulation of nitrogen, this would have already resulted in severe hypoxia in a living organism. Thus, this model simulates relative differences between oxygenation methods but does not provide correct values with regard to rate of oxygen concentration decrease in living organisms. Therefore, we point out that a technical simulation will always have limitations. However, as described previously it should be a good method to compare the effects of different methods of oxygen application in a human airway .
Human experiments resulting in deliberate desaturation are unethical by any standard, animal experiments are not realistic due to differences of anatomy, and attaching a living organism to a manikin in a hybrid model does not provide any further information. This latter technique would indeed simulate apnoeic oxygenation more precisely but that principle had already been tested over 100 years ago . Thus, our simulation model should be realistic enough and, although a compromise, the best available model to test the formulated hypothesis.
The introduction of video laryngoscopes in clinical routine has changed airway management strategies, as this technology facilitates intubation also in many difficult airway scenarios. Thus, the combination of an oxygenating laryngoscope with video laryngoscopy technology could be a useful invention for future airway management.