The efficacy of apneic oxygenation during intubation using a prototype of an oxygenation laryngoscope - a technical simulation

In this model of apneic oxygenation using different prototypes of oxygenation laryngoscopes, both the distal and medium position of the oxygen supply line were highly efficient in maintaining a 100% oxygen content in the test lungs. The main goal of this model was to assess the ability of the new oxygenation laryngoscope prototypes to facilitate apneic oxygenation using different positions of the oxygen outlet; more proximal outlet positions seem to be easier to build, and might be better for handling of the laryngoscope itself.

To evaluate our hypothesis, a realistic model of the anatomy of the human airway was necessary. In this regard, our anatomically correctly shaped manikin was reasonably reliable to achieve this purpose. We deliberately did not simulate a real apneic oxygenation scenario which either would have meant a human study or at least an animal experiment in a hybrid model connected to a human airway manikin. Our prototype is not a registered medical device and must not be used in humans; furthermore, the anatomy of a usual animal model is completely different from the human airway, so results from animal airway model studies cannot be transferred 1:1 to humans. The prerequisite of continuous denitrogenization of a preoxygenated airway to maintain apneic oxygenation is known for a long time, and our goal was to simulate the ability of different oxygenation laryngoscope prototypes in an anatomically realistic model [6‐8]. Thus, we deemed our model suitable for simulating oxygen consumption without using living organisms and that we could spare animal lives by using the stream of 200mL/min of the oxygen meter instead to address the hypothesis of this study.

The main finding of this study is that for optimal continuous denitrogenization of the airway, the prototypes of the oxygenation laryngoscope with both distal and medium oxygen outlet were highly effective in maintaining apneic oxygenation, whereas a proximal outlet was inferior. Interestingly, maintaining denitrogenization was also possible by using very moderate oxygen flows of only 3 L/min. In theory, this relatively low flow should be more than enough, since an oxygen flow of 3 L/min exceeds oxygen consumption of 200mL/min by the factor 15. Controversely, using higher flow rates, we observed in the past that choosing classical high-flow rates even had adverse effects, as this leads to mixing effects with ambient air, most probably caused by venturi effects of the turbulent flow that these high flow rates created [9, 10]. Furthermore, higher gas flows are also known to have negative effects as they may induce direct pressure-related tissue damage and mucosal inflammation [11]. Thus, this study emphasizes again that the optimal point of oxygen insufflation may be an important factor to maintain apneic oxygenation. In this regard, oxygenation laryngoscopes seem to be promising, since they automatically direct the oxygen where it is needed, without the risk of gas mixtures or drawing ambient air into the lungs [11‐14]. (Fig. 2).

The possibility of supplying oxygen via a laryngoscope is not entirely new and has been described by others. Using a Miller-shaped blade in neonates has shown to decrease the incidence of severe desaturation during neonatal and infant intubations in a study by Dias et al. [15] However, the use of Miller-shaped blades outside the neonatological and pediatric setting is extremely rare. Since the Macintosh blade type is standard and the by far most widespread laryngoscope type in Europe, we decided to create a modified Macintosh blade. Laryngeal oxygen insufflation has also shown to increase the time to 1% desaturation and reduce the overall rate of desaturation during laryngoscopy in children in a study by Steiner et al.[16]. However, this group used a standard laryngoscope blade to which a tracheal tube was taped with sterile dressing. Unfortunately, their manuscript does not provide photos or an exact description where the tip was placed, so the reproducibility of their experiments is limited. Furthermore, legal restraints would not allow the use of such a self-made device in most countries. Windpassinger et al. [17] used an Airtraq and placed a tracheal tube into the tube guidance channel, which was either used for supplying pharyngeal oxygen or not. The found that supplying oxygen prolonged the time to desaturation. Again, the study was performed in infants and young children, and the tip position was not standardized, so it may have included various positions where the tube had ended, and the optimum position remains unclear. However, the importance of the position has been identified in several publications [18].

From what is known about the few commercially available oxygenation laryngoscopes, based on pictures from the manufacturers, we hypothesize that the oxygen ports of these devices are ending at the medium or proximal end of the laryngoscope blades [19, 20]. In this regard, it could be discussed based on our results if a deeper point of oxygen supply on the blade could improve these devices as well and should be subject of further studies. A more distal position may be helpful in regard of blocking room air from entering the airway. A more proximal ending of the oxygen supply may be less effective in preventing room air from entering into the airway which reduces efficiency of apneic oxygenation but may improve sight on the glottis. In our model, we were not able to decide whether a medium or distal supply point is more effective, as the difference between these two points may have been too small to detect a difference.

This version of the oxygenation laryngoscope may have some theoretical advantages over the previous version concerning the handling, since the shape of the conventional Macintosh blade itself has not been altered and only an oxygen line was added; the oxygen port might be integrated into the blade in a future version so that the handling remains exactly the same. Furthermore, the use of a disposable blade in our experiments has theoretical advantages over the reusable prototype used in previous studies, since cross-contamination of infectious pathogens cannot occur any more. In a next step, to prove these assumptions, it may be promising to assess handling quality of both concepts.