This study shows that FFI can be performed with a higher success rate and a better first pass success in inexperienced providers than conventional intubation by a MacIntosh blade in swine. It is the first trial to prospectively evaluate the potential benefits towards airway management using FFI in an experimental design in swine that support FFI as a reasonable method for intubation. Furthermore, a practical and efficient approach to student and resident training at the same time is described and evaluated. Our results suggest significantly less problems with the establishment of a secure airway when provided with a FFI device compared to CI. This effect proved significant in inexperienced providers as shown by the limited amount of attempts needed to place a tracheal tube. Simultaneously, first pass success rate was distinctly increased when a FFI device was used, suggesting less stress for the animal, less hypoxic episodes during induction and, subsequently, less potentially confounding factors tainting research results.
FFI required a longer intubation time when compared to CI. However, this had no clinically relevant effect. The described prolongation was most likely due to device handling, coordination with the video monitor as well as the time needed to verify tube positioning, since ventilation in our experimental setup could not be initiated before the endoscope had been removed from the tube. A longer time to intubation using endoscopic devices is common when compared to CI in humans [20,21]. However, the time to intubation using FFI significantly improves through training and expertise [21,22]. Furthermore, a shorter time to ventilation by FFI is possible by using respiratory adapters to facilitate ventilation during endoscopy [23].
Teaching and training health care providers to adequately apply flexible endoscopes for intubation purposes is technically challenging, expensive and time-consuming[16,20]. Manikins as well as virtual reality simulators are reliable training methods for FFI and can facilitate skill maintenance [15,24]. In inexperienced providers, reported times until successful FFI compass 80 seconds {Boet, 2010 #2549}[24] up to 260 seconds [15], whereas success rates are cited from 50 percent before training and 80 percent after virtual reality training [24]. On the one hand, this data is based on different study designs and obviously cannot be compared with the porcine model. On the other hand, there is a trend that swine can be intubated more quickly using FFI than the airway models currently used.
One common training opportunity in clinical routine for residents is the awake FFI in patients with anticipated difficult airway. This collective is found in ear-nose-throat and maxillofacial surgery to a higher degree than in other surgical disciplines[25]. However, various national guidelines considerably differ concerning the indication of awake FFI. Moreover, local protocols and clinical routine often does not foster FFI. Thus, structured and sufficient training programs have to rely on manikins and other models as well[22]. Animal models are perceived as more realistic compared to manikins, suggesting benefits during resident training [17]. Our model could not only provide the opportunity to perform airway management training, but, in contrast to clinical approaches, more than one provider could also be trained on the same animal subsequently and repeat the procedure multiple times, offering a more efficient way to gain proficiency in device handling. Additionally, the use of a video monitor allows a supervisor to directly teach the procedure, thus potentially improving the experience even further. Single-use bronchoscopes, as shown in our trial, can also be used repeatedly to decrease material costs. Alternatively, following our own protocol, an established research facility could schedule regular short intubation trainings during the induction of their animals with the possibility to further use the animal for protocol purposes as needed afterwards. This obviously depends on experimental setups and possible confounding effects on specific studies but might be valid in some cases. Since this would decrease animal numbers by dual-use in research and education, institutional approval to respective protocol addenda should not be problematic.
Using endoscopic techniques for tracheal intubation in animals is sophisticated and rarely used, but reports in rodents[26], ruminants[27] , swine[17] and more exotic animals[28] have been previously published. Interestingly, the last - and to the best of our knowledge - only published scientific use of FFI in swine was 30 years ago by Forbes et al., who not only proved feasibility, but concluded that training with a live porcine model was more realistic and had a greater clinical benefit for students and residents[17]. Unfortunately, the topic was never properly examined again, although porcine models have become an invaluable asset in translational research, especially of systemic diseases like sepsis and ARDS[29,30] and are regularly established at university hospitals and research facilities. Most of these models usually rely on tracheal intubation[31] [10] [12], although some either resort to surgical airways, i.e. tracheostomy[32,33], or supraglottic devices[34,35]. However, tracheal intubation of pigs can be difficult and success depends on experience, expertise and correct preparation[12] [19]. Laryngeal anatomy of supine piglets can be challenging due to a hypermobile larynx, a long snout, deep perilaryngeal recesses and a long epiglottis that usually blocks direct access to the airway in sedated animals[9,10]. The significantly longer oral cavity compared to humans can make it hard to visualize the epiglottis, which is long, U-shaped and often lodged on the soft palate. However, from the authors´ experience, it is usually feasible to use conventional laryngoscopes (size 4) for adequate visualization and then mobilize the lodged epiglottis with a guide rod by carefully inserting it along the soft palate in the right or left piriform recess and then perform a scooping motion to the opposite side to mobilize it. Once mobilized, as long as the animal is not repositioned, the epiglottis usually does not lodge again and intubation and visualization should be easily feasible.
Complications and mortality associated with intubation in pigs have not been comprehensively described yet, but difficult airway management is regularly mentioned[9,10] [11]. This includes the loss of research animals during induction[13] [14] suggesting underreporting and maybe the basis of a general confounder of animal airway studies. The determined failure rate to intubate on the first attempt of over 50% in our study seems high, but retrospective analysis of our own research projects also suggests rates between 25-40%. The additional increase in this trial can easily be attributed to the lack of intubation experience of the performing participants. Furthermore, TTs with a diameter of 7mm are relatively large compared to standard procedures suggested by other research groups[8,19]. While this may have caused an increased failure rate in the CI group compared to smaller tubes, successful placement following FFI suggests a technique-related problem and not an anatomical one. Especially research protocols relying on low ventilation pressures and decreased lung stress could benefit from the possibility to use larger bore tubes. Since piglets can be easily ventilated non-invasively with a suitable mask, intubation problems rather represent a time factor than an actual hazard, delaying eventual tube placement. However, as more intensive manipulation is necessary to establish the airway during CI, increased stress and hypoxia-induced changes might affect the results of planned projects. Since no data on this cause-effect relation exist, this remains speculative. Nonetheless, our study suggests a potentially systematic benefit of a FFI approach to porcine models, which could improve scientific accuracy of experimental results. This would simultaneously decrease the animal numbers needed, thus warranting the additional economical effort of establishing the infrastructure necessary.