Development of endotracheal intubation devices for patients with tumors

Abstract

The incidence and mortality of malignant tumors are rapidly increasing in the world. Patients with malignant tumors often need surgery for treatment. Endotracheal intubation is a necessary technique for surgical patients undergoing general anesthesia. It is also an important procedure for critically ill patients in the emergency room or ICU. Most patients with head and neck tumors and some specific patients have difficult airways, so the operator may need to use a variety of intubation devices. The commonly used devices for endotracheal intubation include endotracheal tube, direct laryngoscope, video laryngoscope, introducer, optical stylet, fiberoptic bronchoscope. With the advancement in science and technology, the endotracheal intubation devices have been improved, and new devices have been developed. These devices are safer and more feasible in clinical practice. In this review, we summarized the features and applications of some of the currently used devices. Each device has its own uniqueness and meets different needs. The devices and their respective properties are strongly suggested to be mastered by the anesthesiologists as well as related staffs, so as to select the appropriate device for intubation.

Introduction

Malignant tumor is one of the main causes of morbidity and mortality. There are many types of malignant tumors, including lung cancer, liver cancer, stomach cancer, colon cancer, and head and neck cancer. Head and neck malignant tumors have high morbidity and mortality, with more than 380,000 new cases occurring worldwide per year, accounting for about 3% of all malignant tumors [1]. About 196,000 of these cases are fatal, making it one of the leading causes of cancer deaths. Most patients with tumors need to undergo surgery under general anesthesia. Head and neck malignant tumors usually involve the airway and cervical spine, resulting endotracheal intubation much more difficult and risk in these patients than that of conventional airway. Therefore, it is necessary to pay close attention to the safety of airway management in these patients.

Endotracheal intubation is also a necessary technique for general anesthesia, and an essential measure for treatment of critically ill patients in the emergency department or ICU. It has been reported that the annual number of surgeries in the world is over 320 million and is increasing every year [2]. In the United States, approximately 1.6 million critically ill patients in ICU require endotracheal intubation to establish artificial airway per year [3]. The latest data from the Fourth National Audit Project (NAP4) of the Royal College of Anesthetists and the Difficult Airway Society show that 38.4% of patients undergoing general anesthesia use tracheal intubation for airway management, with approximately 1.1 million cases requiring endotracheal intubation each year [4]. Therefore, there is a tremendous need for successful endotracheal intubation.

During intubation, the operator may encounter anticipated or unexpected difficult airways. Difficult airway intubation or failed intubation is closely related to hypoxemia, cardiovascular and cerebrovascular accidents, and other complications, resulting in an increased risk of death [3]. According to an incomplete statistical analysis, the proportion of difficult intubation accounts for 1% to 6% of the total intubation, and the incidence of failed intubation accounted for 0.1% to 0.3% of the total intubation [5,6]. Most patients in ICU and emergency department are hospitalized for critical conditions; hence, medical staff need to complete endotracheal intubation quickly and, more importantly, to provide airway management in those situations where no adequate emergency preparation and facilities are available [7]. To accomplish endotracheal intubation more quickly and accurately, the technology and equipment are constantly developed.

The commonly used devices for endotracheal intubation are endotracheal tube, direct laryngoscope, video laryngoscope, introducer, fiberoptic laryngoscope, fiberoptic bronchoscope, etc. [8-11]. To provide more useful information for the endotracheal intubation of patients with tumors, especially those with potentially difficult airways, such as patients with head and neck tumors, in this review, we compared and discussed the application, the advantage, and the limitation of these commonly used devices.

Endotracheal tube (ETT)

Traditional ETT

The origin of the ETT dates back to the 18th century, when Cullen suggested endotracheal intubation and ventilation with bellows to save the life of dying patients, which was the early form of mechanical ventilation [12]. Charles Kite was the first to use endotracheal intubation for resuscitation in drowning patients and to describe the use of oral and nasal intubation [13]. Guedel and Ralph Waters made tracheal tubes with inflatable cuff to prevent leakage and aspiration [12]. Sir Ivan Whiteside Magill further applied the innovation of curved ETT to general anesthesia surgery, and many of his inventions are still in use today [14].

An ETT with a cuff is the most reliable device for maintaining the airway, ensuring an effective oxygenation and ventilation, and preventing aspiration during general anesthesia. Currently, the design, shape, and material of the cuff have been optimized to reduce airway complications. For example, the cuff can automatically inflate and deflate to adjust pressure. However, the automatic control management has not shown an advantage in reducing airway injury [15]. There are two types of single-lumen tracheal tube: curved tube and straight tube, both of which have a oblique opening on the tip. The average radius of the curved type is 14 ± 2 cm [16]. There is a Murphy eye opposite to the beveled opening on the tip of the tube, which is mainly used to prevent airway obstruction caused by the tube tip sticking to the airway wall. The common single-lumen tubes include spiral steel wire reinforced tube, flexible tracheal tube, LITA tube, and subglottic suction tube [17-19]. These ETTs have a hard tip and a soft body and need to be preshaped with a stylet before intubation.

Ralph Waters developed the technique of lung isolation to perform thoracic surgery with a double-cuff, single-lumen tube [20], while E. Carlens and G. White developed a double-lumen tube for single-lung intubation [21,22]. The Carlens tube is a left double-lumen tube with the tip of the tube inserted into the left main bronchus [23]. The structure of the White tube is similar to that of the Carlens tube; however, the tip of the White tube is inserted into the right main bronchus, with a side hole aligned to the opening of the right upper bronchus. Another one is Robertshaw double-lumen tube, which is divided into left and right [24]. Its left tube, although similar to the Carlens tube, has an optimized structural design, which makes the intubation easier. In addition, the distal opening of its right tube is not closed, and there is a lateral opening between the front and the back cuffs on the opening of the right upper lobe bronchus.

The major risks associated with endotracheal intubation are tissue injury, bleeding, laryngeal edema, ventilator-associated pneumonia, etc. To successfully complete endotracheal intubation, manage difficult airway, and reduce complications, the traditional ETT has been improved in aspects such as fabrication materials, structural design, and additional functions. A variety of products, such as mucus shaving tube, temperature-measuring tube, laryngeal nerve injury prevention tube, cardiac pacing tube and biofilm coating tube, have been designed and gradually integrated to achieve the multi-functions of endotracheal tube. However, these novel tubes need to be evaluated in large, multicenter studies.

ETView tracheoscopic ventilation tube (ETView TVT)

ETView TVT is a single-use endotracheal tube integrated with video camera technology, which assists the operator in intubation by displaying the anatomy of the larynx, vocal cords, and the proximal part of the trachea, and enables continuous observation of the airway during surgery. There are currently two types of ETView TVT: VivaSight-DLT and VivaSight-SLT [25,26]. Based on the conventional single-lumen tube and double-lumen tube, the visual ETT is embedded with a high-resolution camera, data transmission system, and a flushing system. The high-resolution camera with PIXELS CIF of 320 × 240 (76,800 Pixels) provide a field depth of 12-60 mm and a field of view of 100° [27]. The camera has two white light sources that do not burn the airway, and the image sensor is made of Complementary Metal Oxide Semiconductor (CMOS). The camera is cylindrical with 2 mm in diameter and 16 mm in length, located at the oval opening on the tip of the tube (the camera of the double-lumen tube is located at the opening between the two cuffs). Micro-cables embedded in the wall of the tube transmit images to a high-definition monitor or other portable video device and power the camera and the light source, so that the operator can view real-time images for precise positioning and operation. When the secretions or blood obscure the field of vision, the camera lens can be rinsed by an additional injection interface; however, the efficiency is limited due to the tube structure and the viscosity of the secretions.

ETView TVT has advantages in situations with difficult airway intubation and one-lung ventilation [28,29]. The continuous visualization and the real-time positioning of the airway enables a faster and more accurate intubation and ensures the timely detection and correction of tube displacement, which increases the quality and safety of airway management. VivaSight-SLT can also be used to accurately place the bronchial blocker and to correct blocker displacement during surgery [26]. Patients with ETView TVT intubation may not need fiberoptic bronchoscopy during intubation or surgery, which avoid some of the limitations of fiberoptic bronchoscope [30]. Indeed, randomized controlled trials have demonstrated that these features help improve the intubation fluency and safety, and effectively reduce the incidence of complications from endotracheal intubation, which is ultimately beneficial to patients [31]. Therefore, ETView TVT has the value of long-term application.

Laryngoscope for endotracheal intubation

Direct laryngoscope

The singer Manuel Garcia invented the direct laryngoscope and performed the first direct laryngoscopy in 1854 [12]. Direct laryngoscope is a great innovation in the history of endotracheal intubation and a key device to ensure the establishment of an artificial airway for patients undergoing general anesthesia. It has been the standard technique for endotracheal intubation for nearly a century. Later, Robert A Miller and Robert Reynolds Macintosh refined the direct laryngoscope. Miller invented the straight laryngoscope, the Miller laryngoscope, in 1941 [32]. In 1943, Macintosh introduced the classic and most widely used curved direct laryngoscope, the Macintosh laryngoscope [33]. In addition, Polio laryngoscope, Alberts laryngoscope and Mccoy laryngoscope have also been developed to meet the special intubation needs of patients.

A direct laryngoscope usually consists of a battery-equipped handle and a detachable metal blade. During intubation, the blade slides into the throat through mouth along the tongue, moves the soft tissues of mouth and throat, lifts the epiglottis, and exposes the glottis for intubation. Curved laryingosope indirectly raises the epiglottis by pressing the hyoid epiglottis ligament, while straight laryingosope directly raises the epiglottis. Although direct laryngoscopy can effectively avoid interference with peripheral soft tissues, intubation still requires the patient to be in a “sniffing position” to ensure the mouth, pharynx, and larynx are aligned to provide a good visual field.

Miller laryngoscope blade is straight; the tip of the blade is curved, while the proximal root of the blade and the handle connector are relatively thin [34]. The blade has a narrow tongue depressor and a low baffle. Miller laryngoscope has advantages in pediatric patients and patients with prominent mandible, long epiglottis, and limited mouth opening since it can easily lift the jaw to observe epiglottis [35]. However, this device provides a smaller field of vision, which is difficult to expose the glottis to implement intubation and may even cause epiglottis injury [36]. Patients with limited cervical mobility such as cervical spine injury or instability may further aggravate the injury. Therefore, it can be used as a supplementary means to curved laryngoscope.

Notably, the curved blade of the Macintosh laryngoscope is wider and thicker than that of the Miller laryngoscope. Furthermore, a higher baffle in the curved blade helps patients open their mouth and expose their epiglottis more easily and expands the field of view of the throat, which facilitates the intubation. The Macintosh laryngoscope allows the operator’s axis of vision to rotate with the line of sight according to the height of the baffle, the arc of the blade, and the patient’s physiological characteristics. Thus, a curved laryngoscope is more suitable for intubation than a straight laryngoscope. Alter et al. found that the effectiveness of the first intubation attempt was 86% for Macintosh blades and 73% for Miller blades [37]. Despite the obvious advantages and applications of Macintosh laryngoscope, there are some limitations. It is easy to cause tooth damage, and the difficulty of intubation increases in patients with limited mouth opening, difficult epiglottis exposure, and obesity [38].

At present, direct laryngoscope is still the main intubation device in undeveloped regions/countries or outside the operating room [37]. With the development of video laryngoscope, direct laryngoscope will be gradually replaced. Compared with direct laryngoscope, video laryngoscope has more advantages: (1) It exposes the structure of the throat more clearly, which facilitates the operator to operate more smoothly and improves the success rate of intubation [39]; (2) The chance of damage to airway, teeth and oral soft tissue is reduced [40]; (3) The airway grade of patients is lowered, and video laryngoscopy can provide laryngeal vision for patients whose exposure is not ideal with direct laryngoscopy [41]; (4) The operator can observe the real-time image, which can be used as a teaching tool.

Video laryngoscope

Video laryngoscopy was applied in clinic in the early 21st century [42-44]. Video laryngoscope can move the soft tissue of oral cavity and throat and transmit laryngeal image to eye lens or monitor by optical technology, camera technology or digital imaging technology, so that the operator can observe the real-time operation of intubation. The invention of the video laryngoscope overcomes the limitation on field of vision and lowers the airway grade. Moreover, it can alleviate the agitation of intubation and reduce the incidence of intubation failure and airway trauma. Therefore, it becomes a new method for partially difficult airway intubation. Indeed, studies have shown that the video laryngoscopy can significantly optimize Cormack-Lehane grade, with the vast majority of airway grades I-II [45]. Overall, video laryngoscopy can greatly improve the success rate of intubation. The operator does not need to look directly into the glottis when using the video laryngoscope, which simplifies the operation of direct laryngoscopy-assisted intubation and makes the operator more relaxed and comfortable. Additionally, the long distance between the operator and the patient can reduce the contact with the patient’s exhaled aerosol. Moreover, the blades of the video laryngoscope are mostly disposable, which can effectively reduce the risk of cross infection. Therefore, video laryngoscope, as an intubation device that benefits both doctors and patients at the same time, has been favored and developed rapidly in recent 20 years.

There are a variety of video laryngoscopes, which have been invented according to different intubation needs. The popular video laryngoscopes include GlideScope, McGrath video laryngoscope, Storz C-MAC/V-Mac video laryngoscope, Pentax Airway scope and AirTraq laryngoscope [44,46-48].

The Glidescope family has three video laryngoscopes: the Glidescope, the Glidescope Cobalt, and the Glidescope Ranger. The distal end of a typical Glidescope blade is bent at an angle of 60°, and a camera is embedded in the blade [44]. A video monitor is installed on the handle of the laryngoscope. The Glidescope Cobalt is modified to use disposable plastic blades for intubation to reduce cross infection [49], while the Glidescope Ranger is a portable video laryngoscope designed to be fog proof for tracheal intubation outside the operating room [50].

The most popular laryngoscopes in the McGrath family are the McGrath series5 and McGrath MAC [51]. The blade length of McGrath Series5 can be adjusted according to the airway, and the tip of the blade is angled at 60° to facilitate intubation [52]. The McGrath MAC has some improvements over the McGrath Series5, adding the regular Macintosh type blade and expanding the LCD screen [51]. The McGrath video laryngoscope is also portable and can be used for intubation under various airway conditions. Moreover, the blade is for single use to prevent cross infection.

Karl Storz developed the V-MAC, C-MAC, and C-MAC-D-Blade video laryngoscopes, which are similar in structure to Macintosh blade [53,54]. These laryngoscopes optimize the angle of the blades but can also be used as a direct laryngoscope.

Pentax Airway scope (Pentax-AWS) is a new rigid video laryngoscope whose fixed structure is more like a short video stylet [48]. The single-use blade has two guide channels parallel to the video system. The main channel is used to insert the tracheal tube, while the subchannel can accept tube ≤ 4.0 mm in diameter for suction or local anesthesia.

AirTraq laryngoscope is an optical laryngoscope consisting of a lens, a prism, and an eyepiece [47]. The eye lens can be equipped with an optical accessory to transmit real-time images to an external monitor. The Airtraq has an adapter installed in its viewfinder to connect to smartphones, iPad, and computer.

Compared with the direct laryngoscope, the vision of the video laryngoscope is determined by the direction of the camera, and the scope and the quality of the vision are significantly improved, which improves the effectiveness of the intubation greatly, usually, the success rate is 98-99% [55]. The application of video laryngoscope and direct laryngoscope has been reviewed in a Cochrane Systematic Review [39]. It showed that the number of failed intubations was significantly reduced when using video laryngoscopy, and fewer intubations failed in the anticipated difficult airway. In summary, video laryngoscope can provide a wider field of view and allow the operator to directly observe the movement of the glottis and the position of the tracheal tube, in some cases replacing fiberoptic bronchoscopy for difficult airways [56]. Video laryngoscopy simplifies the traditional endotracheal intubation methods and is an ideal choice for both inexperienced medical staff and experienced senior physicians.

Optical stylet

In 1979, the concept of “optical stylet” was first proposed by American endoscopists G. Katz and R. Berci [57]. When they performed endotracheal intubation with a straight rigid metal endoscope, they had to position the patient and used a direct laryngoscope to lift the base of the tongue and epiglottis, which was complicated and had many limitations [58]. Hence, as a feasible, new intubation method, optical stylet has been used in clinical practice for nearly half a century. The optical stylet consists of a stylet carrying optical fibers, light source, and a handle, which can be divided into straight or curved, and rigid or semi-rigid devices. The tip of the most stylets is usually bent at an angle to facilitate intubation. The distal end of the semi-rigid stylet is malleable and can be preshaped according to the anatomical structure before endotracheal intubation.

There are several types of optical stylet. In 1983, Bonfils designed the rigid fiberoptic laryngoscope (Bonfils Retromolar Intubation Fiberscope), and the distal end of the stylet was bent by 40° [59]. This improvement was more in line with the anatomical curvature of the larynx to facilitate intubation and resulted in a new technique for retromolar endotracheal intubation [60].

The Shikani optical stylet consists of a “J” shaped fiberoptic stylet, a detachable eyepiece, a handle, and a light source, and its first clinical application was reported in 1999 [61]. The stylet is made of malleable rust-proof metal, and the distal end of the stylet can be shaped according to the anatomical structure of the airway. An adapter can be mounted on the eyepiece to transmit a real-time image of the airway to the monitor.

Levitan FPS stylet is similar to the Shikani optical stylet, but it is 10 cm shorter and is used in direct laryngoscopy or endotracheal intubation [62]. In contrast, SensaScope is an “S” shaped rigid fiber optic laryngoscope with an overall length of 43 cm [63]. The 3 cm distal end of the stylet allows for a flexible manipulation to facilitate endotracheal intubation.

These optical stylets have the advantages of high intubation success rate (all around 99-100%), less complications and a steep learning curve and can be used as a tool for difficult airway management [59,62,64]. It is suitable for patients with small opening and reduced cervical hyperextension movement, which is beneficial to patients with suspected or existing cervical injury [64-66]. The success rate of awake intubation is comparable to that of fiberoptic bronchoscopy, but with faster operation time [9,67]. For conventional airway intubation, this device is more feasible than direct laryngoscope and is as good as video laryngoscope [64]. Nevertheless, its downsides include blurred field of vision due to contamination of secretions and blood and inability to observe tracheal tube movement during intubation.

In recent ten years, several new malleable optical stylets have been developed, which are more convenient and reliable than traditional optical stylets. Their video systems use complementary metal oxide semiconductor (CMOS) video sensors with monitors mounted on handles or directly connected to smartphones and other mobile devices. Clarus Video System (Trachway intubating stylet) was approved for clinical intubation in 2009 [68]. This device is composed of a detachable stylet, a rechargeable handle, and an adjustable color monitor. The camera and light source are located on the atraumatic tip of stylet. The monitor can be rotated to different angles using the monitor rotary control knob. Oxygen is supplied through an attached ventilation connector during intubation. Trachway Intubating Stylet has been used for oral intubation, nasal intubation, double-lumen intubation, and emergency intubation with satisfactory results [69-72]. Lee et al. found that awake nasal endotracheal intubation using Trachway intubating stylet was easier to perform and faster than fiber bronchoscope [70]. In addition, Trachway Intubating Stylet is also a less invasive and highly successful intubation method in patients with difficult airways such as head, neck and facial malformations, and large upper respiratory tumors [73]. The main drawback of this device is the failure of intubation due to blurred vision and improper tube delivery, resulting in trauma and bleeding of throat tissue.

The C-MAC Video Stylet is a straight rigid intubation stylet [74]. The video stylet is 41 cm long and has a flexible tip (length: approx. 4 cm), which allows the tip to deflect up to 60° when intubating. StyletViu Inc is another video intubation stylet with free rotation angle and detachable display, which can be used for intubation and exchanging the ETT [75]. Moreover, the stylet is basically not restricted by the application object, and can be used in newborns, children, and adults. In recent years, more optical stylets such as the Intular Scope and the UE scope have also been introduced [75,76]. However, their application needs to be validated by a large number of clinical studies and supported by evidence-based medicine.

Introducer

Bougie and stylet are two effective introducers to assist tracheal intubation. Bougie-guided intubation and stylet-preshaped tracheal tube can significantly increase the success rate of endotracheal intubation. With further improvement, some introducers can provide oxygen, suck sputum and exchange tracheal tubes, reducing the risk of hypoxemia, laryngeal spasm, and cardiovascular and cerebrovascular accidents [77]. Since the invention of video laryngoscope, its combination with the introducers has greatly optimized the endotracheal intubation technology.

The bougie-guided intubation was first developed by Macintosh through modifying a urethral dilatation catheter in 1949, and it was called “Gum Elastic Bougie” [78]. In the 1970s, a specialized introducer, the Eschmann Tracheal Introducer was invented [79]. Since then, Bougie-guided intubation has been widely used in clinical practice. Common devices are the Eschmann Tracheal Introducer, the Frova Intubating Introducer, the Flexible Tip Bougie, the Airway Exchange Catheter (AEC), the VBM Tube Catheter, the Aintree Intubating Catheter, etc. [80-82]. Studies have proved that the Bougie has a good auxiliary role in tracheal intubation, which can improve the success rate of first attempt and reduce complications, in both prehospital or hospital care [83-85]. The main advantage of the bougie is its slender and flexible, enabling successful intubation in the case of insufficient mouth opening and poor glottis exposure by laryngoscope, and many guidelines recommend it for the management of difficult airways [86-88]. The Eschmann Tracheal Introducer, the Frova Intubating Introducer, AEC, and other introducers are expensive; some of them are reusable and require repeated disinfection, so the usage rate is low in less developed regions/countries.

Stylets are mainly divided into rigid stylets and plastic stylets. Rigid stylets are made of malleable rust-resistant metal with a smooth surface and are commonly used for rapid sequence intubation and pre-hospital emergency intubation [89]. Plastic stylet is a single-use stylet made of plastic, and the representative product is Parker-Flex-It Directional Stylet, which is not required to preshape before intubation [90]. During intubation, the operator’s thumb pushes the lever to change the curvature of the stylet, leading to the alteration in the direction of the tube top, which is useful for oral and nasal intubation. However, some stylets are not firm enough; the tube may deform easily during intubation. On the other hand, some stylets are hard and sharp, which tends to cause airway damage. The endotracheal intubation with stylet can usually be performed rapidly by a single person, unlike bougie-guided intubation, resulting in faster intubation.

Complications associated with introducer include cardiovascular reactions due to intubation agitation, soft tissue damage, bleeding, perforation of the trachea, bronchial rupture, and pneumothorax [89]. There are also reports that the device fractured resulting in an intratracheal foreign body and pulmonary migration of a sheared fragment [91]. In addition, using an introducer to provide oxygenation and ventilation for the patient can cause the barotrauma [92]. And there is a risk of cross infection when the reusable introducer is applied [93].

SEEK^flex (Safe Easy Endotracheal Kit-Flexible) is a new single-use endotracheal intubation kit that was developed by our team in the early days of COVID-19 [94]. It is an inexpensive introducer with characteristics of malleable, stretchable, detachable, lockable, and diverse functions. The main structure of the device includes a stainless metal guide, a flexible catheter, a handle, a fixed lock connector and a detachable transition connector (Figure 1). The lock connector, transition connector and catheter are designed to tight easily and separate freely according to the operation demands. Because the metal guide can be stretched in the catheter, the length can be adjusted (the stretchable length ranges from 43 to 81 cm), which can guide tracheal intubation and exchange tracheal tubes. The tip of the catheter has three vents and three rows of small side holes, and the proximal end of the catheter can be fitted with a matching adapter (with standard 15 mm connector and Luer lock connector functions) after removing the connector. It can be connected to respiratory loop connector and nasal catheter through adapter and can also be used for drug and oxygen delivery through catheter.

Figure 1.

Structure and function diagram of SEEK^flex. A. Structure of SEEK^flex after disassembly; B. The initial state of SEEK^flex; C. SEEK^flex after shaping. It was extended by stretching the metal guide in the catheter; D. Connection of the catheter to the adapter; E. The catheter is connected to the ventilator circuit through the adapter; F. The catheter is connected to the nasal catheter through the adapter.

Despite the similarities to both AEC and Frova Intubating Introducer in construction and functionality, SEEK^flex has its unique advantages (Table 1). Its flexible design allows for a wide range of functions and a much lower cost than the other two devices, making it a potential alternative to the AEC and the Frova Intubating Introducer. After training, the operator can quickly master the operation essentials of SEEK^flex and can easily complete tracheal intubation. A preshaped SEEK^flex can also be used as a stylet for intubation without changing the operating routines of some anesthesiologist, which is another advantage over the bougie.

Table 1.

The characteristics of AEC, Frova Intubating Introducer, and SEEK^flex

	AEC	Frova Intubating Introducer	SEEKflex
Function	Exchange tracheal tube	Endotracheal intubation	Endotracheal intubation, Exchange tracheal tube
Structure	Single catheter with no metal guide	Single catheter with metal guide	Single catheter with stretchable metal guide (detachable and reconnected)
Length	45 cm/83 cm	35 cm/70 cm	43 cm-81 cm
Malleability	unmalleable	Malleable	Malleable
Vent	single hole at the tip, small vents on the side	Small vents on the side of the catheter tip	3 small vents at the tip of the catheter and multiple side holes at the end
Connector	15 mm connector and Luer lock connector	15 mm connector and Luer lock connector	An adapter has the function of both 15 mm connector and Luer lock connector
Usage	Single use	Single use	Single use

In clinical application, we found that the device has a high success rate of guiding tracheal intubation, minimal agitation to the airway, and low incidence of adverse reactions. However, our device needs to be further tested in a large sample size and in hospitals of different levels by doctors of different proficiency.

Fiberoptic bronchoscope

Fiberoptic bronchoscopy is the gold standard for assessing difficult airways and one of the most used auxiliary tools for anesthesiologists, emergency physicians, and critical care physicians to manage difficult airways. Traditional rigid bronchoscope has limited visual field and is painful for many patients during operation. In 1967, P. Murphy first described the use of fiberoptic choledochoscope for nasal intubation, an innovation that led to the development of fiberoptic bronchoscope and a breakthrough in the management of difficult airways [95]. The fiberoptic bronchoscopes have been refined to provide an increased visibility, allowing physicians to operate even when the bronchial tubes are twisted, displaced, or narrowed. The HD monitor and the internal storage of the device enable easy observation as well as the storage of a large amount of valuable video data. In addition to airway assessment and endotracheal intubation, the fiberoptic bronchoscope is also commonly used for airway examination, alveolar lavage, retention of specimens and airway therapy.

Conventional reusable flexible fiberoptic bronchoscopes use fiber-optic technology to transmit the view to an eyepiece, which can be connected to a video system to display the image on a monitor [96]. The recently developed, more popular video flexible bronchoscopes are equipped with charge-coupled video-chip sensors instead of fiber-optic imaging and are illuminated steadily by LED [97]. In anesthesiology, single-use flexible bronchoscopes, the aScope and C-Mac Five S series gradually become mainstream devices because they maintain flexible capabilities, unlike reusable fiberoptic bronchoscopes, and don’t require disinfection and maintenance costs, such as to avoid possible cross-contamination risks [75,98]. Although Karl Storz is a leader in the endoscopic industry, the C-Mac Five S is still inferior to the aScope in terms of performance and cost control management in the field of disposable flexible bronchoscopes.

Earlier studies suggested that there was a gap between endotracheal intubation using the old aScope and conventional fiberoptic bronchoscope [98,99]. However, the researchers compared the new disposable bronchoscope, the aScope 4, with conventional fiberoptic bronchoscope and found that the aScope 4 had greater flexibility, maneuverability, and clearer field of vision, suggesting that the device should be the first choice for awake intubation, diagnostic and therapeutic procedures [96]. In addition, some studies have proved that it also has an ideal effect in alveolar lavage [100]. Cost analysis shows that disposable flexible bronchoscope consumes more than reusable flexible bronchoscope, but it has economic and social benefits in hospitals with few patients [101]. However, considering the infection risk and mobility, disposable bronchoscope is still highly practical.

The use of fiberoptic bronchoscope has certain limitations: (1) Sputum, blood and secretions can easily blur the field of vision and affect the operation [96]; (2) The learning curve is long, requiring skilled doctors to operate; (3) During awake intubation, the tracheal tube is likely to be obstructed at the vocal cords, resulting in a strong stimulus response, an unpleasant patient experience, and the risk of cardiovascular and cerebrovascular accidents. Moreover, reflux may occur during operation, and hypoxemia may result from interruption of ventilation during intubation; (4) The placement of the ETT may require repeated operation, which can cause damage to the glottis and airway, even bleeding and pneumothorax; (5) Fiberoptic bronchoscopes are expensive and easily damaged by improper handling, and traditional devices require costly disinfection and maintenance; Besides, the disinfection is not always effective [98]; (6) Due to the long operation time, patients cough and exhale large amounts of aerosols, increasing the risk of cross infection. In addition, reusable fiberoptic bronchoscope is also a risk factor for increasing the probability of infection.

Recently, some guidelines recommended that patients with suspected and confirmed COVID-19 should be intubated with disposable bronchoscope whenever possible, with adequate protection and airway preparation [102,103]. Furthermore, video laryngoscope is recommended for intubation when possible, mainly because of the reduced aerosol exposure and faster intubation [103]. Although the introduction of video laryngoscope, introducer and other auxiliary tools has partially replaced its role in difficult airway management, fiberoptic bronchoscope is still the first choice for most of the difficult airway. In the context of the COVID-19 pandemic, we need to increase the use of disposable devices, which is essential to reduce the spread of infectious diseases.

Future perspectives

With the development in technology, new approaches have been used in the clinical practice of endotracheal intubation. For example, for the concern of how to accurately evaluate the airway and implement intubation, chest X-rays and chest CT have been used to select the size of tracheal tube by measuring the diameter of the trachea. However, their general use in the clinic remains limited due to the radiation exposure [104]. Ultrasound examination has advantages in choosing appropriate tube size in pediatric patients, but there are still limitations [105]. Artificial intelligence (AI) has attracted much attention in health care in recent years because of its power over traditional statistical methods [106]. AI has been applied to many aspects of airway management research, including diagnosis, assessment, management, and risk prediction. With the progress in AI research, AI may help achieve rapid diagnosis and recommend treatment options in emergency situations, which is very important in pre-hospital setting and for difficult airway assessment and management. Furthermore, robots can be developed to assist people to complete endotracheal intubation in urgent circumstances and further relieve the pressure of medical treatment. Therefore, the development of AI endotracheal intubation technology is of great significance to patients and society.

Summary

Endotracheal intubation is an important rescue technique for both surgical and critically ill tumor patients. Different intubation methods are suitable for different patients. For example, head and neck tumors are one of the most common causes of difficult airways, often associated with forced position, airway compression, and ventilation dysfunction. The operator may need to use several devices, such as video laryngoscope, Fiberoptic bronchoscope and introducer, to improve the success rate and to reduce the risk of difficult intubation. We also hope to develop more devices like SEEK^flex that can be used to perform conventional intubations and difficult airway management. In clinical practice, an appropriate device should be selected after comprehensive evaluation based on the actual situation and personal experience. Otherwise, it may cause a certain degree of injury to the patient, or even failed intubation, resulting in irreparable loss and social impact.

Disclosure of conflict of interest

None.