Department of Anesthesiology, University of Florida College of Medicine, PO Box 100254, Gainesville, FL 32610-0254, USA
Corresponding author. E-mail: didi@anest4.anest.ufl.edu Declaration of interest. Dr Gravenstein has commercial interests in a company associated with intubating devices. The University of Florida owns patents in intubating devices.
Abstract
Br J Anaesth 2003; 91: 40818
Keywords: airway, difficult; equipment, fibreoptic stylet; intubation, tracheal
Fibreoptic and video technologies are widely used for airway management. Fibreoptics have been used in most airway intubation devices, including rigid laryngoscopes and in the tracheal tube wall itself.10 22 Optical stylets, which incorporate flexible fibreoptic imaging elements in an intubation stylet, have now also become available.
More than 10 new optical stylet devices have been introduced since 1995, which shows that combining viewing capability with the familiar handling of a stylet is an appealing concept (Table 1). But are these useful tools? Are their clinical performances similar or is one best? We survey the evolution of optical stylets, describe their properties, summarize their clinical performance and describe an ideal system.
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Almost 35 years ago, Murphy reported successful nasal intubation using a flexible choledochoscope placed through a tracheal tube.19 Flexible fibreoptic technology was then used to assist with intubation. The Flexible Fiberoptic Laryngoscope was a short flexible bronchoscope with a pistol-grip handle, angulation control lever and eyepiece (Fig. 1). The use of this device was reported in 197232 and additional reports soon followed.3 9 23 33 40 Over the years other similar devices were developedthe Port-O-Scope (Mercury Medical, Clearwater, FL, USA) and the Rapiscope (Cook Critical Care, Bloomington, IN, USA)but unlike the flexible bronchoscope, they never found widespread use.8 28
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Despite their popularity, the first optical stylets developed are no longer commercially available. More recent devices with a Schroeder stylet design18 36 (Aeroview, Imagyn, Newport Beach, CA, USA; Video-optical Schroeder Stylet, Volpi AG, Schlieren, Switzerland; Flexguide, Scientific Sales International Inc., Kalamazoo, MI, USA) have also met with commercial failure. The Schroeder stylet does not hold a specific shape but can be flexed anteriorly by depressing a lever on the proximal end of the device (Fig. 5). The position of the lever means that these devices must be held at their proximal end like a dagger.
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The characteristics of all commercially available fibreoptic stylet devices are summarized in Table 2.
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Type of fibres/image resolution (pixel count)
Most optical stylets use glass fibres to transmit the image to an eyepiece or camera. However, the Nanoscope (Nanoptics, Gainesville, FL, USA)11 12 (Fig. 10) and the StyletScope (Nihon Koden Corp., Tokyo, Japan)15 16 (Fig. 11), have image guides made of plastic. Plastic fibreoptic fibres are quite resistant to breakage compared with glass, but often have a lower pixel count, which reduces image resolution. The image brightness also decreases more rapidly in plastic fibres as the image guide length increases, because plastic fibres absorb more light than do glass fibres. More recently, plastic optical fibres with 7000 or more pixels and high clarity have begun to rival their glass counterparts. A comparative clinical trial of a 3000-pixel Nanoscope graded the view as 8 on a 110 scale, compared with 10 by direct laryngoscopy and flexible fibreoptic endoscopy.12 The poorer image quality did not affect the success rate for intubation in this comparative study. The image quality of the StyletScope with 3500-pixel resolution appeared to be inferior compared with glass fibrescopes, which typically use 10 000 or more pixels.16 However, the 3500-pixel resolution of the StyletScope was clear enough for successful intubation. The minimum number of pixels needed for a clinically useful image is not known. Both pixel count and display size affect image quality. Low-pixel-count devices tend to use eyepieces or smaller display screens (15 cm diagonal or less) because they pack pixels more tightly and give the illusion of a sharper image.
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Channels
Stylet channels allow oxygen insufflation, drug administration, and suctioning. Few optical stylets have channels (Aeroview, Bonfils fiberscope, Fiberlightview Shuttle (Fig. 12), Optical Stylet) because they increase stylet diameter and limit the size of the tracheal tube that can be mounted. In reusable systems, these channels can transmit infection if they are not properly cleaned.24 The Shikani Seeing Stylet (Fig. 13) lacks an oxygen insufflation channel but allows for insufflation through the tracheal tube itself. The VETT has a large unobstructed channel for selective suction under indirect vision because the optical fibres are in the tracheal tube wall.
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In contrast, the Nanoscope, TrachView Videoscope (Parker Medical, Englewood, CO, USA), VOIS (Fig. 14) and VETT do not use a traditional monocular eyepiece with a proximal lens. Instead, they use a long image guide that connects to a charge-coupled device, which transmits the image directly to a monitor. This avoids adding weight to the stylettracheal tube assembly, but needs special connectors, a custom light source and a monitor.
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Light source
Many fibreoptic stylets have a battery-powered light source. This is convenient because the user is no longer tethered to the fibreoptic light source. However, a lithium battery can increase cost if batteries have to be replaced frequently.
Flexibility and radius of curvature
If a stylet cannot be bent forwards, it may not be useful if neck extension or mouth opening is limited, or if the larynx is anterior. The Bonfils Retromolar Fiberscope is a rigid scope but has a 45° curvature built into its distal end. The newest Styletscope is rigid proximally but the tip has a 90° anterior flexibility.14 The remaining fibreoptic stylets have varying degrees of malleability with an anterior curvature of at least 90°.
Clinical performance review
True difficult airwayscase reports
The first detailed case report was of the Fiberoptic Stylet Laryngoscope for intubation in a patient with a difficult airway caused by an anterior mediastinal mass and superior vena cava syndrome.29 Having a hand free to position the head and palpate and manipulate the larynx was noted to be an advantage. An optical stylet can be used in stand-by mode for rescue when conventional direct laryngoscopy fails, as shown with the VOIS. 34 39 Two patients with unexpected Cormack grade III views were successfully intubated using the VOIS to display the vocal cords on a video screen. The head, neck and larynx did not need manipulation, nor was help needed from an assistant. Gravenstein and colleagues reported a similar experience with a Nanoscope prototype.12 Two abstracts describe the intubation of 35 emergency patients with a VETT.21 22 Eleven of the airways were difficult because of anatomy, blood or secretions but all were intubated successfully. The VETT aided success in eight of these intubations.
True difficult airwaysclinical trials
The Bonfils Retromolar Intubation Fiberscope was used in a clinical trial of 103 patients with unexpected difficult airways.25 Initial airway evaluation was made under general anaesthesia. Most of the patients in this trial (80%) were undergoing ENT or oro-maxillo-facial surgery and 17% had Cormack grade III and IV airways. All the patients were intubated successfully by the mouth (not retromolar) using the Bonfils Retromolar Intubation Fiberscope. The optical stylet was the sole intubation device in 80% of patients. However, a Macintosh blade was still used in conjunction with the Bonfils Fiberscope in the remaining 20% of patients. No patient with limited mouth opening or severely limited neck extension was intubated during this study. Presumably the fixed anterior curvature of the device would make it difficult to negotiate the oro-tracheal angle in these patients, and the authors themselves remarked that the device was probably not suitable for such patients.
The Machida Fiberscope (Fig. 15) was used with patients with Cormack grade III and IV (16 of 34 patients) on conventional direct laryngoscopy.27 The optical stylet was inserted beyond the tongue along the blade of the laryngoscope. Visualization and intubation was successful in all patients, but it was not clear if the Machida Fiberscope could be used as a sole intubation device. This is the only study that reports the duration of intubation. The 2830 s duration of intubation was not significantly greater than in patients with Cormack grade I, II or IIIa airways (where a denotes that the epiglottis does not touch the posterior pharynx). Duration of intubation was longer in some patients with Cormack grade IIIb and IVa airways (b indicates that the epiglottis is in contact with the posterior pharynx), taking up to 64 s. The jaw-thrust manoeuvre might move the epiglottis away from the posterior pharyngeal wall and is better than conventional direct laryngoscopy.2
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In all of these clinical trials, the sniffing position was used when possible and conventional laryngoscopy was often used as well. Success rates might be different if these devices were the sole intubation device in patients with limited neck extension, limited mouth opening or recognized difficult airway.
Simulated difficult airways clinical trials
The Nanoscope study was the only clinical trial in which a prospective randomized comparison was made between optical stylet intubation, conventional direct laryngoscopy and flexible fibreoptic endoscopy.12 The Nanoscope was used as a sole device, without conventional direct laryngoscopy. Patients with known or suspected difficult airways (Mallampati class IV26) were excluded from the study, but a difficult airway was partially simulated by intubating patients while keeping the head position neutral. Novice users obtained a success rate of 96%, with one failure due to bleeding. This compared well with flexible fibreoptic endoscope intubations. Intubation time was longer than direct laryngoscopy but shorter than with flexible fibreoptic bronchoscopy.
In the StyletScope clinical trial, head positions were also deliberately kept neutral to simulate a difficult airway.16 The device was used in conjunction with a Macintosh blade, which was used to help elevate the tongue. The successful intubation rate was 94% after one attempt and 100% after two attempts. The pre-operative Mallampati classification was not related to the time for intubation (29 (SD 14) s). Similar results were obtained in a study when patients were intubated in the sniffing position with no attempt to simulate a difficult airway.15
Weiss and colleagues37 used a manikin with a simulated difficult airway by limiting head extension to give a Cormack grade III view. Forty anaesthesia personnel compared, in randomized order, the Bullard laryngoscope and VOIS. Mean intubation times were similar about 19 s but the VOIS was associated with fewer failed intubations (8/400 vs 41/400). The VOIS was also compared with a conventional malleable stylet in this way.6 While the VOIS needed 20.4 (7.7) s for intubation, 10 s longer than with a conventional stylet, it avoided oesophageal and endobronchial intubations (both 39.2% of times with the conventional stylet). Another manikin-based difficult-airway study of the VOIS had no failures, and a mean intubation time of 17.4 (6.8) s.5
Learning requirements
Extensive learning for optical stylets was not noted in studies, 31 except for the Aeroview system. For the Shikani Stylet, a brief initial learning period was noted for a single operator.30 An intubation success rate of 96% was achieved in another study of a device used by 16 novice operators,12 but these intubations were done under the guidance of more experienced users. For the Stylet Scope trial, no preliminary training specific to the technique was made and no significant difference was found between intubation times for the first five patients (33 (8) s) compared with the last five patients (29 (14) s) on a total of 32 intubations.16 The lack of significant further decrease in intubation time after 27 uses suggests that operation of the device was easy to learn.
With a simulated difficult airway in manikins, the number of failed attempts using a Bullard laryngoscope decreased from the first five intubation attempts (30/200 failures/attempts) to the last five (11/200).37 For the VOIS in the same study, failed attempts were few and did not decrease from attempts 15 to attempts 610 (6/200 and 2/200, respectively). However, the mean intubation time improved by 6 s (27%).37 The fewer failures and shorter intubation times suggest that the VOIS was easier to use. Simulating a Cormack grade III laryngeal view in 50 paediatric patients, a 92% successful intubation rate within 40 s was obtained with the VOIS.35 Intubation time improved by the fifth patient (20.8 (10.9) s) compared with the first (24.5 (17.3) s) but this difference was not significant.
When four operators with over 3 years experience used the Shikani Seeing Stylet as a sole intubation device,1 the intubation success rates were lower than in the original trial by Shikani, which suggests that training is needed.
Generally, learning with the optical stylets seems rapid, but most studies were by the original inventors or by people working under their guidance.
Complications
Failure to intubate
Failure to intubate had several causes, but was rarely because of device malfunction. The Bonfils Fiberscope trial had a 3.7% failure in the initial part of the study, caused by a problem with the fixation of the tracheal tube onto the stylet. This problem was quickly solved. A study that tested for failure using a prototype plastic optical fibre system reported a partial distal lens separation after 40 bend-and-straighten cycles.11 No other study found technical difficulties with the optical stylet devices.
Secretions can obscure the lens, which with the Shikani Seeing Stylet caused a 20% failure rate on first attempt.1 Kitamura and colleagues15 16 reported two failures resulting from mucus on the lens, requiring additional attempts. The Nanoscope study had a failed intubation caused by blood on the recessed lens.12 Secretions caused complications with the Shikani Seeing Stylet, and successful intubation required a change from the original stylet curvature to a hockey-stick shape in 10% of patients.1 The curve of the stylet required adjustment in one patient in the Machida Fiberscope trial.27 This shows that perhaps up to 100° of anterior flexion may be needed for successful intubation in some patients. Not all fibreoptic stylets provide this amount of flexion. In the Nanoscope trial, one patient was described where no view was obtained.12 Conventional direct laryngoscopy with a Miller 3 blade found a true difficult airway with Cormack grade IV view. The optical stylet was then passed under the retracted epiglottis to show the vocal cords and the patient was intubated successfully.
Sore throat and hoarseness
The StyletScope trial reported minor sore throat in 28% patients and minor hoarseness in 25% on the first post-operative day.16 The optical stylet in the Nanoscope trial caused less sore throat (median 1.0 on a scale of 010, with 10 being the worst) than direct laryngoscopy (median 3.0) but not statistically less than with fibreoptic bronchoscopy (median 2.0).12 Avoiding direct laryngoscopy may limit pharyngeal trauma and reduce the severity of postoperative sore throat.
The ideal optical stylet
Optical characteristics
The ideal optical stylet provides an image of high clarity, which requires more than 10 000 pixels. The stylet must provide illumination. Orientation and steering are best with a lens of long focal length (50 mm) and large angle of view (90°).
Physical characteristics
The ideal optical stylet system will be available in paediatric and adult sizes. Compared with a conventional stylet, the optical stylet will provide features such as imaging, illumination, an accessible channel, malleability and additional tip flexion without altering the weight, diameter, balance or handling. It produces an image that can be viewed from an eyepiece or displayed on a monitor. Re-usable systems will be robust and tolerate multiple bend-and-straighten cycles. Temperatures in excess of 60°C, needed for re-sterilization, will not affect components.
Cost
Cost varies, and is not considered here because no cost-per-use analysis of an optical stylet has been reported. All systems except the VETT are reusable, but no information about the average number of uses before mechanical failure or maintenance and repair costs is available for any device.
Recommendation
A review of the reports of clinical experience with the intubating fibreoptic stylet scopes suggests they are useful devices whose operation is easy to learn. It seems likely they will find an expanding role in the management of the airway.
The features of different stylets reflect different preferences and practices among clinicians. We found that the most important features are image quality and familiar handling. Availability of an extra channel or tip flexion were less important. The device whose use and feel is most like a conventional stylet, gives a view from the tip of the tracheal tube, and can be bent to view around an obstruction will need least time and effort to learn. The VOIS is the only commercially available device that has good optics and can be used very much like a conventional stylet. The Stylet Scope is an alternative but has a lower resolution image. Unfortunately, because experience of re-use of these systems has not been described, we cannot offer an opinion on the value of these devices in comparison with other methods.
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