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Summary

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. Competing interests
  8. References

We conducted a crossover randomised study to evaluate the performance of a novel optical stylet, the InnoScope, for tracheal intubation in simulated normal and difficult airways. Twenty-five anaesthetists attempted tracheal intubation on a SimMan 3G simulator using the InnoScope first followed by the Macintosh laryngoscope or vice versa. Three airway scenarios were tested: (1) normal airway; (2) difficult airway with swollen pharynx; and (3) limited neck movement. In each scenario, the laryngeal view, duration of and success rate for tracheal intubation were recorded. Compared with the Macintosh laryngoscope, the use of InnoScope increased the percentage of glottic opening seen by 17% in normal airway, 23% in the difficult airway and 32% with limited neck movement, p < 0.01. Despite this better laryngeal view, successful tracheal intubation achieved with the InnoScope (88.0%) was lower than that for the Macintosh laryngoscope (98.7%), p = 0.008. Using the InnoScope, tracheal intubation during the first attempt was only successful in 48% of cases with difficult airway. In this scenario, the median (interquartile range [range]) duration of tracheal intubation was significantly longer with the InnoScope (53 (20–100 [15–120]) s) compared with the Macintosh laryngoscope (27 (20–62 [15–120]) s), p = 0.01. We conclude that an improved laryngeal view with the use of the InnoScope did not translate into better conditions for tracheal intubation.

Tracheal intubation using the Macintosh laryngoscope can be difficult because it requires perfect alignment of the oral, pharyngeal and laryngeal axes [1-3]. To overcome the difficulty, a number of videolaryngoscopes has been developed to facilitate indirect viewing of the larynx [4, 5].

The InnoScope (Medlume, New Taipei City, Taiwan), also known as the KiaScope or ViewStylet, is a recent addition to the family of videolaryngoscopes. The InnoScope is a lightweight, reusable, malleable optical stylet that has a miniature video camera incorporated at its tip. This provides high-resolution real-time video images that can be displayed in a battery-operated liquid crystal display monitor attached to the handle of the stylet (Fig. 1). To use the InnoScope, the stylet is loaded with a tracheal tube (internal diameter ≥ 6.5 mm) and is bent into a desirable curvature of up to 90°. This is then inserted in the midline and advanced until a view of the glottic opening is identified and optimised. The tracheal tube is then passed through the vocal cords.

image

Figure 1. InnoScopes loaded with tracheal tube at distal angulation of (a) 50° and (b) maximum 90°.

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The purpose of this study was to evaluate the performance of the InnoScope in a simulated difficult airway. Using a high-fidelity patient simulator, we compared the efficacy of the InnoScope with the Macintosh laryngoscope for performing tracheal intubation in three airway scenarios.

Methods

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. Competing interests
  8. References

We conducted a two-period, two-treatment, crossover randomised study in three airway scenarios: (1) normal airway; (2) difficult airway with swollen pharynx; and (3) normal airway with limited neck movement. In the first scenario (normal airway), 25 anaesthetists with varying experience were asked to perform tracheal intubation on a SimMan 3G simulator (Laerdal Medical, Stavanger, Norway) using the InnoScope and Macintosh laryngoscope. The treatment sequence was randomised according to a computer-generated random number. In Sequence 1, anaesthetists used the InnoScope first for tracheal intubation (Period 1), followed 3 h later by the Macintosh laryngoscope, (Period 2). The order of treatment was reversed in Sequence 2.

In the second scenario (difficult airway), the same group of anaesthetists repeated the study with the simulator programmed to produce a difficult airway with swollen tongue and pharynx. In the last scenario (limited neck movement), we tested the effect of cervical immobilisation on tracheal intubation by applying a rigid neck collar. The same treatment sequence was adopted in all study scenarios.

Before the study, all participants, who had never use the InnoScope previously, were given a standardised demonstration on the use of the device. Participants were allowed to practise tracheal intubation on the simulator using the InnoScope and the Macintosh laryngoscope for up to 50 attempts in the manikin, until they were satisfied with the use of both devices. All tracheal intubations were performed with a 7.5-mm cuffed tracheal tube, and a size-3 Macintosh blade was used in this study. The study was approved by the Clinical Research Ethics Committee, and all participants gave written informed consent.

The primary endpoint of the study was the time required for successful tracheal intubation. This was the time from insertion of the device between the teeth to successful ventilation of the lungs. Tracheal intubation was considered as unsuccessful if it required > 3 attempts or > 120 s to accomplish. The number of attempts at laryngoscopy was noted, and we recorded the manoeuvres required to facilitate tracheal intubation. These included external laryngeal pressure, use of a gum-elastic bougie, repositioning of the head and the need for additional assistants.

After tracheal intubation, all participants were asked to quantify the best laryngeal view obtained using the percentage of glottic opening (POGO) scale [6]. A POGO score of 100% indicated a complete view of the glottic opening from the anterior commissure to the interarytenoid notch, and 0% indicated that none of the structures of the glottic opening could be seen during laryngoscopy.

The sample size was determined based on a 30% difference in the duration of tracheal intubation between the two devices. Prior studies have shown that the mean (SD) duration of tracheal intubation with the Macintosh laryngoscope on the SimMan simulator was 15 (5) s [7-11]. We therefore calculated that 25 subjects were required to achieve a power of 80%, with a type-1 error of 0.05, in a two-period, two-treatment, crossover study.

To determine whether the sequence of treatment affected observations in each period, we calculated the treatment (InnoScope vs Macintosh laryngoscope) period (Period 1 vs Period 2) and carryover (treatment × period interaction) effects using the method proposed by Hills and Armitage [12]. We also estimated the minimum laryngeal view that would permit successful tracheal intubation using a generalised estimation equation (GEE) approach [13]. In this analysis, the dependent variable was successful tracheal intubation during the first attempt that did not require additional optimisation manoeuvres such as use of a gum-elastic bougie or external laryngeal pressure. The independent variables were the type of airway device (InnoScope vs Macintosh laryngoscope), treatment sequence (Sequences 1 vs 2) and anaesthetic experience (in years). Data were modelled according to a logistic function. A SAS procedure PROC GENMOD (Release 8.02; SAS Institute, Cary, NC, USA) was used to fit the GEE models. This model accounts for the intracorrelation between measurements during the two study periods and adjusted for the period and carry-over effect. The final model was selected according to quasi-likelihood criterion [14]. A two-sided p value < 0.05 was considered statistically significant.

Results

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. Competing interests
  8. References

All 25 participants completed the study in the assigned order of treatments (Fig. 2). Fourteen participants were allocated to the InnoScope/Macintosch laryngoscope group and 11 to the Macintosh laryngoscope/InnoScope group. The median (interquartile range [range]) anaesthetic experience among the participants was 8.0 (3–16 [1–32]) years.

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Figure 2. Study flow diagram.

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The conditions of laryngoscopy and tracheal intubation in different simulated airway scenarios using both devices are shown in Table 1.

Table 1. Conditions of laryngoscopy and tracheal intubation using the InnoScope or Macintosh laryngoscope on a SimMan simulator with a normal airway, a difficult airway and limited neck movement. Values are median (IQR [range]) or number
 InnoScopeMacintosh laryngoscopep value
  1. POGO, percentage of glottic opening.

Normal airway
 POGO score; %100 (90–100 [70–100])80 (65–100 [30–100])0.003
 Duration of tracheal intubation20 (14–25 [13–33])18 (14–21 [10–32])0.11
 Number of intubation attempts
124251.00
210
300
Failed intubation00
Difficult airway
 POGO score; %80 (70–98 [30–100])25 (10–65 [0–95])0.04
 Duration of tracheal intubation22 (17–65 [15–109])30 (20–51 [16–72])0.01
 Number of intubation attempts
112240.002
210
340
Failed intubation81
Limited neck movement
 POGO score; %90 (80–100 [30–100])55 (30–78 [0–100])< 0.001
 Duration of tracheal intubation16 (13–25 [10–77])16 (13.3–24 [10–31])0.14
 Number of intubation attempts
112250.20
220
300
Failed intubation10

With the normal airway, tracheal intubations were successful in all cases except one occasion when a second attempt was required with the InnoScope. None of the participants used optimisation manoeuvres to facilitate tracheal intubation with the InnoScope. In contrast, external laryngeal pressure was used during one intubation attempt with the Macintosh laryngoscope. Overall, there was no statistically significant difference in the duration of tracheal intubation (Fig. 3). The use of the InnoScope significantly improved the laryngeal view: the POGO seen with the InnoScope was increased by 17% (p = 0.0003). Our crossover analysis showed that the results are unchanged with different orders of treatment (period effect, p = 0.47). There was also no carry-over effect, so that laryngoscopy in Period 1 did not affect the subsequent observations in Period 2 (p = 0.72).

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Figure 3. Duration of tracheal intubation using the InnoScope and then crossoverr to the Macintosh laryngoscope (left panels) or vice versa (right panels) in normal airways, difficult airways and the scenario with cervical immbolisation. The line within the box is the median value; the ends of the box show the IQR, and the whiskers represent the 10th and 90th percentiles; the dots represent values outside 10th and 90th percentiles range.

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Even with simulated tongue oedema and pharyngeal obstruction, the InnoScope produced a better view of the larynx than did the Macintosh laryngoscope (Table 1). The POGO seen with InnoScope was 23% more than that with the Macintosh laryngoscope (p = 0.04). The improved views, however, did not provide better conditions for tracheal intubation. When using the InnoScope, 13 participants (52%) required multiple attempts for tracheal intubation and eight (32%) were unsuccessful after three attempts. The relative risk for unsuccessful tracheal intubation with the InnoScope compared with the Macintosh laryngoscope was 2.1 (95% CI 1.4–3.3), p = 0.01. Despite the higher failure rate, participants used fewer optimisation manoeuvres with the InnoScope (four repositioned the head, one used external laryngeal pressure) compared with the Macintosh laryngoscope (10 required a gum-elastic bougie; six used external laryngeal pressure), p < 0.001. The duration of tracheal intubation was also significantly longer with the InnoScope than with the Macintosh laryngoscope (p = 0.01). There was no period (p = 0.68) effect, and no carry-over effect was detected (p = 0.19).

The percentage of glottic opening was again superior with the InnoScope during neck immobilisation (Table 1). The mean increase in POGO scores was 32%, p < 0.001. Tracheal intubation was unsuccessful on one occasion with InnoScope, and two other participants required a second attempt; no optimisation manoeuvre was used. In contrast, all tracheal intubations were successful with the Macintosh laryngoscope during the first attempt. However, the gum-elastic bougie (n = 1) and external laryngeal pressure (n = 2) were used to facilitate tracheal intubation with the Macintosh laryngoscope. The duration of tracheal intubation was, however, similar between devices (p = 0.14). There was no demonstrable period (p = 0.69) or carry-over effect (p = 0.85).

Taking all observations together, the median POGO score required for successful tracheal intubation with the InnoScope, 51% (95% CI 35–67%), was more than that with the Macintosh laryngoscope, 23% (95% CI 11–33%; p = 0.002; Fig. 4).

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Figure 4. Probability of successful tracheal intubation with varying percentage of glottic opening seen during laryngoscopy using the InnoScope (solid line) and Macintosh laryngoscope (broken line). The 95% CI for 50% and 95% chances of successful tracheal intubation are also displayed. Raw data are presented in the upper part of the figure. The numbers listed immediately above or below the vertical bars indicate the number of observations.

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Discussion

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. Competing interests
  8. References

We have shown a dissociation between an improved glottic view and poorer performance with the InnoScope compared with the Macintosh laryngoscope.

Our findings are consistent with results using several other optical stylets. The average success rate for tracheal intubation using the Bonfils intubating fibrescope was 92% [5, 15-24]. However, the success rate was substantially reduced to 79% (range 55–98%) in simulated [19, 20] and predicted difficult airways [5, 18, 22, 23]. Similarly, the reported average success rate for the Shikani Seeing Stylet in normal [5, 25] and difficult [5, 26] airways was 89% and 77%, respectively. Two studies have evaluated the Levitan stylet in unselected patients [24, 27]. Successful tracheal intubation during the first attempt was between 57% and 62% [24].

A number of factors may limit the usefulness of optical stylets in the management of a difficult airway. Although the pre-formed distal angulation would provide a better view of the larynx, it does not account for the anatomical variations associated with difficult airways. The POGO appears critical for successful tracheal intubation when using optical stylets, but less so for the Macintosh. Figure 4 even suggests that when the glottis cannot be seen, there can still be a 20% chance of successful intubation with the Macintosh and this may reflect familiarity with the device.

Interestingly, in 60 patients undergoing orotracheal intubation using the Bonfils intubating fibrescope, Halligan and Charters found that the usual optimisation manoeuvres such as laryngeal pressure, use of a bougie and neck flexion did not enhance the laryngeal view [28]. In contrast, jaw lifting, tongue retraction and neck extension improved the chances of successful tracheal intubation [28]. A novel optical stylet (Video RIFL; AI Medical Devices, Inc., Williamston, MI, USA) that permits real-time angulation of the distal tip has recently been introduced to accommodate unexpected anatomical variations [29]. This device should avoid the need for withdrawal of the optical stylet for readjustment of the distal curvature for subsequent intubation attempts. Further study is required to evaluate the performance of this device in difficult airway management.

The view using an optical stylet may be obscured by blood, secretions and swollen pharyngeal tissues, especially in paediatric patients [15], and can only serve to make the InnoScope even less effective. In contrast, the Macintosh laryngoscope is able to displace the swollen tongue and laryngeal tissue laterally and should be useful to facilitate tracheal intubation.

There were a number of limitations to our study. Although we allowed the participants to become familiar with both airway devices in the manikin before the study, their previous experience may contribute to the better success with the Macintosh laryngoscope.

One limitation of the POGO score is that its denominator is the part of the glottis that cannot be seen; however, an experienced anaesthetist who is familiar with the Macintosh laryngoscope, and who knows the structure and shape of a normal larynx, could probably estimate the glottic opening even with a limited view of the larynx.

It is plausible that the success rate for the InnoScope and other optical stylets could be improved with further practice and training. However, it remains unclear as to the minimum practice required before one is confident with its use. Nevertheless, if the optical stylets are to be used in handling difficult airways, we believe these devices must be used regularly in routine airway management, so that anaesthetists will be able to maintain their skills before facing a crisis.

We conducted the study in a manikin, so that a standardised environment was provided for each intubation attempt. However, there is ongoing debate as to whether findings in manikins can be extrapolated to humans [30]. The recent ‘ADEPT’ guidance from the Difficult Airway Society recommends that each new device undergoes at least one relevant clinical trial (not a manikin study) before being adopted for use [31]. However, Cook has noted that manikin studies could be useful in identifying problems before widespread clinical use [32]. In this regard, on the basis of our study, anaesthetists should be cautious in choosing the InnoScope for the management of a difficult airway.

We designed this study as a two-period, crossover trial, allowing each participant to serve as his or her own control [12]. This requires fewer subjects compared with a standard parallel study design. However, the carry-over effect from the first observation may influence the subsequent recordings and could have either minimised or exaggerated the overall effect. Previous workers using a similar study design have attempted to balance the carry-over effect by adopting a randomisation procedure [33]. We, however, instituted a washout period between observations, and our analysis confirmed that there was no period or carry-over effect.

In conclusion, the laryngeal view was improved with the use of the InnoScope during simulated normal and difficult airways. However, this benefit did not confer to faster tracheal intubation or a higher success rate. When choosing devices for a difficult airway, anaesthetists should be aware of the problems associated with the use of the InnoScope.

Acknowledgements

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. Competing interests
  8. References

The authors thank the Clinical Skills Training Centre, Tuen Mun Hospital, for allowing us to use the SimMan simulator. We also thank Innotronik Hong Kong Limited for providing the InnoScope. The device was returned after the completion of the study.

Competing interests

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. Competing interests
  8. References

No external funding and no competing interests declared.

References

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. Competing interests
  8. References