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Predictors of Difficult Intubation with the Bonfils Rigid Fiberscope

Nowakowski, Michal MD; Williams, Stephan MD, PhD; Gallant, Jason MD; Ruel, Monique Inf CCRP; Robitaille, Arnaud MD, FRCPC

doi: 10.1213/ANE.0000000000001258
Patient Safety: Research Report

BACKGROUND: Endotracheal intubation is commonly performed via direct laryngoscopy (DL). However, in certain patients, DL may be difficult or impossible. The Bonfils Rigid Fiberscope® (BRF) is an alternative intubation device, the design of which raises the question of whether factors that predict difficult DL also predict difficult BRF. We undertook this study to determine which demographic, morphologic, and morphometric factors predict difficult intubation with the BRF.

METHODS: Four hundred adult patients scheduled for elective surgery were recruited. Patients were excluded if awake intubation, rapid sequence induction, or induction without neuromuscular blocking agents was planned. Data were recorded, including age, sex, weight, height, American Society of Anesthesiologist classification, history of snoring and sleep apnea, Mallampati class, upper lip bite test score, interincisor, thyromental and sternothyroid distances, manubriomental distances in flexion and extension, neck circumference, maximal neck flexion and extension, neck skinfold thickness at the cricoid cartilage, and Cormack and Lehane grade obtained via DL after paralysis was confirmed. Quality of glottic visualization (good or poor), as well as the number of intubation attempts and time to successful intubation with the BRF, was noted. Univariate analyses were performed to evaluate the association between patient characteristics and time required for intubation. Variables that exhibited a significant correlation were included in a multivariate analysis using a standard least squares model. A P < 0.05 was considered significant.

RESULTS: Glottic visualization with the BRF was good in 396 of 400 (99%) cases. On the first attempt, 390 patients were successfully intubated with the BRF; 6 patients required >1 attempt; 4 patients could not be intubated by using the BRF alone. These 4 patients were intubated by using a combination of DL and BRF (2 patients), DL and a Frova bougie (1 patient), and DL and an endotracheal tube shaped with a semirigid stylet (1 patient). Mean time for successful intubation was 26 ± 13 seconds. Multivariate analysis showed that decreased mouth opening (P = 0.008), increased body mass index (P = 0.011), and higher Cormack and Lehane grade (P = 0.038) predicted longer intubation times, whereas shorter thyromental distance predicted slightly shorter intubation times (P < 0.0001).

CONCLUSIONS: Mouth opening, body mass index, and high Cormack and Lehane grade predict longer intubation times, as with DL. Decreasing thyromental distance predicts slightly shorter intubation times with the BRF, possibly because of a design initially optimized for a pediatric population with receding chins. These findings, along with the high success rate of BRF in this study, and the possibility of further increasing success rates by combining BRF with DL, help define the role of BRF intubation in contemporary airway management.

Published ahead of print March 29, 2016

From the Anesthesia Department of the University of Montreal Health Center, Université de Montréal, Montreal, Quebec, Canada.

Accepted for publication February 8, 2016.

Published ahead of print March 29, 2016

Funding: Internal. Equipment loan provided by Storz Medical US.

The authors declare no conflicts of interest.

This report was previously presented, in part, at the Whistler Anesthesiology Summit 2015, Association des Anesthesiologistes du Quebec Annual Conference 2015

Reprints will not be available from the authors.

Address correspondence to Michal Nowakowski, MD, Anesthesia Department, University of Montreal Health Center, Universite de Montreal, 5623 Av. des Erables appt B, Montreal, Qc H2G-2L9 Canada. Address e-mail to michal.k.nowakowski@gmail.com.

General anesthesia often requires endotracheal intubation, most commonly performed via direct laryngoscopy (DL). In some patients, however, DL can be difficult or even impossible,1–4 potentially leading to significant morbidity. When difficulty with DL is anticipated or encountered after induction of anesthesia, the anesthesiologist must choose among several alternative methods for securing the airway.5 One successful alternative intubation device is the Bonfils Rigid Fiberscope® (BRF),6,7 a rigid fiberscope with a 40° curved tip and a 110° angle of view through a proximal eyepiece or camera/monitor system. Intubation with the BRF is described in patients with normal airways, patients with obstructing airway tumors, and patients with anticipated and nonanticipated difficult airways, including patients with failed intubation attempts with DL.8–11 The very different design and high success rate of the BRF in intubation situations where DL is difficult raise the question of whether demographic, morphologic, or morphometric predictors of difficult DL also predict difficult BRF. A prospective study was therefore designed to determine which patient characteristics, if any, predict difficult BRF intubation in an elective surgical population.

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METHODS

After approval by the IRB, 400 adult patients, based on BRF and patient availability, were recruited at the Centre Hospitalier de l’Université de Montréal from January to July 2014, after having signed a written informed consent. The estimated sample size for linear regression assuming small effect size of 0.05, 90% power, and 1% significance level was estimated to be 420 patients.12–14 Using this number, as well as a previously published study by the same group of investigators15 as a guide, the Institutional Scientific Review Board agreed that a 400-patient sample was considered satisfactory. All were scheduled to undergo elective neurosurgical, otorhinolaryngologic, neurologic, thoracic, gynecologic, orthopedic, plastic, or general surgery under general anesthesia. Patients were excluded if they could not provide informed consent or if awake intubation, a rapid sequence induction, or an induction without neuromuscular blocking agents was planned. Demographic and morphometric data were recorded before induction of anesthesia, including age, sex, weight, height, American Society of Anesthesiologist classification, history of snoring and sleep apnea, Mallampati class, and upper lip bite test score. Interincisor, thyromental, and sternothyroid distances; manubriomental distances in flexion and extension; neck circumference; and neck skinfold thickness at the cricoid cartilage were noted in centimeters and were analyzed as continuous data. Finally, maximal neck flexion and extension were measured in degrees.

The induction technique and agents were chosen by the attending anesthesiologist but had to include a neuromuscular blocking agent. Two senior anesthesiology residents, who had intubated >40 patients each6,16 with the BRF before the study began, performed all the intubations. Paralysis was confirmed by the absence of motor response to train-of-four stimulation at the ulnar nerve. With the patient in the sniffing position,17 DL was performed with a Macintosh 3 or 4 blade, using backward upward rightward pressure18 on the larynx as necessary when the vocal cords were not initially visualized; the best resulting glottic visualization Cormack and Lehane grade was noted. The patient’s head was thereafter placed in a neutral position, and oral aspiration of secretions was performed. The BRF, previously treated with Fred™ 2 (Covidien, Mansfield, MA) antifogging agent and mounted with a size 7.0 (female patients) or 8.0 (male patients) endotracheal tube, was inserted into the mouth, and intubation was performed via a midline or retromolar approach, facilitated by the use of a jaw lift. A laryngoscope was not used to assist BRF.7 Correct placement of the tube was confirmed by capnography.

Several measures were collected to grade ease of intubation with the BRF. Glottic visualization with the BRF was graded as good or poor when the glottis was visualized or not. The number of attempts required was recorded (an attempt began with insertion of the BRF into the mouth and ended with BRF withdrawal). The time from insertion of the BRF into the mouth to capnographic confirmation of endotracheal intubation was used as the primary outcome. After 120 seconds of intubation attempts, the BRF technique was considered a failure, and the attending anesthesiologist at his discretion decided whether to continue BRF intubation with or without a laryngoscope or intubate the patient using another device.

After consultation with the statistical service of the Centre de recherche du Centre hospitalier de l’Université de Montréal, a logarithmic transformation of the dependent variable was completed. Univariate analyses were performed to evaluate the association between patient characteristics and time required for intubation. Pearson and Spearman coefficients were calculated (GraphPad Prism® version 6.00 for Windows, GraphPad Software, La Jolla, CA) and were used, respectively, for continuous and ordinal data. Variables that exhibited a significant correlation with time required for intubation in the univariate analysis were included in a subsequent multivariate analysis using a standard least squares model (JMP® version 11, SAS Institute Inc., Cary, NC) to identify those that remained significantly and independently correlated with time to intubate. Five values were missing for final analysis; 4 upper lip bite test scores because of lack of patient comprehension of the task and 1 neck circumference because the value was not noted during data collection. To minimize the number of excluded variables, the multivariate analysis was recomputed while replacing the missing values by the extremes of their ranges. Because no difference in the analysis results was established with this test, the median values in each category were used as replacements for the missing ones. A linear regression of the variables having exhibited a positive correlation in the multivariate analysis was completed to better describe their effects on the dependent variable. To better describe the relationship between variables, 2 × 2 interactions were completed. Unless otherwise stated, data are presented as percentages or means ± 1 SD. A P value of <0.05 was considered significant.

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RESULTS

Patient characteristics, including the Cormack and Lehane grade with DL, are included in Table 1. The first attempt with the BRF was successful in 390 patients, 6 patients required >1 attempt to achieve successful tracheal intubation with the BRF, and 4 patients could not be intubated with the BRF alone because of the inability to visualize the glottis. Of those 4 patients, 2 were intubated using DL simultaneously with BRF. In the other 2 patients, BRF with DL was not attempted; 1 was intubated using DL and a malleable stylet to shape the endotracheal tube and the other with DL and a tracheal introducer. The mean intubation time for all successful intubations was 26 ± 13 seconds. Glottic visualization with the BRF was good in 396 of 400 (99%) of cases. In the 6 patients with a grade III Cormack and Lehane grade on DL, BRF was successful on the first attempt in 4, on the second attempt in 1, and was not successful in 1 patient. Fogging of the lens hampered intubation in 11 cases (3%). Secretions also complicated visualization in 16 cases (4%).

Table 1

Table 1

Multiple patient characteristics were correlated with time required for intubation using univariate analysis (Table 2). These included patient weight, body mass index (BMI), high Cormack and Lehane grade, upper lip bite test, sternothyroid distance, thyromental distance (TMD), mouth opening, neck circumference and skinfold thickness, history of snoring, and female sex. After multivariate analysis, only 4 variables remained correlated with intubation times. Decreasing TMD appeared to be correlated with shorter intubation times, whereas decreasing mouth opening, increasing BMI, and higher Cormack and Lehane grade predicted longer intubation times (Table 2, Fig. 1). There was no evidence that interaction could be present from the univariate and multivariate results; 2 × 2 interactions were verified, and none was significant. Interaction with Cormack and Lehane grade could not be tested because there was insufficient power to detect such an association.

Table 2

Table 2

Figure 1

Figure 1

In 12 patients, a size 6.5 or 9.0 endotracheal tube was substituted for the standard size in the study, at the request of the surgeon or the attending anesthesiologist, without impact on the intubation technique. Data collection was incomplete in 5 patients: 4 patients were unable to reliably execute the upper lip bite test and, in 1 patient, the measurement of neck circumference was accidentally omitted. No patients and no data were excluded from the analysis. No adverse events or complications associated with an increase in the use of the BRF were noted during this study.

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DISCUSSION

In this study, successful tracheal intubation with the BRF occurred in 396 (99%) cases. In all 4 cases where intubation with the BRF alone was not successful, failure to visualize the glottis appeared to be the result of difficulty in mobilizing the epiglottis to create an opening between it and the posterior pharyngeal wall without the BRF passing directly into the esophagus. DL performed simultaneously with BRF is a technique that has been described before10 and that allows the anesthesiologist to lift the epiglottis off the posterior pharynx. When attempted in 2 of the cases of failed BRF in this study, this technique proved successful. In the other 2 failures with BRF alone, the anesthesia team chose to intubate using DL and another intubation aide; DL + BRF was not attempted (but may also have been a successful alternative). The success rate for BRF intubation in this study is similar to the 98.3% success rate previously reported in a case series of 60 patients6 and somewhat better than the 91.7% success rate with the BRF reported by Wahlen and Gercek19 (12 patients) or the 86.1% reported by Wong20 (36 patients). The success rate for BRF in this study is comparable with the 99.75% success obtained in a similar study by this group using the GlideScope videolaryngoscope15 and with the 99.7% success rate reported for DL.21

When the study was designed, analysis of the correlation between number of intubation attempts and patient characteristics was planned.15 However, the low number of patients with >1 intubation attempt (10/400) in this study precluded this type of analysis.

In this study, decreased mouth opening, increased BMI, and higher Cormack and Lehane grade were independently associated with time to intubation with the BRF. All these 3 factors are also associated with difficult DL. However, decreasing TMD appears to be correlated with slightly shorter (Fig. 1) intubation times with the BRF, in contrast to the findings of studies evaluating DL or videolaryngoscopy.1,15 This could be attributable to key differences between DL and BRF: with DL, a longer TMD allows proper alignment of the oral, pharyngeal, and laryngeal planes as the operator displaces the anterior pharyngeal structures with the laryngoscope to visualize the glottis22; with the BRF, however, the anterior pharyngeal wall is not displaced by the BRF itself, the jaw being instead lifted by the operator to create an opening beneath the epiglottis to allow passage of the scope and glottic visualization. Although it is not possible to definitely explain from our results why decreased TMD predicted slightly shorter intubation times, it is interesting to note that the BRF was designed for pediatric patients with a receding chin,7 and it is therefore possible that the fixed angle and length of the distal portion of the scope may make it particularly suitable for intubation of patients with short TMDs.

Mouth opening was inversely correlated with time required for intubation with the BRF. A mouth opening of <35 mm has also been predictive of difficult DL in previous studies,23,24 although results of a meta-analysis were less clear.25 One previous study26 compared DL with the BRF in 76 patients with simulated difficult airways (using a rigid cervical collar to restrict cervical movement and mouth opening). In this study, successful tracheal intubation occurred in 82% of the BRF group versus only 40% in the DL group. It may be that, although a small mouth opening does prolong intubation times with the BRF, the BRF’s small size could still make it less susceptible to outright failure than DL.

Increasing BMI also predicted the time required for intubation, as is the case in studies exploring the relationship between BMI and failed tracheal intubation using DL.2,27 Redundant mucosal tissues hampering glottic visualization may explain this finding in both techniques.

Finally, Cormack and Lehane grade III at DL also correlated with longer intubation times at BRF. A Cormack and Lehane grade of III or IV is a well-established criterion for difficult DL.21 The BRF has previously been studied versus flexible fiberoptic bronchoscopy in unanticipated difficult airways, as defined by failed intubation at DL or Cormack and Lehane grade III or IV at DL. Rudolph et al.11 had a 100% success rate with BRF in the unanticipated difficult airway (n = 116), including success with BRF after 2 failed attempts at fiberoptic bronchoscopy intubation. Kim et al.10 had a 90% (18/20) success rate with BRF. In this study, Cormack and Lehane grade III at DL was observed in 6 (1.5%) patients, an incidence similar to previous studies that noted only the best glottic view obtained with or without backward upward rightward pressure28; BRF intubation without assistance with a laryngoscope was successful in 5 of these 6 patients (83%).

Figure 2 presents an algorithmic interpretation of the findings of this study. In addition to having a high success rate as a primary intubation tool, the BRF is an attractive alternative to DL in patients with short TMDs, with or without reduced mouth opening. Conversely, patients with a high BMI, or in which BRF alone fails, may benefit from a combined BRF and DL technique. Other maneuvers such as head tilt or neck extension may also be useful in modifying the spatial relationship between the epiglottis and the anterior pharyngeal wall. Further studies could evaluate the clinical utility of the addition of DL to BRF, as well as other possible maneuvers.

Figure 2

Figure 2

Our study has several limitations. The use of end-tidal CO2 as a confirmation for successful intubation led to slightly longer intubation times when the capnograph sampling line was longer (when the head of the patient was far from the anesthesia machine). Another limitation is that the Cormack and Lehane grade at DL was determined by the same person who subsequently performed BRF, which could potentially have influenced the performance of the second technique. Furthermore, because only patients receiving neuromuscular blockade were studied, patients with nonreassuring airways in whom awake intubation or intubation without paralysis was selected were not included. Anticipated difficult airways using DL may therefore be underrepresented in our patient group.

In conclusion, mouth opening, BMI, and high Cormack and Lehane grade predict a longer time to successful intubation, as has already been noted with other intubation techniques, such as DL and videolarygoscopy. Decreasing TMD predicts slightly shorter intubation times with BRF, a relation that may reflect the original patient population for which BRF was designed. The high success rate of BRF in this study, the unique relationship between TMD and intubation times, and the possibility of further increasing BRF success rates by combining it with DL help define the role of BRF in the management of difficult intubation.

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DISCLOSURES

Name: Michal Nowakowski, MD.

Contribution: This author helped with study design and conduct, as well as data collection and analysis and manuscript preparation.

Attestation: Michal Nowakowski attests to the integrity of the original data and the analysis reported in this manuscript and approves the final manuscript.

Name: Stephan Williams, MD, PhD.

Contribution: This author helped with study design and conduct, as well as data analysis and manuscript preparation.

Attestation: Stephan Williams attests to the integrity of the original data and the analysis reported in this manuscript and is the archival author.

Name: Jason Gallant, MD.

Contribution: This author helped with study design and conduct, as well as data collection.

Attestation: Jason Gallant approved the final manuscript.

Name: Monique Ruel, Inf CCRP.

Contribution: This author helped with study design and conduct.

Attestation: Monique Ruel approved the final manuscript.

Name: Arnaud Robitaille, MD, FRCPC.

Contribution: This author helped with study design and conduct, as well as data analysis and manuscript preparation.

Attestation: Arnaud Robitaille attests to the integrity of the original data and the analysis reported in this manuscript.

This manuscript was handled by: Sorin J. Brull, MD.

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ACKNOWLEDGMENTS

The authors would like to thank Martin Ladouceur, PhD, Principal Advisor in Biostatistics, CR-CHUM.

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