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Original article

Clinical experience of airway management and tracheal intubation under general anesthesia in patients with scar contracture of the neck

XUE, Fu-shan; LIAO, Xu; LI, Cheng-wen; XU, Ya-chao; YANG, Quan-yong; LIU, Yi; LIU, Jian-hua; LUO, Mao-ping; ZHANG, Yan-ming

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Abstract

The airway management of patients with scar contracture of the neck often presents unique challenges to the anesthetists because limitation of the head extension can affect the airway and influence airway management. These patients are not only very difficult to intubate, but also at high risk of inability to ventilate via facemask after failed intubation.1 The traditional choices advocated for airway management have included awake intubation, pre-induction neck scar release under local anesthesia, elective tracheostomy, or in some cases, discouragement of surgery altogether.2,3 Awake intubation is often regarded as the safest option for patients with known difficult airway,2,4,5 but it is a severe stimulating and painful procedure.6 Some patients refuse awake intubation and even abandon surgical treatment because of anxiety and trepidation, especially for the patients who had this experience in the past surgery.1 In addition, awake intubation is also more difficult than the intubation under general anesthesia.7

The new supraglottic airway devices, such as the laryngeal mask airway (LMA), the esophageal-tracheal Combitube and the laryngeal tube, have proven to be very useful options in managing a difficult airway because they are able to re-establish the patent airway and maintain adequate ventilation when the facemask ventilation proves difficult or impossible.2,5 This has not only greatly facilitated management of the unanticipated difficult airway, but also provided the prerequisite to safely perform tracheal intubation under general anesthesia in patients with a known difficult airway. In the past few years numerous studies have reported the favorable experiences in this respect.7–12 However, there is no available published data about the airway management and tracheal intubation under general anesthesia in patients with scar contracture of the neck. This retrospective clinical study was conducted to determine possibility, safety and efficacy of airway management and tracheal intubation under general anesthesia in such patients.

METHODS

Patients' selection

After the Ethics Committee of Plastic Surgery Hospital approved the study, we collected and analyzed data from 1683 patients (959 male and 724 female) who were scheduled for scar resection and reconstructive surgery requiring general anesthesia and tracheal intubation for airway management in our hospital from January 1994 to December 2006. Of all patients, 181 were children aged 1.5—6 years, 279 were children aged 7—14 years, 1171 were patients aged 15—50 years, and 52 were patients aged >50 years. They all suffered from postburn scars of the neck and some also combined scars of the face, the trunk and the other body sites (Figures 1 and 2).

Figure 1.
Figure 1.:
Adult patients with postburn scar of the neck, face and other body sites. The left female patient suffered from severe limitation of head extension due to scar contracture of the neck. The right male patient had a combination of limited head extension and mouth opening due to scar contracture of the face and neck.
Figure 2.
Figure 2.:
Pediatric patients with postburn scar of the neck, face and other body sites. The girl on the left suffered from the severe limitation of head extension due to scar contracture of the neck. The boy on the right had a combination of limited head extension and mouth opening due to scar contracture of the face and neck.

Patients were excluded if they had an interincisor distance of <18 mm, were ASA physical status 3 or more, had respiratory tract pathology or a history of sleep apnea and were at risk of regurgitation-aspiration (e.g., known hiatus hernia and esophageal reflux). If children aged <6 years had an atlanto-occipital extension of <20° or children could not cooperate to allow adequate preoperative airway assessment, they were also excluded.

Preoperative airway evaluation

Seven senior anesthetists in our department were involved in this study. They all had received the same training program in difficult airway management during the resident training and had been working in this respect for more than 5 years. Before surgery, one of them performed the full-scale airway evaluation for each patient in an attempt to determine difficult degrees of facemask ventilation and tracheal intubation. During the airway evaluation, Mallampati classification was defined with the patient sitting upright, with the head extension as soon as possible, the mouth open as wide as possible and the tongue protruded maximally as previously recommended.13 To test extension of the atlanto-occipital joint, the patient maximally flexed his/her cervical spine and then tilted the head up as far as possible without moving the neck. The rater's hand was placed on the neck to ensure immobilization of the lower cervical spine and a goniometer was used to measure the angle transversed by the occlusal surfaces of the maxillary teeth from complete head flexion to maximal head extension.14 The interincisor distance of the patient was defined as the maximum distance between the upper and lower incisors or between the gingival margins in the edentulous with the mouth widely opened.

Patient's group assignment

Based on the preoperative airway assessment, all the patients were predicted to have a known difficult airway due to limitation of the head extension caused by scar contracture of the neck. In addition, 312 patients also combined moderately limited mouth opening with an interincisor distance of 18—30 mm due to their face scar (right patients in Figures 1 and 2) and 56 were Mallampatti's grade III or IV.

After discussing with each adult patient and parents of each pediatric patient the potential risks and benefits of awake and non-awake intubations, they preferred airway management under general anesthesia and gave written informed consent to participate in this study. To safely control the airway after anesthesia, when the schemes of anesthesia and airway management were formulated before surgery, the patients were classified into the two groups based on the preoperative measuring results of the atlanto-occipital extension and the Mallampati's classification. Group 1 included 1375 patients with the atlanto-occipital extension of >20° and a Mallampatti's grade I or II. Group 2 contained 308 cases with the atlanto-occipital extension of <20° and a Mallampatti's grade III or IV.

Preoperative preparation

Patients were routinely fasted for 4—6 hours and were premedicated with intramuscular injection of midazolam 0.05 mg/kg 1 hour prior to anesthesia. On arrival in the operating room, an IV catheter was inserted and atropine 0.01 mg/kg was given for its antisialogogue effect. Noninvasive blood pressure, heart rate, electrocardiogram and pulse oxygen saturation (SpO2) were monitored with a multifunction monitor. Appropriately sized facemasks, endotracheal tubes (ETT), oropharyngeal and nasopharyngeal airway, LMA, Macintosh laryngoscope, intubating stylet, fiberoptic bronchoscope (FOB), fiberoptic stylet laryngoscope (FOSL) (Olympus Optical Corporation, Tokyo, Japan), gum-elastic bougie, tube changer, emergency tracheotomy or cricothyroidotomy kit and suction devices were prepared and all the apparatus were immediately available.15 In group 1, a lubricated intubating stylet was inserted into the ETT. The distal end of the ETT with an intubating stylet was bent anteriorly to an angle of 50° to 80°. In group 2, the lubricated FOSL was inserted into the ETT until its anterior end was closed to but not beyond the distal end of the ETT.16 The ETT-FOSL unit was bent to an angle of 70° to 90° angle at 6–10 cm from the distal end according to the anatomical abnormities of the individual patient's airway (Figure 3). The distal lens of the FOSL was wiped with a sterile anti-fog solution.

Figure 3.
Figure 3.:
The ET-FOSL unit.

Anesthesia protocol and airway management

After the patients in group 1 were preoxygenated for 5 minutes anesthesia was induced with intravenous injection of thiopentone 4 mg/kg or propofol 2 mg/kg. When any response to verbal command was lost, patients were ventilated via a facemask with 100% oxygen. After confirmation of adequate facemask ventilation, intravenous injection of succinylcholine 1 mg/kg was administered for muscle relaxation. After anesthesia induction, the patient's head was placed in the sniffing position with a firm pillow under the occiput as soon as possible. Laryngoscopy and blind intubation were carried out with the appropriately sized Macintosh blade using a modified Macintosh technique,17 with the external laryngeal manipulation performed by an assistant if possible. After visualization of the epiglottis, the precurved ETT with an intubating stylet was placed upward against the epiglottis and advanced 2 cm in the midline. No resistance in advancement of the ETT indicated that its tip had passed through the glottis. Then the stylet was gently withdrawn from the ETT and the ETT was further advanced downwards into the trachea. In the patients with combined limited mouth opening, if the epiglottis was only partially visualized through a narrow tunnel during the laryngoscopy, the gum-elastic bougie guided intubation under the help of a Macintosh laryngoscope was also used after the first intubation attempt failed.18 After the intubation, the correct position of the ETT in the trachea was confirmed with bilateral chest auscultation and detection of carbon dioxide in the expired gas.

After the patients in group 2 were routinely preoxygenated, one of the following two anesthesia methods was used. The one was total intravenous anesthesia (TIVA), in which thiopentone 4 mg/kg or propofol 2 mg/kg and fentanyl 2 μg/kg were slowly injected intravenously for induction, and then lidocaine 4.8 mg·kg−1·h−1 and fentanyl 1.2 μg·kg−1·h−1 were injected intravenously for maintenance.19 The other was sevoflurane inhalational anesthesia via a semi-closed circuit. Inspired concentration of sevoflurane was increased in 0.5% increments every 4 breathes until an end-tidal concentration of 5% was reached and maintained for 5 minutes.7,15 In this study, TIVA and sevoflurane inhalational anesthesia were used in 185 and 123 patients, respectively. After the desired level of anesthesia was achieved, the patient was placed in the supine position and the intubation was performed with the ETT-FOSL unit. During the intubation, the operator was located over the head or at a side of the patient, and the assistant was situated at a side of the head of the patient to do jaw thrust and pull the patient's tongue out for maintaining the patent airway (Figure 4). The ETT-FOSL unit was inserted in the midline through the mouth into the upper esophagus. Then the ETT-FOSL unit was slowly withdrawn with gentle anterior traction. As soon as the ETT-FOSL unit was drawn out of the esophagus, a click could often be felt with posterior displacement of the larynx. At this time, the tip of the ETT-FOSL unit was usually just under the epiglottis and the clear vocal cords came into view. After the tip of the ETT-FOSL unit was positioned at the laryngeal aperture, the ETT was advanced through the glottis into the trachea along the FOSL and the FOSL was then removed.20

Figure 4.
Figure 4.:
Trachea intubation was performed with a fiberoptic stylet laryngoscope in patients with scar contracture of the neck. The operator was located over the head (A) or at side of patient (B). An assistant was situated at a side of the head of the patient to do jaw thrust (A) and pull the patient's tongue out (B) for keeping the airway patency.

All intubation was performed by the senior anesthetists experienced in the two intubation methods. If complete airway obstruction occurred after anesthesia and could not be effectively relieved by routine airway maneuvers, a LMA was immediately inserted. Emergency tracheotomy or cricothyroidotomy was done if necessary. If hypoxemia (SpO2 of 90% or less) occurred during the intubation, the procedure was stopped and 100% oxygen was immediately supplied via facemask. Laryngospasm was treated with positive pressure ventilation, deepening anesthetic depth and 10—20 mg of succinylcholine injected intravenously if necessary.1,19 If the intubation attempt failed, the facemask ventilation was done before the reintubation.

Observed variables

After successful intubation, the patients were examined immediately for any traumatic injury to teeth, lips and tongue. During this study a detailed work sheet was completed by the operator or the assistant. The recorded data included: patient's characteristics, results of the preoperative airway evaluation, methods of anesthesia, the time required for intubation (namely the period from first removing facemask to successful intubation, including the time required for intermittent facemask ventilation and repeating laryngoscopy), number of attempts taken for successful intubation, auxiliary maneuvers adopted during the laryngoscopy and intubation; locations of injuries, arrhythmia, respiratory depression, laryngospasm, vomiting, aspiration, hypoxemia, cardiorespiratory arrest brain damage and etc.

Statistical analysis

Data were analyzed with a SPSS 10.1 statistically software (SPSS Inc., Chicago, USA). The differences in difficult degrees of the laryngoscopy and intubation and incidences of complications between patients with the laryngoscopic views of Cormack and Lehane (C&L) grades II and III in group 1 were compared using a 2×2 contingency test. The incidences of adverse respiratory events between the patients using the TIVA and the sevoflurane inhalational anesthesia in group 2 were compared using a Chi-square test. Data are expressed as number of patients or incidence (%). P <0.05 was considered statistically significant.

RESULTS

Quality of tracheal intubation

All patients were successfully intubated. In group 1, the intubation was accomplished by one attempt in 1279 cases (93%). Two or more attempts were required to complete intubation in 96 patients (7%). Of those, 7 cases with combined limited mouth open were successfully intubated using the gum-elastic bougie by one or two attempts after the first intubation attempt failed. In group 2, one hundred and fourteen patients (37%) were successfully intubated by one attempt. 194 patients (63%) required two or more attempts to achieve successful intubation (Table 1).

Table 1
Table 1:
Time and number of attempts required for the successful intubation in the two groups

Causes of failed intubation attempts

In group 1, the reasons of 105 failed intubation attempts were an inappropriate precurved angle of the ETT (56.2%), a tunnel view due to limited mouth open (31.5%), the ETT extruded from the glottis during withdrawal of the stylet (7.6%) and an unexpected oesophageal intubation (4.7%). In group 2, the causes of the 298 failed intubation attempts were the obscure vision of FOSL due to blood and secretions in the airway or fogging of the lens (76.8%), difficulty to introduce FOSL towards the glottis because of severe airway anatomical abnormalities (17.5%), hypoxemia (3.7%) and the ETT extruded from the trachea during withdrawal of the FOSL (0.7%).

Complications associated with laryngoscopy and intubation

A total incidence of the complication was 6.7%. Neither cardiac arrest nor brain damage occurred in either group. Of all complications, airway trauma was most common. The traumatic complication included 35 lip injuries, 27 mouth mucous injuries, 19 posterior pharyngeal mucous injuries, 5 dental injuries and 3 others. The incidence of traumatic complication was 2.6% and 9.7% with one intubation attempt in groups 1 and 2, respectively, 12.5% and 17% with multiple intubation attempts (one vs multiple attempts in both groups, P <0.001) (Table 2).

Table 2
Table 2:
Complications associated with the laryngoscopy and intubation

The nontrauamatic complications occurred only in group 2 (Table 2). Laryngospasm occurred in 4 patients using the TIVA and was relieved without any adverse sequences. Hypoxemia occurred in 11 patients (5 children and 6 adults) due to a long duration of the intubation attempt, respiratory depression and airway obstruction. The incidence of hypoxemia was 5% (9/185) and 1.6% (2/123) in patients using the TIVA and sevoflurane inhalational anesthesia, respectively. Hypoxemia was corrected by routine airway maneuvers and the facemask ventilation with 100% oxygen in 7 patients. In the remaining 4 cases using the TIVA, the LMA was used because of total airway obstruction and difficult facemask ventilation. No patients required emergency tracheotomy or cricothyroidotomy. Laryngospasm and hypoxemia were more common in the patients using the TIVA compared to those using the sevoflurane inhalational anesthesia (P <0.001).

DISCUSSION

Difficulty in airway management has been associated with serious complications, particularly when failed intubation has occurred.4 Occasionally in a patient with a difficult airway, the anesthetist is faced with the situation where the facemask ventilation proves difficult or impossible. This is called a “cannot intubate - cannot ventilate” (CICV) situation and is one of the most critical emergencies in clinical anesthesia. If this situation continues for a few minutes severe outcomes, such as brain damage or death, may occur.2,5 Therefore, it is generally believed that awake intubation is the safest option for patients with a known difficult airway. Possible advantages of awake intubation include the facts that the patient is able to cooperate with the operator, is able to breathe spontaneously throughout the procedure and is able to maintain the airway patency though conscious control of the airway muscles.6

Because awake intubation is very stimulating and painful, its success depends on patients' cooperation to great extent.1,3 Expect for the psychological preparation and sedation management, adequate local anesthesia of the airway is key to obtain patients' cooperation. In patients with scar contracture of the neck, however, the anatomical abnormities of the face and neck may result in difficult or impossible to perform local anesthesia of the airway using the routine methods, such as glossopharyngeal block (oropharynx), superior laryngeal block (larynx above the cords), and translaryngeal block (larynx below the cords and trachea).6 If the patient combines the limited mouth opening due to the face scar, even simple airway spray of local anesthetics may be very difficult. Moreover, psychological anxiety and trepidation due to the repeated surgery in such patients can also make them very sensitive to adverse stimuli so that they are often not able to tolerate any airway procedure.1,4 When awake intubation is attempted in these patients, they can often not satisfactory cooperate with anesthetists and struggle against the ETT approaching the glottis because of active airway reflexes. This may further increase difficulty in the intubation procedure. Therefore, awake intubation is generally time-consuming for the anesthetist and unpleasant for the patient. The patients who had unpleased experience of awake intubation in the past anesthetic history may absolutely refuse this technique. Obviously, awake intubation is also not a suitable option for pediatric patients because of inability to cooperate. Under these circumstances the need for general anaesthesia becomes necessary.7,15

When the intubation is performed under general anesthesia in patient with a known difficult airway, the problem that the anesthetist is extremely concerns about is whether the facemask ventilation is difficult.2 When there is no difficulty in the facemask ventilation, the airway is easy to manage using the facemask ventilation even if the larynx proves difficult to visualize or tracheal intubation is a failure.21 For the patients with scar contracture of the neck, we recommend to estimate whether the facemask ventilation is difficult from the history before anesthesia. If the patient does not snore or experience sleep apnea, it is likely that his/her airway will remain patent during anesthesia. However, if the patient has a history that indicates lack of airway patency during sleep, a successful facemask ventilation is unlikely and in consequence an awake intubation technique must be used.22 In the patients with a known difficult airway, a successful facemask ventilation must be confirmed before IV administration of muscular relaxants during induction. In this study, succinylcholine was chosen for neuromuscular blockade as it is rapid onset and has short duration of action.1 According to our experience, the patients in group 1, i.e. who have an atlanto-occipital extension of >20° and a Mallampatti's grade I or II, can be successfully ventilated via a facemask after anesthesia. Although the partial airway obstruction does occur, this can be effectively relieved by routine airway maneuvers such as jaw thrust and use of the artificial airways, changing patient's position etc.1,14,19

During anesthesia, however, the patients in group 2, i.e. who have an atlanto-occipital extension of <20° and a Mallampatti's grade III or IV, are at a high risk of difficult facemask ventilation due to complete airway obstruction that is not relieved by routine airway maneuvers. This is mainly due to the limited mandibular space and reduced displaceability of the oropharyngeal soft tissues caused by scar contracture of the neck. After anesthesia, loss of muscle tone can also facilitate the relevant upper airway structures such as the base of the tongue, the epiglottis, the larynx and the pharyngeal wall to collapse toward one another.1,5,10,19 If tracheal intubation is attempted using direct laryngoscope under these circumstances, the laryngoscopy may displace the base of the tongue upwards and obstruct the laryngeal aperture to cause complete airway obstruction.15 Considering a high risk of loss of the airway control in the patients of group 2, the spontaneous breathing must be reserved before successful intubation and the patient's airway should be tested by gradually deepening anesthetic depth.2,7,21 If a difficult facemask ventilation occurs after anesthesia in such patients the following methods may be attempted. The precurved ETT with a metal stylet, looking like a hockey stick, is inserted into the oropharynx to lift the base of the tongue upwards and make the airway open. In clinical practice, we usually also use the ETT-FOSL unit as the precurved ETT with a metal stylet. If this technique can not maintain a patent airway the LMA is a very useful rescue airway device and must be inserted in time.2,10,21 Despite the existence of obvious anatomical abnormities of the upper airway in patients with severe scar contracture of the neck, we found that as long as the patient's interincisor distance was >18 mm, the LMA could be successfully inserted to provide a patent airway and maintain adequate ventilation. If the LMA is not available, the esophageal-tracheal Combitube or the laryngeal tube is another option.23 Emergency tracheotomy or cricothyroidotomy is a last choice.2,5,21

According to the experience and preference of the different senior anesthetists in our hospital, both the TIVA and sevoflurane inhalational anesthesia had been used in group 2. But the complete airway obstruction that was not relieved by routine airway maneuvers occurred in 4 patients using the TIVA. It suggested that the TIVA was likely to precipitate sudden loss of the airway control and the CICV situation. In addition, all laryngospasm also occurred in the patients using the TIVA. This may be due to the increased laryngeal sensitivity caused by thiopentone.24 Therefore, it might be prudence to use the TIVA in patients with a known difficult airway. Sevoflurane has been shown to be a useful agent for inhalational anesthesia in the management of the difficult pediatric and adult airway,9,10,24 as it has a blood gas solubility of 0.69 and is least irritating to the airway.7 During the sevoflurane inhalational anesthesia, the patient's airway can be tested by a gradual onset of anesthesia and the spontaneous breathing may be well maintained. Should hypoventilation and airway obstruction arise, sevoflurane can be turned off and the patient may be woken up. After the adequate level of anesthesia is achieved tracheal intubation is attempted. If the intubation attempt is unsuccessful, then sevoflurane inhalation and ventilation are continued using the facemask. In our study, the inspired concentration of sevoflurane was initially increased gradually until an end-tidal concentration of 5% was reached and maintained for 5 minutes before fiberscopy and intubation.15 We found no episodes of severe airway obstruction, anpea, laryngospasm and coughing at any stage of anesthesia induction, fiberscopy and intubation. Moreover, the incidence of hypoxemia was also significantly lower in patients using the sevoflurane inhalational anesthesia than in those using the TIVA. These results indicate that using sevoflurane inhalational anesthesia can achieve adequate depth of anesthesia and decrease risk of sudden loss of the airway control in patients with a known difficult airway.

Difficult intubation is mainly due to inadequate visualization of the larynx using the direct laryngoscope.13,17 If the anesthetist can predict in which patient it is likely to prove difficult to view the larynx during the laryngoscopy, the risks of anesthesia may be significantly reduced.25 While there are many predictors of difficult laryngoscopy, they all have a low positive predictive value when used alone.26,27 The sensitivity and specificity of predictive tests may also be considerably affected by the patients' age and habitus, causes of laryngoscopy problems and measuring methods. It is usually thought that the quality of predicting difficult laryngoscopy can be improved by associating several tests, such as the Mallampati's classification, the thyromental distance, the ability to protrude the mandible, the atlanto-occipital extension etc.13,17,25–27 However, the thyromental distance and the ability to protrude the mandible are impossible to precisely measure in patients with scar contracture of the neck. It has long been well appreciated that an adequate extension of the atlanto-occipital joint is a key to bring the oral, the pharyngeal, and the laryngeal axes into a straight line during the laryngoscopy. A normal person can produce 35° of atlanto-occipital extension. When the atlanto-occipital extension decreases, vigorous attempts to do so will cause the convexity of the cervical spine to bulge further anteriorly, which will push the larynx anteriorly and compromise a direct laryngoscopic view.25 Considering the special anatomic abnormalities in patients with scar contracture of the neck, we had suggested a practical bedside test to predict the difficult laryngoscopy using the atlanto-occipital extension based on extensive clinical observation.19 We demonstrate that when the atlanto-occipital extension is >20°, the epiglottis is usually able to be viewed with the Macintosh laryngoscope, i.e. a laryngoscopic view of C&L grade II or III. When the atlanto-occipital extension is <20°, however, the epiglottis can not generally be viewed with the Macintosh laryngoscope, i.e. a laryngoscopic view of C&L grade IV. The predicative accuracy of this test is 87.5%, without false negatives in the patients with a difficult to view larynx during the laryngoscopy. All improper predications (12.5%) are false positives in the patients who are easy to view the larynx during the laryngoscopy. When an atlanto-occipital extension of <20° and a Mallampatti's grade III or IV simultaneously exist, the predictive sensitivity of this test for difficult laryngoscopy is up to 95%, with a specificity of 91%. This is in agreement with the previously reported results in the studies of predicting a difficult laryngoscopy using this two tests.14,26 In addition, the Mallampati's classification has also been shown to be a sensitive test of predicting difficult facemask ventilation.13,17,27 Now these two predicative tests have become a routine part of managing difficult intubation in patients with scar contracture of the neck. Therefore, in this study, the atlanto-occipital extension and Mallampati's classification were routinely measured in every patient during the preoperative airway assessment. The schemes of anesthesia and airway management were also formulated according to the measuring results of the two tests.

An operator who is familiar with modified Macintosh technique can successfully perform tracheal intubation for patients with the laryngoscopic view of C&L grade II or III after anesthesia.17 This method may provide excellent intubating conditions. Our results showed that in group 1, 99.5% of patients were successfully intubated by one or two attempts. Based on our experience some key points of safe and successful intubation using this method must be emphasized. The distal end of the ETT with an intubating stylet must be bent to an appropriate angle according to the anatomical abnormities of patient's airway. This bent angle helps the ETT to negotiate acute oropharynx-tracheal angles and be inserted into the glottis under guide of the epiglottis. In addition, the midpoint of the ETT to right at an angle of 70° to 80° to the distal first bend also helps to maintain a better laryngoscopic view during the intubation.28 An inappropriate precurved angle of the ETT is a primary cause of initial failed intubation attempts in our study. After the tip of the ETT passes the epiglottis, the resistance to further advance of the ETT often suggests that the ETT is snagged in the right vocal cord or anterior laryngeal wall. A slight back and forth rotation of the ETT (about 15°) can usually overcome this difficulty. When the patient combines a limited mouth opening, the visualization of the epiglottis through a narrow tunnel is a problem frequently encountered during the laryngoscopy. Vision is obscured further when part of the narrow tunnel is subsequently occupied by the ETT. With a tunnel view, anesthetists can visualize the epiglottis, but not perform blind intubation using it as a guide. This is a secondary cause of the failed intubation attempts in our study. Under this circumstance, the gum-elastic bougie guided intubation is often helpful.18 When the intubation attempt fails, the priority is to ensure adequate facemask ventilation and oxygenation of the patients rather than repeated intubation attempts. As compared to adult patients, this principle is more important for pediatric patients because they can only tolerate a shorter period of apnea due to high oxygen consumption relative to reserves.24,28 Moreover, multiple intubation attempts may also result in bleeding and edema of the upper airway, making the task even more difficult, particularly at pediatric patients who they are vulnerable to airway trauma.2,5 It is often better to accept failure after a few attempts and move on to a pre-planned failed intubation sequence, such as the intubation using the intubating LMA and FOB.2,9,21,29,30 An operator who is not experience in the difficult airway management must choose awake intubation technique in patients with a known difficult airway.

The modified Macintosh method is not suitable for the patients in group 2 because the epiglottis can not often be adequately viewed during the laryngoscopy. Moreover, most of these patients are also at a high risk of complete airway obstruction after anesthesia.19 For these patients, it is usually recommended that a rigid fiberoptic laryngoscope, such as the FOSL, should be used to perform tracheal intubation.5 In patients with scar contracture of the neck, the tracheal intubation using the FOSL has the following advantages. The FOSL can elevate the base of the patient's tongue with upward force on the handle to keep the airway patency.20 The anterior part of the FOSL can be bent according to the anatomical abnormities of patient's airway. Its advancement and withdrawal are well controlled by manipulating the handle outside the oral cavity because of its rigid stylet.16 Thus the tip of the ETT-FOSL unit is easily positioned at the glottis. In contrast, it is more difficult to introduce the tip of the FOB into the glottis and insert an ETT over the FOB into the trachea because of its flexibility. In anesthetized patients, moreover, posterior movement of the tongue and epiglottis can not only restrict visibility of the FOB, but also result in airway obstruction.15

Our results showed that 79.5% of the patients in group 2 were successfully intubated by one or two attempts. Our experience with the operator located at the right side of the patient was more convenient to perform intubation using the FOSL than locating in front of the patient. We also found that the distal bent length of the FOSL was very important for successful intubation. It is approximately equivalent to the estimated distance between the thyroid prominence and the mandibular angle.20 During the intubation, the FOSL must remain aligned with the airway axis. Angled approaches to the laryngeal aperture frequently result in intubation failure.16 Because obscured vision of the FOSL due to blood and secretions in the airway or fogging of the lens is a primary cause of the failed intubation attempts in group 2, we consider that sufficient administration of antisialogogue drugs, effective cleaning of blood and secretions in the airway and use of anti-fog measures are also key points of successful intubation. What should be specially emphasized is that when the FOSL is used to perform tracheal intubation in the pre-planned airway management strategy, a preliminary direct laryngoscopy attempt to view the laryngeal structures should be avoided as far as possible. This may not only decrease the obscured vision of the FOSL due to the airway bleeding, but also reduce the severity of postoperative sore throat.31 One of main disadvantages of the FOSL is inability to use it in pediatric patients aged <6 years because its stylet, with an external diameter of 5 mm, does not fit the ETT with an inner diameter of <5.5 mm.20 For pediatric patients aged <6 years, the Shikani Optical Stylet is a useful device because its pediatric version can accommodate the ETT with an inner diameter of 2.5 mm or larger.32 The fiberoptic intubation is another useful approach.24 If a thin FOB that is used for infants and small children is chosen, it is best to use a LMA as a conduit for the fiberscopy and intubation because the aperture of a properly positioned LMA aligns itself anatomically with the glottis.33 This technique is now firmly established in the approach to the difficult pediatric airway.24

The patients with difficult intubation are at high risk of producing relative complications because of repeated intubation attempts, blind maneuvers and airway abnormities.2,21,25,34 The possibility in occurrence of the complications increases as the difficulty of airway management, physical force and number of intubation attempts increase.34–36 When anesthesia depth is inadequate and the duration of airway maneuvers is too long, hypoxemia, apnea, arrhythmia, laryngospasm and vomiting may also occur due to airway stimuli and adverse reflexes.1,20 A previous study has confirmed that in patients with anticipated difficult intubation, the incidence of minor upper airway trauma (posterior pharyngeal and lip lacerations and bruises) with laryngoscopy and intubation is 17%.36 In patients in whom the intubation is actually found to be difficult (multiple attempts at laryngoscopy but ultimately successful), the incidence of minor upper airway trauma is up to 63%. Our results showed that in group 1 the incidences of traumatic airway complications with one attempt and multiple attempts were 2.6% and 12.5%, respectively. Our findings are significantly lower than the results of the previous study above. Except for possible differences in patient selection and techniques of anesthesia and intubation between previous and our studies, this difference may reflect the fact that all operators in our study are senior anesthetists experienced in managing the difficult airway.

There are no available data about traumatic airway complications of intubation using the FOSL in patients with a known difficult airway. Our results showed that in group 2, the incidences of traumatic airway complications with one attempt and multiple attempts were 10% and 17%, respectively. The total incidence of traumatic airway complications was 14.3% and significantly lower than previous results with a direct laryngoscope.34–36 These results suggest that in patients with a known difficult airway, the intubation using the FOSL is likely to produce a lower incidence of the upper airway trauma compared to using the direct laryngoscope. The results from the previous studies on the other rigid stylet fiberscopes also support this view.4,31 Therefore, a further randomized controlled trial is required to show this clinical difference.

Based on the reasons of complications related to laryngoscopy and intubation in this study, we emphasize the key measures to decrease and prevent the occurrence of complications as follow. First, a preformulated strategy to safely secure the airway must be made before anesthesia. This strategy depends on the surgery, the condition of the patient, and the skills and preferences of the anesthetist.2 Second, the anesthetist must ascertain that the correct equipment is immediately available and there is at least one additional individual who is immediately available to serve as an assistant in management of the difficult airway.21,37 Third, sufficient preoxygenation should be done and adequate anesthetic depth must be achieved before the intubation. Fourth, excessive physical force to the upper airway and teeth must be avoided during the laryngoscopy.33 Fifth, the duration of each intubation attempt can not last too long, and SpO2 and ECG must be closely monitored, particular for pediatric patients.24,36 Sixth, if the CICV situation occurs, the immediate use of the rescue airway devices such as LMA for emergency ventilation can reduce or even avoid severe airway-related adverse outcomes.2,4,5,38

In conclusion, our results showed that with a full-scale airway evaluation, an adequate preoperative preparation and a pre-planned failed intubation strategy, the anesthetist who was experienced in management of the difficult airway could safely perform airway control and tracheal intubation under general anesthesia in patients with scar contracture of the neck. This study demonstrates the growing wealth of information regarding the applicability of airway management under general anesthesia in patients with a known difficult airway. The common teaching that the patients with a known difficult airway must be managed by an awake intubation technique may be challenged. Therefore, we believe that this technique may be very valuable for management of a known difficult airway because it is comfortable for the patient and saves time for the anesthetist. This has certain very important significance to improve the quality of clinical anesthesia and satisfy patients' needs and expectations.

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Keywords:

scar contracture of the neck; limitation of head extension; difficult intubation; airway management; general anesthesia

© 2008 Chinese Medical Association