The last decade has witnessed an increase in the level of knowledge and skills in airway management by anaesthetists. The American Society of Anesthesiologists has formulated widely accepted strategies for management of the difficult airway (the ASA Difficult Airway Algorithm). There has been much interest in the development of new and modified pre-existing equipment. The Difficult Airway Association (DAS, UK) is the first specialist society on this side of the Atlantic to concentrate on this important area of anaesthesia. Since 1996 the Departments of Anaesthesia at the Academic Hospitals of Groningen and Maastricht have held a 2-day airway management course (‘Access to the Airway’), three times per year. This course is the first of its kind in The Netherlands. It includes didactic teaching in anatomy of the airway, the ASA Difficult Airway Algorithm and techniques for awake tracheal intubation. The techniques of flexible fibreoptic intubation, stylet-aided intubation [e.g. the gum elastic bougie (Eschmann/Huikeshoven medical BV, Ophemert, The Netherlands), jet stylets, etc.], the use of the Trachlight (Laerdal BV, Groessen, The Netherlands), Fastrach (The Surgical Company, Amersfoort, The Netherlands), Combitube® (Kendall Ltd, Den Bosch, The Netherlands) and percutaneous access to the airway via the cricothyroid membrane and jet ventilation are taught in a hands-on setting using simulators. The purpose of the survey was to investigate current practice in airway management amongst Dutch anaesthetists. As organizers of the course we were curious as to what role, if any, our course has played in the development of airway management practice over the last decade. Partly because of the low medico-legal pressure there are no exact nationwide Dutch figures concerning morbidity and mortality related to airway management in anaesthetic practice.
A questionnaire was constructed in Dutch, containing 15 general questions and 12 questions pertaining to adjunctive techniques. The three-page questionnaire (the Appendix contains a short translated version) was sent in May 1999 to all practising anaesthetists (trainee and specialist grade) in The Netherlands. For this purpose a file of addresses was kindly supplied by the Dutch Anaesthesia Society (holder of the Register of all Dutch physician anaesthetists). The anaesthetists were asked about their training in airway management, method of recognizing a difficult airway, airway management strategy and use of adjunct or alternative techniques to conventional orotracheal intubation using a Macintosh or Magill laryngoscope. To avoid making the questionnaire too complex (and so to increase the response rate) simple questions were asked with an unavoidable loss of detail. The difficult intubation situation, for example, was not divided into anticipated and non-anticipated difficult intubation, and the different ways a laryngeal mask airway can be used in a difficult intubation situation were not detailed. The questionnaires were returned anonymously until September 1999 and were analysed using Pearson’s χ2-test with the SPSS® version 8.0 statistical software program.
The total number of Dutch physician anaesthetists in 1999 was 1063 (664 specialist grade in the non-teaching hospitals, 206 specialist grade in the teaching hospitals and 193 trainees). Of the 1063 questionnaires sent, 447 completed forms were returned in September 1999 (response rate 42%, Table 1). Until May 1999, 196 Dutch anaesthetists (18% of the total of 1063) had attended our course (‘course attenders’). There were significantly more course attenders from the teaching hospitals compared with the non-teaching hospitals (29% vs. 12%, P < 0. 01, Table 1). The questionnaires received from the three subgroups were evenly returned (no significant differences between relative sample sizes, P < 0.18). The proportions of course attenders amongst respondents from the subgroups trainees and teaching hospital specialists was also evenly distributed. However, in the subgroup comprising the non-teaching hospital specialists, there were significantly less respondents amongst those anaesthetists who had not attended the course (non-course attenders;P < 0.0005, Table 1). There were significantly more course attenders amongst those who started their anaesthetic training after 1988 (post 1988 group, P < 0.005).
Airway management training
Thirty-two per cent of respondents indicated having had some kind of training in airway management, apart from their formal general anaesthetic training; 18% attended our course ‘Access to the airway’, 7% attended workshops (mainly during congresses) and 7% trained by self-education or with help from colleagues.
Failed intubation and difficult intubation protocol
In our questionnaire failed intubation was defined as failure to intubate with a conventional laryngoscope. The incidence of self-professed failure of endotracheal intubation by conventional laryngoscopy is variable. The median frequency is two failed intubations per year. Most respondents (63%) indicate having failed to intubate the trachea one to five times per year; five (1%) indicated 50 or more times per year and 19 (3%) indicated that they never had had a failed intubation.
Fifty-four per cent of respondents indicated that they used a special protocol for difficult intubations. Thirty-seven per cent used their own (non-specified) protocol, 19% used the ASA Difficult Airway Algorithm , 5% used the Advanced Trauma and Life Support (ATLS) scheme  and 1% (three respondents) used Tunstall’s failed intubation drill . Forty-eight per cent of the respondents simulate difficult tracheal intubation as a way of airway management training.
Seventy-eight per cent of the respondents claimed that they often, or always, assess the expected degree of difficulty of tracheal intubation as part of their routine preoperative assessment. Twenty-one percentage of respondents say they never, or only occasionally, do so. Mouth opening is used most frequently (81%) as an indicator of possibly difficult intubation, followed by the three ASA advocated tests: neck extension (65%), Mallampati score (58%) and thyromental distance (38%) and by facial structure and dentition (12%) and history of previous tracheal intubation (5%).
Three hundred and sixty-one respondents (81%) considered an awake intubation to be an ethical anaesthetic technique, 16% considered it an unethical choice and 3% gave no opinion. Two hundred and twenty-two respondents (50%) perform awake tracheal intubation. Fifty-four per cent of all respondents considered an anticipated difficult intubation as an indication for an awake intubation. A full stomach, poor general condition and respiratory problems are mentioned as additional indications by 4%, 3% and 3% of respondents respectively.
Table 2 shows the mean frequencies of usage of several alternative techniques per year per respondent. Malleable soft metal tube introducers and non-metal stylets were used by 45% of respondents. Variation in use is very high. Some used them more than a thousand times per year and others just a few times.
The gum elastic bougie  is the most frequently used non-metal stylet (34% of respondents). Only 8% of respondents used other types of non-metal stylets.
The laryngeal mask airway  was used by 79% of respondents. There is considerable variation in its frequency of use. Some used it more than 2000 times per year.
The Trachlight  is used by 29% of respondents, from once to more than 250 times per year.
The Combitube® was used by 25 (5%) of respondents . Most used it once or twice per year, some up to 50 times per year.
The rigid bronchoscope was used very infrequently – by 5% of respondents. Two respondents (0.4%) used it 50 times per year.
Seventy-five per cent of respondents say they started performing flexible fibreoptic intubation (FFI) at some stage of their career (Table 3). Seventy-two per cent of this group started performing flexible fibreoptic intubation after 1989. The anaesthetist is the practitioner who usually performs flexible fibreoptic intubation (69%), followed by ear, nose and throat surgeons (9%) and respiratory physicians (6%). Flexible fibreoptic intubation is used once to 10 times per year by most respondents, three respondents (0.6%) used it more than 250 times per year. Local anaesthesia, frequently combined with sedation, was used by 57% of all respondents when performing a flexible fibreoptic intubation; 55% gave intravenous anaesthesia; 22% used inhalation anaesthesia. Some of the respondents use more than one method. Propofol (20%) is the preferred sedative for flexible fibreoptic intubation, followed by midazolam (7%), fentanyl (2%) and sevoflurane (1%). Lidocaine is by far the most frequently (89%) used local anaesthetic for awake flexible fibreoptic intubation. Cocaine was used in 11% of cases. Local anaesthetic for an awake intubation was usually applied by spray (56%), nebulization (28%) or by intratracheal injection (18%). Neural blockade (10%), gargling (10%) and application on cotton wool patches (10%) is used less frequently. The most frequently used neural blockade is that of the superior laryngeal nerve. Fifty-five per cent of those performing flexible fibreoptic intubation usually used the nasal route.
Retrograde tracheal intubation was performed by 18% of repondents: 84% of these used epidural catheter sets for this purpose; 14% used dedicated Cook retrograde intubation sets (Cook BV, Son, The Netherlands). The retrograde technique was used three times per year at the most.
Orthograde techniques (orthograde cannulation of the airway via the cricothyroid membrane) were used by 18% of respondents; once or twice per year by most, but two respondents (0.4%) used it 50 times a year. The techniques that are used for orthograde ventilation via the cricothyroid membrane are: the Minitrach (9%; Sims Portex Ltd, Hythe, UK), dilatation tracheotomy sets (8%), the Quicktrach (5%, VBM, Robert Bosch Str., Sulz am Neckar, Germany), the Cook Melker set (2%, Cook BV, Son, The Netherlands), the Ravussin needle (, 1%, VBM, Robert Bosch Str., Sulz am Neckar, Germany) and other techniques (1%).
Nineteen per cent of respondents had experience with jet ventilation. Only eight (2%) respondents had ever used a dedicated cricothyroid (Ravussin) needle; most used an oral or nasal catheter. Variation in the frequency of usage is high; from once to more than 300 times per year.
Additional techniques (9% of respondents) included the Fastrach (intubating laryngeal mask airway , 3%) and the laryngeal mask airway as a dedicated airway (for introduction of a bougie, fibrescope, etc.; 2% of respondents) and special laryngoscopes such as the McCoy (, 2%, Penlon Ltd, Abingdon, UK). Blind nasal intubation is near extinct (1%, four respondents). The variation in the frequency of use of additional techniques was very high (once to 500 times per year).
Techniques of choice
Table 4 shows the first choice technique of respondents in instances of a difficult tracheal intubation. No distinction was made in the questionnaire between an anticipated and an unanticipated difficult intubation. Fifteen per cent of respondents failed to mention a first choice technique. Respondents were also asked to state a second choice technique in case of a difficult intubation. Comparing the first choice with the respondent’s second choice, flexible fibreoptic intubation (28% vs. 34%), laryngeal mask airway (18% vs. 17%), Trachlight (5% vs. 7%) and Fasttrach (3% vs. 2%) were about equally popular. The use of stylets reduced from 25% to 4% and techniques via the cricothyroid membrane increased from 3% to 11%. The Combitube® is hardly mentioned (1% and 1%). The second choice of those using flexible fibreoptic intubation as first choice was: the laryngeal mask airway (23%), orthograde techniques (14%), retrograde technique (13%) and Trachlight (9%).
Influence of training
In the group of those individuals who have more than two failed intubations per year there is a significant difference between those who commenced their anaesthetic training before 1989 (pre-1989 group, 54%) and those who started after 1988 (post-1988 group, 72%, P < 0.0005), but no significant difference between course attenders (70%) and non-course attenders (58%, P < 0051). Significantly more of the course attenders are using the ASA Algorithm (34% vs. a responders mean of 19%, P < 0.0005). The post-1988 group used the ASA Algorithm more often than the pre-1989 group (29% vs. 14%, P < 0.0005). Course attenders simulated difficult tracheal intubation more often (59%) than non-course attenders (44%, P < 0.002). The post-1988 group did this more often (61%) than the pre-1989 group (40%;P < 0.0005). In the 21% of responders who claimed never, or only occasionally, to assess the expected degree of difficulty of intubation there were significantly fewer course attenders (14% vs. 26%, P < 0.03) and also significantly fewer post-1988 group members (8% vs. 30%, P < 0005). The post-1988 group did perform awake tracheal intubation significantly more often (56%) than the pre-1989 group (46%, P < 0.04). Fifty-eight per cent of the course attenders and 48% of the non-course attenders performed awake tracheal intubation; the difference is not significant (P < 0.08). Course attenders used the gum elastic bougie more often (42%) than did the non-course attenders (31%, P < 0.04). The Trachlight was used significantly more often by course attenders (42% vs. 26%, P < 0.001). The post-1988 group used it more often than the pre-1989 group (35% vs. 26%, P < 0.04). The post-1988 group performs flexible fibreoptic intubation significantly more often (87%) than the pre-1989 group (74%, P < 0.002). Amongst the respondents who started flexible fibreoptic intubation after 1994 there were significantly more course attenders (51% vs. 35%, P < 0.003).
In 1976, Tunstall presented his failed intubation drill  at the Nottingham meeting of the Obstetric Anaesthetists’ Association. It was a milestone in the development of anaesthetic airway management. Closed malpractice claim studies in the 1980s in the USA suggested that 30% of deaths totally attributable to anaesthesia were due to the inability to successfully manage very difficult airways [11,12]. This worrying figure was the motivating force behind the ASA Task Force’s determination to develop a Difficult Airway Management Algorithm (the ‘ASA algorithm’) in 1991 . This algorithm is revised periodically  and is widely acclaimed. Parallel to this ‘theoretical current’ there has been a development of technical possibilities in the field of airway management in the past 15 years. The introduction of the laryngeal mask airway in 1988 is another milestone in this regard. Flexible fiberoptic intubation became a commonly used technique  and many new instruments have been introduced. The Bullard (, Circon Corp., Santa Barbara, CA, USA) and McCoy laryngoscopes, the Lightwand  and disposable cricothyrotomy sets are such examples.
There are nine hospitals in The Netherlands, which are certified to train anaesthetists. These comprise eight university hospitals and one non-university hospital. In the Netherlands only specialist anaesthetists are allowed to provide anaesthetic care. To certify as a specialist anaesthetist, Dutch doctors (not nurses) follow a 5-year anaesthetic training scheme in a teaching hospital. In addition to the programme provided by the training centre, each trainee must attend a national theoretical course organized by the Dutch Anaesthesia Society over 3 days, every year. In 1992, the ASA algorithm was included in this Central Course of Anesthesiology (Prof. W. W. A. Zuurmond, personal communication), and so it was taught to those who commenced training after 1988. For this reason a pre-1989 and a post-1988 group were instituted in this survey. The airway management course ‘Access to the Airway’ was instituted in 1996. It consists of two mornings of theoretical talks and two afternoons of practice of airway management techniques using mannequins. After 3 years involving 196 course attenders (comprising 18% of all Dutch anaesthetists), we were interested to evaluate the possible role that Dutch formal anaesthetic training and the ‘Access to the Airway’ course played in individual anaesthetists’ practice; and more specifically in their management strategies and their familiarity with alternative techniques. The response rate of 42% was sufficiently high to draw statistical conclusions between subgroups. The 1997 study of Rosenblatt and his colleagues  had a response rate of 47% amongst 1000 ASA anaesthesiologists: this is slightly higher than our 42%, but we did not approach non-responders a second and third time. A simpler questionnaire might have generated a higher response rate, but then more detail would have been lost.
The incidence of failed intubation is about 1:250 in the obstetric population and about five to 35 of 10 000 in the general surgical population [17–20]. The definition of failed intubation varies in the literature. Hawthorne and his colleagues  defined it in their audit of a maternity unit as an ‘intubation that was not accomplished with a single dose of succinylcholine (suxamethonium), and therefore initiated the failed intubation drill’. Their failed intubation rate in obstetric patients increased from 1:300 in 1984 to 1:250 in 1994. Samsoon and Young in their retrospective study observed completely failed tracheal intubation . Their failed tracheal intubation rate was 1:280 (7:1980) in obstetric patients and 1:2230 (6:13 380) in surgical patients. In our questionnaire the question we asked for was: ‘How often do you fail to intubate with a conventional laryngoscope?’, which is different from ‘an intubation that was not accomplished with a single dose of suxamethonium’ or from a ‘completely failed intubation’; a gum elastic bougie may have solved the problem. In our sample survey the median of self-professed failed conventional tracheal intubation is two times per year per anaesthetist in all patients. Dutch anaesthetists perform on average 1040 general anaesthetics per year. About 600 of these involve endotracheal intubation, 36% involve a laryngeal mask airway and 6% involve anaesthesia by conventional facemask. A self-professed failed tracheal intubation rate of two times per year would mean an incidence of 1:300 intubations (33:10 000), which is similar to the obstetric figure (1:280), but seven times higher than the 1:2230 in the general surgical population mentioned by Samsoon and Young. Part of the discrepancy can be explained by the difference in definition of failed intubation. Second, our figure concerns anaesthetics including tracheal intubation in a general anaesthetic population, obstetric and surgical patients combined. Third, and perhaps most importantly, the question is ‘how reliable are figures for failed intubation?’ if they are not scored on the spot. Recent trainees seem to be less successful conventional intubators of the trachea. This may be due to lack of experience (in years, and by the extensive use of the laryngeal mask airway and other new equipment). However, it may also be that they wisely switch to another technique at an earlier stage, as taught by the ASA algorithm, thus avoiding the vicious circle of bleeding, oedema, difficult mask ventilation up to the ‘cannot intubate, cannot ventilate’ situation.
Use of a difficult airway algorithm
A postal survey amongst tutors of the Royal College of Anaesthetists in Great Britain and Northern Ireland  showed that nearly 60% of their departments have a failed intubation protocol. Although departmental policy does not necessarily reflect individual clinical practice, it is nevertheless encouraging that 54% of respondents reported the use of such a protocol, including 19% using the ASA algorithm.
Awake intubation of the trachea
Awake tracheal intubation is a cornerstone of the ASA algorithm. Nevertheless, 16% of respondents think it is an unethical technique, which is an unfortunately high figure. The fact that 21% of respondents ‘never’ or ‘sometimes’ estimate difficulty of tracheal intubation in advance is a figure that deserves the same grave concern as well, because as Ovassapian stated at the 1999 Annual Meeting of the Difficult Airway Society in Edinburgh, UK: ‘Too many patients have lost their lives or have suffered major complications because anaesthesia was induced in the presence of a compromised airway before the airway was secured.’
The use of malleable soft metal stylets has been common in The Netherlands since the early days of Dutch anaesthesia after the Second World War; the gum elastic bougie was rarely used before the 1990s. The gum elastic bougie as an adjunct to tracheal intubation is promoted by the ‘Access to the Airway’ course and perhaps this coincides with its increasing popularity.
Seventy-five per cent of the total of 1063 Dutch anaesthetists perform an average of 15 flexible fibreoptic intubations per year, which would mean 11 959 (= 0.75 × 1063 × 15) flexible fibreoptic tracheal intubations in total per year (1.1% of all general anaesthetics). Seventy-eight per cent of ASA anesthesiologists are familiar with flexible fibreoptic intubation  which is similar to the Dutch 75%.
Techniques of choice
In the questionnaire anaesthetists were asked to name their first and second choices of technique when presented with a difficult intubation scenario. No distinction was made between anticipated and non-anticipated difficult intubations. Flexible fibreoptic intubation (28% first choice and 34% second choice, respectively), the laryngeal mask airway (16% first choice, 17% second choice) and stylets (26% and 4%) were the most popular techniques. The Trachlight (5% first choice, 7% second choice) and Fasttrach (3% first choice, 2% second choice) and methods via the cricothyroid membrane (3% first choice, 11% second choice) were also mentioned. The laryngeal mask airway and techniques via the cricothyroid membrane are mentioned in the ASA algorithm as first and third rescue techniques respectively (the latter as trans-tracheal jet ventilation). Stylets (especially the gum elastic bougie), Trachlight and Fastrach are not mentioned in the ASA algorithm, but are obviously becoming standard practice in The Netherlands. It seems that the Combitube® (a supraglottic instrument) is rarely used in The Netherlands.
Influence of training and course
The significant conclusions of our sample survey are: course attenders and those who started their training after 1988 are more frequent users of the ASA algorithm, of simulation of difficult tracheal intubation as a training tool , of preoperative airway evaluation, of awake intubation, of flexible fibreoptic intubation, of the gum elastic bougie and of the Trachlight.
In the Rosenblatt study  several different difficult intubation scenarios were presented. It was found that respondents who had attended a difficult airway workshop within the previous 5 years, were more likely to use flexible fibreoptic intubation, to use other alternative techniques (both consistent with our study) and to have a retrograde wire kit or flexible fiberoptic bronchoscope available (not included as such in our study). We did not find a significant increase in the use of retrograde or orthograde techniques, but it may be that course attenders and more recently qualified anaesthetists will use them more frequently if necessary. No significant differences in intended practice were found when responders were grouped according to board certification in Rosenblatt and colleagues’ study. This is in contrast with our finding that anaesthesia training starting after 1988 correlates with an increased use of the ASA algorithm and of most alternative techniques.
Overall in our study, Dutch anaesthetists, who commenced anaesthetic training after 1988, and those who attended the airway management course ‘Access to the Airway’ are significantly more likely to follow the ASA Difficult Airway Algorithm and to use a wider variety of adjunctive techniques for airway management. It is difficult to discern which relationship is the stronger. There is a correlation between the two groups though, as there are more course attenders amongst those who started their anaesthetic training after 1988. The causality of all these relations is difficult to elucidate.
Anaesthetists in The Netherlands are using a wide variety of airway management techniques and adherence to the ASA algorithm is encouraging. It will be interesting to perform a similar survey in the future and reassess the changing profile in anaesthetic airway management amongst Dutch physician anaesthetists. The impact of the attendance at a dedicated airway management course and changes in the national anaesthetic curriculum may become clear.
The support in data entry and statistical analysis by Judy Nijman (MEMIC, Centre of Data and Information Management, Universiteit van Maastricht) is greatly acknowledged, as is the statistical advice by Fred H. M. Nieman (KEMTA, Academisch Ziekenhuis Maastricht).
A short translation of the questionnaire.
- • In which year did you start anaesthetic training?
- • Do you practice in an academic hospital?
- • Did you attend the ‘Access to the Airway’ course?
- • Did you train airway management in another way? How?
- • How often do you try to assess the difficulty of intubation during your preoperative assessment?
- • What are your indicators of possibly difficult intubation?
- • Do you ever perform awake intubations?
- • Do you think an awake intubation is an ethical anaesthetic technique?
- • What are your indications for an awake intubation?
- • How often do you not manage to intubate with a conventional laryngoscope?
- • Do you/your Department have a protocol for difficult intubations? Which?
- • Do you simulate difficult intubations as a training tool?
- • Do you use/How often do you use the following techniques:
malleable soft metal introducers/stylets (which?)
non-metal stylets (which?)
laryngeal mask airway
flexible fiberoptic bronchoscope (since when; who usually performs it; which type of anaesthesia, which local anaesthetic, how applied; nasal/oral route?)
retrograde intubation (which technique?)
orthograde technique via the cricothyroid membrane (which technique/kit?)
jet ventilation (which route/technique?)
other technique (which?)
- • In case of a difficult intubation which technique is your first choice and which your second?
1 Practice guidelines for management of the difficult airway. A report by the American Society of Anesthesiologists Task Force on Management of the Difficult Airway. Anesthesiology
1993; 78: 597–602.
2 Williams MJ, Lockey AS, Culshaw MC. Improved trauma management with advanced trauma life support (ATLS) training. J Accid Emerg Med
1997; 14: 81–83.
3 Tunstall ME. Failed intubation drill. Anaesthesia
1976; 31: 850–850.
4 Nolan JP, Wilson ME. Orotracheal intubation in patients with potential cervical spine injuries. An indication for the gum elastic bougie. Anaesthesia
1993; 48: 630–633.
5 Brain AIJ. The laryngeal mask – a new concept in airway management. Br J Anaesth
1983; 55: 801–805.
6 Hung OR, Pytka S, Morris I, et al.
Clinical trial of a new lightwand device (Trachlight) to intubate the trachea. Anesthesiology
1995; 83: 509–514.
7 Bigenzahn W, Pesau B, Frass M. Emergency ventilation using the Combitube in cases of difficult intubation. Eur Arch Otorhinolaryngol
1991; 248: 129–131.
8 Ravussin P, Freeman J. A new transtracheal catheter for ventilation and resuscitation. Can Anaesth Soc J
1985; 32: 60–64.
9 Joo H, Rose K. Fastrach – a new intubating laryngeal mask airway: successful use in patients with difficult airways. Can J Anaesth
1998; 45: 253–256.
10 McCoy EP, Mirakhur RK. The levering laryngoscope. Anaesthesia
1993; 48: 516–519.
11 Caplan RA, Posner KL, Ward RJ, Cheney FW. Adverse respiratory events: a closed claims analysis. Anesthesiology
1990; 72: 828–833.
12 Cheney FW, Posner KL, Caplan RA. Adverse respiratory events infrequently leading to malpractice suits. A closed claims analysis. Anesthesiology
1991; 75: 932–939.
13 Benumof JL. Management of the difficult adult airway. With special emphasis on awake tracheal intubation. Anesthesiology
1991; 75: 1087–1110.
14 Ovassapian A. Fiberoptic Airway Endoscopy in Anesthesia and Critical Care
. New York: Raven Press, 1990.
15 Baraka A, Muallem M, Sibai AN. Facilitation of difficult tracheal intubation by the fiberoptic Bullard laryngoscope. Middle East J Anesthesiol
1991; 11: 73–77.
16 Rosenblatt WH, Wagner PJ, Ovassapian A, Kain ZN. Practice patterns in managing the difficult airway by anesthesiologists in the United States. Anesth Analg
1998; 87: 153–157.
17 Benumof JL. Airway Management; Principles and Practice
. St. Louis: Mosby-Year Book, 1996.
18 King TA, Adams AP. Failed tracheal intubation. Br J Anaesth
1990; 65: 400–414.
19 Hawthorne L, Wilson R, Lyons G, Dresner M. Failed intubation revisited: 17-yr experience in a teaching maternity unit. Br J Anaesth
1996; 76: 680–684.
20 Samsoon GL, Young JR. Difficult tracheal intubation: a retrospective study. Anaesthesia
1987; 42: 487–490.
21 Ratnayake B, Langford RM. A survey of emergency airway management in the United Kingdom. Anaesthesia
1996; 51: 908–911.
22 Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia
1984; 39: 1105–1111.