The intubating laryngeal mask airway with and without fiberoptic guidance

We conducted this feasibility study using the intubating laryngeal mask airway (ILMA) and a polyvinyl chloride tracheal tube to compare success rates, hemodynamic effects, and postoperative morbidity with two methods of tracheal intubation. After ethics approval and informed consent, 90 healthy ASA physical status I or II women with normal airways were enrolled in the randomized, controlled study. After a standardized inhaled anesthesia induction protocol, tracheal intubations using ILMA with fiberoptic guidance (ILMA-FOB) and ILMA inserted blindly without fiberoptic guidance (ILMA-Blind) were compared with the control group of direct laryngoscopy (laryngoscopy group). All 90 patients were successfully ventilated. For tracheal intubation, success rates were equal in all three groups (97%). Total intubation times were longer for the ILMA-FOB group (77 s versus 48.5 s for laryngoscopy and 53.5 s for ILMA-Blind). The laryngoscopy group had a larger increase in mean arterial blood pressure to tracheal intubation. There were no differences in postoperative sore throat or hoarseness among the groups. In conclusion, success rates are equally high for tracheal intubation using ILMA-Blind and ILMA-FOB techniques in women with normal airways. IMPLICATIONS: The intubating laryngeal mask airway (ILMA) can be used as a primary airway for oxygenation and ventilation. Both methods of tracheal intubation using the ILMA were equally successful. Postoperative morbidity in the ILMA groups was similar to that in the laryngoscopy group. For women with normal airways, both the ILMA inserted blindly and the ILMA with fiberoptic guidance are suitable alternatives to laryngoscopy for tracheal intubation.     

Tracheal intubation for two patients with lingual tonsil hyperplasia and prospect of difficult airway management

Lingual tonsil hyperplasia is rare, but may cause difficult or inpossible tracheal intubation. We experienced two cases of tracheal intubation for lingual tonsil hyperplasia. A 71-yr-old man was scheduled for resection and biopsy of symptomatic hypertrophied lingual tonsils. In this patient, we performed oro-tracheal intubation by rigid laryngoscopy from left oral angle, because left hypertrophied lingual tonsils are smaller than those on the right side. A 44-yr-old man was scheduled for resection of symptomatic hypertrophied lingual tonsils after lingual tonsillitis. In this patient, we performed nasotracheal intubation using fiberoptic bronchoscopy with assist of jaw-lift and tongue-extension. When an anesthesiologist can predict the abnormality of lingual tonsils, these methods might be recommended for difficult airway and intubation. However, it is necessary to prepare a difficult airway management set including laryngeal mask airway, intubating laryngeal mask airway, fiberoptic bronchos-copy and transcutaneous tracheotomy set. And most important is preliminary evaluation of airway and cautious planning of tracheal intubation.     

The airway: problems and predictions in 18,500 patients

The purpose of this study was to describe methods, risk factors, and outcomes of airway management in all patients (obstetrics excluded) attended by anaesthetists over 27 months. Preoperatively, anaesthetists recorded patient factors and assessed four airway characteristics. Methods of tracheal intubation and ease of direct laryngoscopy following general anaesthesia (easy, awkward, difficult) were noted. Factors predictive of poor outcome and the value of the preoperative airway examination were determined. For 18,205 patients following a direct laryngoscopy, (GA), tracheal intubation was difficult (> 2 laryngoscopies) in 1.8% and awkward (≤2 laryngoscopies) in 2.5%. This approach was a failure in 0.3%, and surgery was postponed in 0.05%. However, an alternative approach to direct laryngoscopy, (GA) was the first choice in 353 patients. Risk factors for difficult tracheal intubation included male sex, age 40–59 yr and obesity (P≤0.01). For direct laryngoscopy, (GA), airway characteristics predictive of difficult tracheal intubation were decreased mouth opening (relative risk 10.3), shortened thyromental distance (9.7), poor visualization of the hypopharynx (4.5), and limited neck extension (3.2), any two (7.6) and more than two (9.4) (P< 0.01). For 1,856 patients (10.0%) where at least one airway characteristic was abnormal, a direct laryngoscopy, (GA) resulted in 8.3% awkward and 6.0% difficult tracheal intubations. For patients with no abnormal airway characteristics, tracheal intubation was easy in 96.3%. Where tracheal intubation was difficult, 34.3% of patients had one or more abnormal airway characteristics preoperatively. Patients with difficult tracheal intubation had an increased rate of desaturation (< 90%), hypertension (> 200 mmHg) and dental damage on induction of anaesthesia. It is concluded that difficult tracheal intubations occurred infrequently but were associated with increased morbidity. Patient factors and four physical airway characteristics were useful predictors but limited in identifying all problems.     

Evaluation of tracheal intubation difficulty in patients with cervical spine immobilization: fiberoptic (WuScope) versus conventional laryngoscopy.

BACKGROUND: The WuScope is a rigid, fiberoptic laryngoscope designed to facilitate tracheal intubation without the need for head extension. The study evaluated the WuScope in anesthetized patients with neck immobilization. METHODS: Patients were randomized to one of two groups: those receiving fiberoptic laryngoscopy (WuScope, n = 43) and those receiving conventional laryngoscopy (Macintosh blade, n = 44). Manual in-line stablization of the cervical spine was done during intubation. Seven parameters of intubation difficulty were measured (providing an intubation difficulty scale score): number of operators, number of attempts, number of techniques, Cormack view, lifting force, laryngeal pressure, and vocal cord position. RESULTS: Successful intubation occurred in 95% of patients in the fiberoptic group and in 93% of patients in the conventional group. There were no differences in number of attempts. In the fiberoptic group, 79% of patients had an intubation difficulty scale score of 0, representing an ideal intubation: that is, one performed by the first operator on the first attempt using the first technique with full glottic visualization. Only 18% of patients in the conventional group had an intubation difficulty scale score of 0 (P < 0.001). More patients had Cormack grade 3 or 4 views with conventional than with fiberoptic laryngoscopy (39 vs. 2%, P < 0.001). Intubation times in patients with one attempt were slightly longer in the fiberoptic (median, 25th-75th percentiles: 30, 23-53 s) compared with the conventional group (24, 17-30 s, P < 0.05). Corresponding times in patients requiring > one attempt were 155 (range, 112-201) s and 141 (range, 95-186) s in the fiberoptic and conventional groups, respectively (P value not significant). CONCLUSIONS: Compared with conventional laryngoscopy, tracheal intubation using the fiberoptic laryngoscope was associated with lower intubation difficulty scale scores and better views of the laryngeal aperture in patients with cervical imnmobilization. However, there were no differences in success rates or number of intubation attempts.     

Le glottiscope AWS-Pentax™, alternative à la fibroscopie dans deux cas d’intubation difficile prévue du brûlé grave

Intubating patients with facial burn is difficult to most anesthesiologists. Awake flexible fiberoptic intubation is the gold standard for management of anticipated difficult tracheal intubation. However, serious facial burn and dysmorphic syndrome can make fiberoptic intubation more difficult or impossible. We report the use of awake oral intubation using the Pentax-Airway Scope (AWS) in two major burn patients with facial injury, in whom awake fiberoptic intubation was impossible. As shown in morbidly obese patient and in patients with unstable necks, AWS could be useful to facilitate tracheal intubation in awake, facial burn patients presenting with a potentially difficult airway. Awake AWS intubation seems as a potential alternative to awake fiberoptic intubation.     

Morbid obesity and tracheal intubation

The tracheas of obese patients may be more difficult to intubate than those of normal-weight patients. We studied 100 morbidly obese patients (body mass index >40 kg/m(2)) to identify which factors complicate direct laryngoscopy and tracheal intubation. Preoperative measurements (height, weight, neck circumference, width of mouth opening, sternomental distance, and thyromental distance) and Mallampati score were recorded. The view during direct laryngoscopy was graded, and the number of attempts at tracheal intubation was recorded. Neither absolute obesity nor body mass index was associated with intubation difficulties. Large neck circumference and high Mallampati score were the only predictors of potential intubation problems. Because in all but one patient the trachea was intubated successfully by direct laryngoscopy, the neck circumference that requires an intervention such as fiberoptic bronchoscopy to establish an airway remains unknown. We conclude that obesity alone is not predictive of tracheal intubation difficulties. IMPLICATIONS: In 100 morbidly obese patients, neither obesity nor body mass index predicted problems with tracheal intubation. However, a high Mallampati score (greater-than-or-equal to 3) and large neck circumference may increase the potential for difficult laryngoscopy and intubation.     

Validation of a simple algorithm for tracheal intubation: daily practice is the key to success in emergencies--an analysis of 13,248 intubations

A fundamental skill of the anesthesiologist is airway management. We validated a simple endotracheal intubation algorithm with a large proportion of fiberoptic tracheal intubations used for years in daily practice. Over 2 yr, 13,248 intubations (>90% of all intubations, including obstetrics and ear, nose, and throat patients) in a heterogeneous patient population at our acute care hospital were evaluated prospectively. About 80 physician and nurse anesthetists were involved. Once the indication for intubation (oral or nasal) was established, the first step was to choose between the primary conventional technique (laryngoscope with Macintosh blades) and the primary fiberoptic technique. For the conventional technique, a well defined procedure had to be followed (maximum of two attempts at intubation; if unsuccessful, switch to secondary oral fiberoptic intubation). For the primary fiberoptic technique, the anesthesiologist had to decide between nasotracheal intubation in awake patients and oral intubation in anesthetized patients. Fiberoptics were used for 13.5% of the intubations. By following our algorithm, intubation failed in 6 out of 13,248 cases (0.045%; 95% confidence interval 0.02%-0.11%). We demonstrate that a simple algorithm for endotracheal intubation, basically limited to fiberoptics as the only aid, is successful in daily practice. Only methods that are practiced daily can be used successfully in emergencies.     

Management of difficult pediatric airway

Anesthesiologists should be familiar with the management of airway and be able to recognize and identify potential difficult airway. These entities include congenital craniofacial deformities with micrognathia (e.g. Robin sequence, Treacher Collins, Goldenhar's, Crouzon's syndromes) and metabolic diseases causing the deposit of accumulated by-products (e.g., Hurler's, Morquio's, Beckwith-Wiedemann syndromes). Cormack and Lehane grades 3 and 4 at laryngoscopy are an indication for advanced techniques for intubation. The laryngeal mask airway (LMA) and fiberscope with a directable tip are useful and important modalities in handling difficult pediatric airway and intubation. LMA not only offers another mode of securing airway besides face mask and tracheal intubation, but also provides a conduit for tracheal intubation and a rescue airway in the CICV (cannot intubate, cannot ventilate) situations. Intubation with a fiberscope can be utilized through LMA or through a specially designed face mask. Face mask designed for fiberoptic intubation has a 15 mm port for connection with the breathing circuit and another 22 mm port covered with a rubber membrane through which the fiberscope is introduced and directed to the larynx and trachea followed by the tracheal tube while ventilating and anesthetizing the pediatric patients with inhalational anesthetics. Getting used to these two modalities, LMA and fiberoptic intubation of the trachea, gives a great advantage in handling of difficult pediatric airway and intubation.     

创伤患者弹性插管探条引导下紧急逆行气管内插管一例

背景：保证严重颌面部外伤患者的气道安全是有一定难度的。通常情况下,使用喉镜或纤维支气管镜气管插管失败后,可通过紧急外科手术建立气道。但当外科手术仍然不能建立气道时,麻醉医师就必须采用一些新的方法来进行气道管理。但是,现有的文献很少对这些技术进行描述。方法1例创伤后颌面部变形的患者,在使用直接喉镜气管插管失败,环甲膜切开和气管切开同样失败之后,从气管破口处向头侧逆行插入弹性插管探条,使其从患者口腔内伸出。结果运用改良的逆行插管技术,从气管破口处使用弹性插管探条成功完成了插管。结论当通过外科手术仍然无法控制气道时,如果气管存在破损,则可以考虑使用弹性插管探条进行逆行插管。     

Evaluation of Tracheal Intubation Difficulty in Patients with Cervical Spine Immobilization: Fiberoptic (WuScope) Versus Conventional Laryngoscopy

Background: The WuScope is a rigid, fiberoptic laryngoscope designed to facilitate tracheal intubation without the need for head extension. The study evaluated the WuScope in anesthetized patients with neck immobilization.

Methods: Patients were randomized to one of two groups: those receiving fiberoptic laryngoscopy (WuScope, n = 43) and those receiving conventional laryngoscopy (Macintosh blade, n = 44). Manual in-line stablization of the cervical spine was done during intubation. Seven parameters of intubation difficulty were measured (providing an intubation difficulty scale score): number of operators, number of attempts, number of techniques, Cormack view, lifting force, laryngeal pressure, and vocal cord position.

Results: Successful intubation occurred in 95% of patients in the fiberoptic group and in 93% of patients in the conventional group. There were no differences in number of attempts. In the fiberoptic group, 79% of patients had an intubation difficulty scale score of 0, representing an ideal intubation: that is, one performed by the first operator on the first attempt using the first technique with full glottic visualization. Only 18% of patients in the conventional group had an intubation difficulty scale score of 0 (P < 0.001). More patients had Cormack grade 3 or 4 views with conventional than with fiberoptic laryngoscopy (39 vs. 2%, P < 0.001). Intubation times in patients with one attempt were slightly longer in the fiberoptic (median, 25th--75th percentiles: 30, 23-53 s) compared with the conventional group (24, 17-30 s, P < 0.05). Corresponding times in patients requiring > one attempt were 155 (range, 112-201) s and 141 (range, 95-186) s in the fiberoptic and conventional groups, respectively (P value not significant).     

Clinical competence in the performance of fiberoptic laryngoscopy and endotracheal intubation: a study of resident instruction

Anesthesiologists must be competent in the technique of fiberoptic laryngoscopy and intubation in airway management. The goal of this study was to test the hypothesis that an acceptable level of technical expertise in fiberoptic laryngoscopy and intubation may be acquired within 10 intubations while maintaining patient safety. The learning objectives were an intubation time of 2 minutes or less and greater than 90% success on the first intubation attempt. Ninety-one ASA physical status I-II patients with normal laryngeal anatomy had general anesthesia and were intubated orally with an Olympus LF-1 fiberoptic scope; the mean (+/- SD) time for intubation was 1.92 +/- 1.45 minutes. Four residents with no prior experience with fiberoptic laryngoscopy intubated at least 15 patients each. A learning curve was generated using logarithmic analysis of the mean (+/- SD) time for intubation of patients 1 to 15 for all residents combined. The curve showed that the mean (+/- SD) intubation time decreased from 4.00 +/- 2.91 to 1.53 +/- 0.76 minutes within the first 10 intubations. After the tenth intubation, the mean time was 1.53 minutes and the percent success on the first attempt at intubation was greater than 95%. There were no clinically important changes in O2 saturation, mean arterial pressure (MAP), or heart rate (HR) as a consequence of fiberoptic intubation. The results suggest that an acceptable level of technical expertise in fiberoptic intubation can be obtained (as defined by the learning objectives) by the tenth intubation, and patient safety is maintained.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of clonidine premedication upon hemodynamic changes associated with laryngoscopy and tracheal intubation

The authors studied 30 patients undergoing general anesthesia in order to evaluate whether oral clonidine premedication could attenuate the hemodynamic changes associated with laryngoscopy and tracheal intubation. Patients were randomly assigned to one of two groups; clonidine group (n = 15) who received oral clonidine of approximately 5 micrograms.kg-1, or control group (n = 15) who received no clonidine. The magnitude of increases in mean blood pressure from baseline values following laryngoscopy and tracheal intubation in the clonidine group was significantly smaller as compared with that in the control group (20 +/- 12 vs. 31 +/- 14 mmHg, mean +/- SD, P less than 0.05). There was also a significant difference between the two groups in the incidence of systolic blood pressure increases above 180 mmHg following laryngoscopy and tracheal intubation (0% vs. 26%, P less than 0.05). However, no significant difference was noted between the two groups in the heart rate responses to laryngoscopy and tracheal intubation. It is concluded that oral clonidine of 5 micrograms.kg-1 as a preanesthetic medication could attenuate the pressor responses associated with laryngoscopy and tracheal intubation.     

Endotracheal intubation with a lighted stylet in a patient with difficult airway from the first and second brancheal arch syndrome]

A 28 year-old-woman with the first and second brancheal arch syndrome was scheduled for the lift of the inferior part of the right ear. Difficult intubation was expected because of the mandibular hypoplasia. We chose a lightwand stylet for tracheal intubation. Anesthesia was induced with sevoflurane slowly increased to 5% in nitrous oxide 3 l.min-1 with oxygen 3 l.min-1. A spiral tube with 6.5 mm inner diameter (Safety-Flex, Mallinckrodt Medical, Ireland) was attached to the lightwand stylet (Surch-Light, Aaron, U.S.A.) and shaped to fit to pharyngo-laryngeal curve. Under spontaneous breathing, the tube was successfully inserted without laryngoscopy. No traumatic events occurred. Usually fiberoptic laryngoscopy requires more skill, more expensive equipment, and more time to prepare than the lightwand stylet technique. Moreover profuse secretions or blood in the oropharynx sometimes inhibit clear vision by fiberoptic laryngoscopy. The lightwand stylet is simple and inexpensive and it is useful for tracheal intubation in patients with difficult airway from the first and second brancheal arch syndrome.     

Fiberoptic nasal intubation in a patient with huge laryngeal cyst

Huge laryngeal cyst is rare, but may cause difficulty or inability in tracheal intubation during induction of general anesthesia. A 69-year-old patient was scheduled for laryngomicroscopic cystectomy. In this patient, we examined two methods of oro-tracheal intubation either with rigid laryngoscopy or flexible fiberscopy using transnasal fiberoptic monitoring. Direct laryngoscopy failed to expose the epiglottis because of large cyst being fragile and easy to bleed. And even oral fiberscopy intubation was also difficult since a large mass hindered acquiring a suitable view. However, trans-nasal fiberscopy monitoring could guide the oro-tracheal fiber into the trachea for intubation. When an anesthesiologist can predict the abnormality of epiglottis, this combination might be recommended for difficult airway and intubation. Postoperative respiratory management under intubating state was necessary because of bleeding, airway edema, and deviation of the larynx after tumor resection. We reported anesthetic management of a patient with epiglottis gigantic cyst occupying the laryngopharyngeal airway. It is a rare tumor leading to difficulty of induction of anesthesia and necessitating postoperative intubated respiratory care.     

Lightwand intubation using the Trachlight™: a brief review of current knowledge

PURPOSE: About 1% to 3% of laryngoscopic intubations can be difficult or impossible. Light-guided intubation has been proven to be an effective, safe, and simple technique. This article reviews current knowledge about the newer version lightwand: the Trachlight (TL). SOURCE: To determine its clinical utility and limitations, we reviewed the current literature (book and journal articles) on the TL since its introduction in 1995. PRINCIPAL FINDINGS: TL has been shown to be useful both in oral and nasal intubation for patients with difficult airways. It may also be useful in "emergency" situations or when direct laryngoscopy or fiberoptic endoscopy is not effective, such as with patients who have copious secretions or blood in the oropharynx. TL can also be used for tracheal intubation in conjunction with other devices (laryngeal mask airway -LMA-, intubating LMA, direct laryngoscopy). However, TL should be avoided in patients with tumours, infections, trauma or foreign bodies in the upper airway. CONCLUSIONS: Based on the clinical reports available, the TL has proven to be a useful option for tracheal intubation. In addition, the device can also be used together with other intubating devices, such as the intubating LMA and the laryngoscope, to improve intubating success rates. A clear understanding of the principle of transillumination of the TL, and an appreciation of its indications, contraindications, and limitations, will improve the effectiveness of the device as well as reducing the likelihood of complications. Finally, regular practice with the TL with routine surgical patients requiring tracheal intubation will further improve intubation success rates.     

Hemodynamic and catecholamine response to tracheal intubation: direct laryngoscopy compared with fiberoptic intubation

STUDY OBJECTIVE: To compare the stress response following tracheal intubation using direct laryngoscopy to that using fiberoptic bronchoscopy technique. DESIGN: Randomized, prospective study. SETTING: Operating rooms in a teaching hospital. PATIENTS: 51 ASA physical status I and II patients who were scheduled for an elective surgery with general anesthesia. INTERVENTIONS: Patients were randomly assigned to receive either direct laryngoscopy or fiberoptic orotracheal intubation, as part of general anesthesia. A uniform protocol of anesthetic medications was used. MEASUREMENTS: Blood pressure and heart rate were measured before induction, before endotracheal intubation, and 1, 2, 3, and 5 minutes afterwards. Catecholamine (epinephrine and norepinephrine) blood samples were drawn before the induction, and 1 and 5 minutes after intubation. MAIN RESULTS: Duration of intubation was shorter in the direct laryngoscopy group (16.9 (16.9 +/- 7.0 sec, range 8 to 40) compared with the fiberoptic intubation group (55.0 +/- 22.5 sec, range 29 to 120), p < 0.0,001. In both groups, blood pressure and heart rate were significantly increased at 1, 2, and 3 minutes after intubation, but there was no significant difference between the two study groups. Catecholamine levels did not increase after intubation and did not correlate with the hemodynamic changes. CONCLUSIONS: The use of either direct laryngoscopy or fiberoptic bronchoscopy produces a comparable stress response to tracheal intubation. Catecholamine levels do not correlate with the hemodynamic changes.     

Use of the bougie in simulated difficult intubation. 1. Comparison of the single-use bougie with the fibrescope

We studied the success rates for tracheal intubation in 64 healthy patients during simulated grade III laryngoscopy after induction of anaesthesia, using either the single-use bougie or oral flexible intubating fibrescope, both in conjunction with conventional Macintosh laryngoscopy. Patients were randomly allocated to either simulated grade IIIa or grade IIIb laryngoscopy, and also to one of the two study devices. Success rates for tracheal intubation (primary outcome measure) and times taken to achieve intubation (secondary outcome measure) were recorded. For the simulated grade IIIa laryngoscopy group, the fibreoptic scope was more successful than the bougie (16/16 successful intubations vs. 8/16; p = 0.02). For the simulated grade IIIb laryngoscopy group, the fibreoptic scope was also more successful than the bougie (8/16 successful intubations vs. 1/16; p = 0.02), but clearly use of the fibreoptic scope was not as successful as it had been in simulated grade IIIa laryngoscopy (p = 0.04). With either device, median (range) total tracheal intubation times for successful attempts with either grade of laryngoscopy were less than 60 s (19-109) and there were no clinically important differences. We conclude that the fibrescope used in conjunction with Macintosh laryngoscopy is a more reliable method of tracheal intubation than the single-use bougie in both types of grade III laryngoscopy. This finding has implications for the management of patients in whom grade III laryngoscopy is encountered unexpectedly after induction of anaesthesia, and also for the management of patients previously known to have grade III view at laryngoscopy.     

Tracheal intubation using the Bonfils intubation fibrescope after failed direct laryngoscopy

Failed tracheal intubation due to a difficult airway is an important cause of anaesthetic morbidity and mortality. This study was undertaken to evaluate the effectiveness of the Bonfils intubation fibrescope for tracheal intubation after failed direct laryngoscopy. Twenty-five patients undergoing coronary artery bypass grafting were enrolled in the study after two attempts at conventional laryngoscopy by a board certified anaesthetist had failed. Intubation with the Bonfils fibrescope was successful on the first attempt in 22 patients (88%) and on the first or second attempt in 24 patients (96%); in one patient intubation was impossible. Median (IQR [range]) time to intubation using the Bonfils intubation fibrescope was 47.5 (30-80 [20-200]) s. Tracheal intubation using the Bonfils intubation fibrescope appears to be a simple and effective technique for the management of a difficult intubation.     

Subcutaneous emphysema and pneumomediastinum after translaryngeal intubation: tracheal perforation due to unsuccessful fiberoptic tracheal intubation

A 77-year-old man was scheduled to undergo a cervical lymph node biopsy under general anesthesia. Although awake, nasotracheal fiberoptic intubation was initially planned because of an anticipated difficult airway, the attempt was unsuccessful. Orotracheal intubation was subsequently performed under direct laryngoscopy without difficulty. After initiating positive pressure mechanical ventilation, subcutaneous and mediastinal emphysema developed. The cause of this emphysema was considered to be tracheal perforation after an unsuccessful attempt at fiberoptic tracheal intubation.     

The haemodynamic response to the insertion of the laryngeal mask airway: a comparison with laryngoscopy and tracheal intubation

The haemodynamic response to the insertion of the laryngeal mask airway (LMA) was assessed and compared to that of laryngoscopy and tracheal intubation in a study of forty patients (ASA 1) randomly allocated into two groups and anaesthetised using a standard balanced anaesthetic technique. The results show that the changes in all cardiovascular parameters measured following LMA insertion were significantly less (P<0.05) when compared with those following laryngoscopy and tracheal intubation. We conclude that airway management with the LMA may be used to avoid the haemodynamic response to tracheal intubation where such a response is undersirable.