Prediction Of Difficult Intubation Research Articles

Prediction of difficult intubation in apparently normal patients by combining modified mallampati test and thyromental distance: A prospective observational study

Introduction: Difficult intubation, often unexpected, remains a primary concern for the anesthesiologist. None of the bedside airway assessment tests have proven to be efficacious and highly predictive. This study was done to determine the sensitivity and specificity of Modified Mallampati test alone, Thyromental distance alone and in combination of both for predicting difficult intubation. Material and Methods: A prospective observational study was conducted in Universal College of Medical Sciences Teaching Hospital (UCMSTH), which involved 80 American Society of Anesthesiologist Physical Status (ASA PS) I patients undergoing elective surgeries under general anesthesia with endotracheal intubation. All patients were included for preoperative assessment by Modified Mallampati test alone and Thyromental distance measurement. The correlation between these tests and Cormack and Lehane laryngoscopic grading was done to find out the result. Results: Prediction of difficult intubation among 80 patients by Modified Mallampati test alone showed sensitivity of 72.7% and specificity of 98.6% whereas with Thyromental distance alone, showed sensitivity of 36.4% and specificity of 100%. When both predictors were combined, result showed sensitivity of 72.7% and specificity of 98.6% which was similar to that of Modified Mallampati test alone. Conclusion: This study concluded that sensitivity to predict difficult intubation was higher with Modified Mallampati test alone or in combination with Thyromental distance test whereas specificity was highest with Thyromental distance test alone.

Comparison of palm print test with other difficult intubation prediction tests in patients with diabetes mellitus

Prediction of difficult intubation has critical importance in anesthesia practice. Diabetic patients usually have limited joint mobility which limits the head and neck extension during laryngoscopy. On the day prior to surgery, the patients who had diagnosis of diabetes mellitus at least for one year, were examined by an investigator by using palm print test (PPT) and other standard assessment indices. The investigator assigned a score to each evaluation indices. PPT was found to have better sensitivity (0.944), spesivity (0.692), positive predictive value (0.895) and negative predictive value (0.818) than standard assessment tests (p < 0.01). PPT should be kept in mind as a viable and easy-to-perform test for predicting difficult intubation of patients with Diabetes Mellitus.

Prediction of Difficult Intubation with the Modified Mallampati and Upper Lip Bite Test: A Cross-Sectional Analysis

Difficult intubation is a term used when insertion of the endotracheal tube with conventional laryngoscopy technique requires more than 10 minutes or greater than three attempts. This study sought to determine the predictive value of two commonly used tests; Mallampati test vs. Upper Lip Bite Test (ULBT). A cross-sectional study conducted on 196 patients aged 18-60 years in lady reading hospital Peshawar presented for elective surgeries planned for general anaesthesia and requiring endotracheal intubation were included during March 2018 to May 2018. Both tests (Mallampati and Upper lip bite test) were performed on each sample. In the total 196 participant 180 (91.8%) were found easy cases and 16 (8.2%) were difficult cases. Out of 16 (8.2%) difficult cases 5 (31.3%) difficult cases were underclass III Upper Lip Bite Test (ULBT) which are true positive. From Modified Mallampati Test 8 (50%) were attributed to MMT class III which is 21.1% of total class III and 3 cases (18.8% of total difficult cases) with MMT class IV which is 75% of MMT class IV are both true positive. During anaesthesia and intubation, most common cause of death and brain damage is encounter because of difficult intubation. In this study, we have performed two tests i.e. ULBT and MMT on total of 196 patients, out of which 16 (8.2%) were found difficult intubation which is in the range (0.5- 17.5%) mention in the previous studies. The study has shown most of the difficult were predicted incorrectly. Most cases resulted in difficult intubation, which were predicted easily by two tests (Criteria set by Hoda et al.), so to only rely on ULBT and MMT for predictive measure is not standard.

Development of a scoring system for predicting difficult intubation using ultrasonography.

Background and Aims: Ultrasound measurement of anterior neck soft tissue thickness by skin to epiglottis distance (SED) has been shown to predict difficult laryngoscopy. In this study, we developed an airway scoring system incorporating SED into three clinical predictors and assessed whether it would improve accuracy in prediction of difficult intubation.Methods:Mentohyoid distance, mandibular subluxation, head extension and ultrasound measurement of skin to epiglottis distance were measured a day before surgery in 310 adult patients. During direct laryngoscopy, Cormack–Lehane grading was noted (Grade 1 and 2 = Easy, Grade 3 and 4 = Difficult). We constructed a score named MSH, which included mentohyoid distance, mandibular subluxation and head extension. Then, SED was added to the MSH score to form another new score named USED-MSH. Student's t-test, Mann-Whitney U test and Chi-square test or Fisher exact tests were used. Both scoring systems were compared under the receiver-operating characteristic curve and area under the curve (AUC) were calculated.Results:Difficult intubation was observed in 62/310 patients (20%). The AUC for USED-MSH score was greater than the MSH score (0.93, 95% CI [0.89–0.97] vs 0.76, 95% CI [0.69–0.84], P value <0.001). USED-MSH score had higher sensitivity (93.6% vs 59.7%) and lower specificity (85.9% vs 91.1%) with similar positive predictive value (62.7% vs 62.4%) in comparison with MSH score.Conclusion: An airway scoring system using the ultrasound measurements of skin-to-epiglottis distance along with the clinical predictors would be helpful in the prediction of difficult intubation.

Diagnostic Value of Upper Lip Bite Test combined with Hyomental Distance Ratio in assessing the airway

INTRODUCTION Airway assessment before any type of anesthesia is mandatory. Upper lip bite test (ULBT) is a relatively new test for assessing the airway. Range of freedom of mandible and dental architecture has a significant role in facilitating intubation, both of which are assessed by this single test. ULBT with good neck movements are sufficient criteria for assessing the airway. Hyomental distance ratio (HMDR) assesses the range of neck movements. So by combining these two tests we expect an increase in reliability and validity. METHODS Preoperatively we assessed the airway using ULBT, HMDR, and Mallampati (MP) tests of 300 patients. The anesthesiologist doing laryngoscopy is blinded to the results of pre op airway evaluation. Direct laryngoscopies are graded using the modified Cormack and Lehane scale. Difficult laryngoscopy was defined as Grade 3 or 4 view. ULBT class 3, and HMDR with optimal cutoff point 1.2 are considered as difficult intubation predictors. Sensitivity, Specificity, Positive Predictive Value (PPV), Negative Predictive Value (NPV) and Accuracy of both ULBT and HMDR separately and then both in combination are calculated and compared with those of MP. RESULTS ULBT:-Sensitivity - 88.2%, Specificity - 99.62%, PPV - 96.77%, NPV - 98.51%, Accuracy - 98.33% HMDR: - Sensitivity - 88.2%, Specificity – 99.25%, PPV - 93.75%, NPV - 98.50%, Accuracy - 98.00% MP: - Sensitivity - 17.65%, specificity - 88.72%, PPV - 16.67%, NPV - 89.39%, Accuracy - 80.66% ULBT+HMDR: - Sensitivity - 82.35%, specificity-100%, PPV100%, NPV97.75%, Accuracy - 98.00 % CONCLUSION Upper Lip Bite test is good screening test for assessing the airway. ULBT in combination with HMDR increased the specificity and positive predictive value.

Role of ultrasound in airway assessment in the respiratory ICUs

BackgroundAirway evaluation and its management remain an emerging clinical science. Ultrasound (US) provides point-of-care dynamic views of the airway in perioperative, emergency, and critical care settings. Identification of a difficult airway before intubation allows for optimal preparation, equipment selection, and participation of experienced personnel.ObjectiveThe aim of this study was to evaluate the role of US in the assessment of airways and to determine whether US has the potential to serve as an effective, noninvasive method for the diagnosis of tracheomalacia.Patients and methodsA prospective cross-sectional study was carried out on patients admitted at the respiratory ICU. US examination of the airways and diaphragm was performed together with either fiberoptic bronchoscopy (FOB) or dynamic expiratory computed tomography chest. Dynamic expiratory computed tomography chest and FOB were done within 24h of US examination.ResultsA total of 53 patients were included. US could successfully confirm endotracheal tube (ETT) placement in all patients. ETT was endotracheal in 30 (94%) patients, whereas it was esophageal in two (6%) patients. Hyomental distance at a cut-off of up to 4.51cm was a good predictor of difficult intubation with 100% sensitivity and 87.5% specificity. Subglottic airway transverse diameter was used as a predictor of ETT size. Patients with tracheomalacia by FOB had a significantly longer duration of mechanical ventilation. Lateral pharyngeal wall thickness was used as a predictor of obstructive sleep apnea, a new cut-off point was used at more than 4.1 cm in the intubated group of patients with 87.5% sensitivity and 95.8% specificity, whereas a cut-off point more than 4.2 cm in the nonintubated patients had 100% sensitivity and 100% specificity. In the intubated group, out of the seven cases diagnosed with tracheomalacia by FOB, five patients were missed by US with 40% sensitivity, whereas in the nonintubated group, the results were significantly better, where only one case was missed by US with 80% sensitivity.ConclusionUS has many advantages for imaging the airway; it is safe, quick, repeatable, portable, widely available, and provides real-time dynamic images relevant for several aspects of management of the airway. Thus, it seems reasonable to consider the routine use of airway US in the ICU.

A clinical prediction rule to identify difficult intubation in children with Robin sequence requiring mandibular distraction osteogenesis based on craniofacial CT measures

BackgroundAirway management is challenging in children with Robin sequence (RS) requiring mandibular distraction osteogenesis (MDO). We derived and validated a prediction rule to identify difficult intubation before MDO for children with RS based on craniofacial computed tomography (CT) images.MethodThis was a retrospective study of 69 children with RS requiring MDO from November 2016 to June 2018. Multiple CT imaging parameters and baseline characteristic (sex, age, gestational age, body mass index [BMI]) were compared between children with normal and difficult intubation according to Cormack−Lehane classification. A clinical prediction rule was established to identify difficult intubation using group differences in CT parameters (eleven distances, six angles, one section cross-sectional area, and three segment volumes) and clinicodemographic characteristics. Predictive accuracy was evaluated by receiver operating characteristic (ROC) curve analysis.ResultsThe overall incidence of difficult intubation was 56.52%, and there was no significant difference in sex ratio, age, weight, height, BMI, or gestational age between groups. The distance between the root of the tongue and posterior pharyngeal wall was significantly shorter, the bilateral mandibular angle shallower, and the cross-sectional area at the epiglottis tip smaller in the difficult intubation group. A clinical prediction rule based on airway cross-sectional area at the tip of the epiglottis was established. Area > 36.97 mm2 predicted difficult intubation while area < 36.97 mm2 predicted normal intubation with 100% sensitivity, 62.5% specificity, 78.6% positive predictive value, and 100% negative predictive value (area under the ROC curve = 0.8125).ConclusionComputed tomography measures can objectively evaluate upper airway morphology in patients with RS for prediction of difficult intubation. If validated in a larger series, the measures identified could be incorporated into airway assessment tools to guide treatment decisions.This was a retrospective study and was granted permission to access and use these medical records by the ethics committee of Guangzhou Women and Children’s Medical Center.Trials registrationRegistration No. ChiCTR1800018252, NaZhang, Sept 7 2018.

Neural network prediction of difficult tracheal intubation risk by using the patient’s face image

Background. The prognosis of the difficult tracheal intubation remains an essential problem. The effectiveness of using predictors does not allow to foreseen such situation accurately.&#x0D; The purpose of the study was to develop a predictive system and evaluate its effectiveness in difficult tracheal intubation based on facial image analysis combined with the most significant predictors of difficult intubation.&#x0D; Materials and methods. A database based on the registration of difficult intubation predictors was developed. It was based on the patients face images with marked reference points. It allowed to estimate the information signs associated with the difficult tracheal intubation. The degree of intubation severity was determined directly during the intubation process according to the proposed original scale of severity.&#x0D; Results. The classifier was synthesized by using the self-organization neural network method. The trained neural network was the basis of the classifier model implemented as a computer application. The sensitivity of the difficult tracheal intubation prognosis was 90.90%, specificity was 97.02%, the prognostic value of the positive result was 58.82%, the negative one was 99.56%.&#x0D; Conclusions. The proposed decision support system allows patients to be stratified into groups according to the degree of difficult tracheal intubation risk. In addition, the self-learning process of the system continues as the new data become available. This allows to improve its efficiency continuously.

Predicting difficult intubation: A comparison between upper lip bite test (ULBT) and Modified Mallampati test (MMT)

Introduction: No anaesthesiologist like to face scenario of unanticipated difficult intubation, as it may cost patients life. Many tests are there to predict difficult intubation, amongst those tests Modified Mallampati test [MMT] is a gold standard test. Upper lip bite test [ULBT] is an acceptable option for predicting difficult intubation. Our study aimed to compare both the tests to predict difficult intubation. Objectives: To analyse positive predictive value (PPV), negative predictive value (NPV), sensitivity, specificity and accuracy of ULBT and MMT. To compare the results of both the tests to predict difficult intubation. Aim: To ascertain whether ULBT can be incorporated in standard protocol of airway assessment along with other tests to increase predictive accuracy of difficult endotracheal intubation Materials and Methods: It was prospective randomised comparative observational study carried out at single centre. Three hundred patients of either sex, aged between 16-60 yrs scheduled for elective surgery under general anaesthesia with endotracheal intubation were enrolled in the study. Preoperative evaluation of airway was done with ULBT and MMT and findings were documented.MMT class III,IV and ULBT class III were considered as predictors of difficult intubation. On the day of the surgery after direct laryngoscopy laryngeal view was noted and was classified according to Cormack and Lehane classification. Patients with Cormack Lehane class III,IV considered as difficult to intubate. Cormack Lehane classification (C &L class) redings were compared with ULBT and MMT. Observations and Results: Demographic data and ASA grade was same for both the groups as participants were same. By comparing ULBT with Cormack and Lehane score we got 88.46% sensitivity, 92.74% specificity, 71.87% Positive predictive value (PPV), 97.45% ne gative predictive value (NPV) and 92% accuracy. For MMT we go t 19.23% sensitivity, 91.93% specificity, 33.33% PPV, 84.44% NPV a nd 79.33% acc

Impact of a Critical Care Specialist Intervention on First Pass Success for Emergency Airway Management Outside the ICU.

There has been limited investigation into the procedural outcomes of patients undergoing emergent endotracheal intubation (EEI) by a critical care medicine (CCM) specialist outside the intensive care unit (ICU). We hypothesized that EEI outside an ICU would be associated with lower rates of first pass success (FPS) as compared to inside an ICU. We performed a retrospective cohort study of all adult patients admitted to our academic medical center between January 1, 2016, and July 31, 2018, who underwent EEI by a CCM practitioner. The primary outcome of FPS was identified in the EEI procedure note. Secondary outcomes included difficult intubation (> 2 attempts at laryngoscopy) and mortality following EEI. In total, 1958 patients (1035 [52.9%] inside ICU and 923 [47.1%]) outside an ICU) were included in the final cohort. Unadjusted rate of FPS was not different between patients intubated out of the ICU and patients intubated inside of the ICU (689 [74.7%] vs 775 [74.9%]; P = .91). There was also no difference in FPS between groups after adjusting for predictors of difficult intubation and baseline covariates (odds ratio: 0.95; 95% confidence interval, 0.75-1.2, P = .65). Mortality of patients undergoing EEI out of the ICU was higher at each examined time interval following EEI. For EEI done by CCM practitioners, rate of FPS is not different between patients undergoing EEI outside an ICU as compared to inside an ICU. Despite the lack of difference between rates of procedural success, patient mortality following EEI outside an ICU is higher than EEI inside an ICU at all examined time points during hospitalization.

Urgent intubation without neuromuscular blocking agents and the risk of tracheostomy.

Neuromuscular blocking agents play a significant role in improving the success rate for urgent intubation, although there is limited evidence about the effect on subsequent outcomes, such as the incidence of tracheostomy. In this retrospective cohort study, we aimed to examine the association between avoidance of neuromuscular blocking agents for urgent tracheal intubation and incidence of tracheostomy among patients in the intensive care unit (ICU). The setting of this study was an eight-bed ICU at a tertiary-care hospital in Okayama, Japan. We included patients who underwent urgent tracheal intubation at the emergency department or the ICU and were admitted to the ICU between April 2013 and November 2017. We extracted data on methods and medications of intubation, predictors for difficult intubation, Cormack–Lehane grade, patient demographics, primary diagnoses, reintubation. We estimated odds ratios and their 95% confidence intervals for elective tracheostomy during the ICU stay using logistic regression models. Of 411 patients, 46 patients underwent intubation without neuromuscular blocking agents and 61 patients underwent tracheostomy. After adjusting for potential confounders, patients who avoided neuromuscular blocking agents had more than double the odds of tracheostomy (odds ratio 2.59, 95% confidence interval 1.06–6.34, p value = 0.04). When stratifying the subjects by risk status for tracheostomy, the association was more pronounced in high-risk group, while we observed less significant association in the low-risk group. Avoidance of neuromuscular blocking agents for urgent intubation increases the risk of tracheostomy among emergency patients, especially those who have a higher risk for tracheostomy.Electronic supplementary materialThe online version of this article (10.1007/s11739-019-02214-0) contains supplementary material, which is available to authorized users.

Tracheal intubation with the McGrath MAC X-blade videolaryngoscope in morbidly obese and nonobese patients

Background/aimIncreased body mass index (BMI) and neck circumference are the two independent predictors of difficult intubation. McGrath MAC X-Blade is a videolaryngoscope specifically designed for difficult intubations.Materials and methodsEighty patients with the American Society of Anesthesiologists (ASA) physical status I–III undergoing elective surgery requiring endotracheal intubation were enrolled in the study. Patients were divided into two groups, nonobese (BMI < 30) and morbidly obese (BMI > 35). All patients were intubated with the McGrath MAC X-Blade in both groups. View optimization and tube insertion maneuvers such as reinsertion of the device, slight removal of the device, cricoid pressure, handling force, 90° anticlockwise rotation of the tube, use of stylet, and head flexion maneuvers were recorded. Cormack–Lehane grades, insertion times, intubation, and total intubation times were recorded. The hemodynamic changes and postoperative minor complications were also recorded.ResultsBody mass index, neck circumference, Mallampati scores, and ASA physical status were statistically higher in the morbidly obese group (P < 0.001 and P < 0.05). Sternomental distances were shorter in the morbidly obese (P < 0.05). Cormack–Lehane grades were comparable among the groups. The morbidly obese patients required more reinsertion attempts and cricoid pressure maneuvers during intubation than the nonobese patients (P = 0.019 versus P = 0.012, respectively). Slight removal of the device, handling force, use of the stylet, 90° anticlockwise rotation of the tube, and head flexion maneuvers were also helpful in both groups. Although device insertion times were similar between the groups, intubation and total intubation times were longer in the morbidly obese group (P = 0.009 and P = 0.034, respectively). The groups were comparable in hemodynamic changes and postoperative minor complications.ConclusionThe McGrath MAC X-Blade videolaryngoscope could safely be used both in nonobese (BMI < 30) and morbidly obese (BMI > 35) patients with the aid of some key maneuvers and with a statistically significant but clinically negligible prolongation of the intubation time.

A Study to Predict Difficult Intubation Using Simple Non-invasive test: A Prospective Observational Study

Background: Airway Management is prime importance to the anaesthesiologist of all the anaesthetic deaths 30 % to 40 % are attributed to the inability to manage a difficult airway. The various simple bedside predictors of difficult airway like Mallampati test, Upper lip bite test, Thyromental, Hyomental, Thyrosternal & Sternomental distances, mandibular length, Neck circumferences etc. are easy to perform with No extra cost or inconvenience to patients. Aim: To assess the predictability of Upper lip bite test, Hyomental distance, Thyromental distance, Thyrosternal distance & Mandibular length. Objective: 1) To find out the best predictor of difficult intubation amongst the Upper Lip Bite Test, Hyomental Distance, Thyromental Distance, Thyrosternal Distance & Mandibular Length on the basis of the test. Methods: This is a prospective observational study of 110 patients are randomly selected & age caring between 20 to 59 yrs both male and female are presented in anaesthesia of routine per anaesthetic check-up. After from mouth opening & mallampati test which are routinely done airway assessment included ULBT, HMD, TMD, TSD & ML will be done. Results: After analysing study state that the age (P = 0.031) were significant , & sex (P = 0.213) ASA (P = 0.091) were not significant with difficult intubation on the basis of Cormack lehane grading. All these factors were applicant on next set of 110 patients to found sensitivity & specificity to be ULBT ( SE = 50.00%, SP =100.00%), HMD (SE = 10.00%, SP = 97.00%), TSD (SE = 0.00%, SP = 97.00%), ML (SE = 20.00%, SP = 97.00%) & TMD (SE = 70.00%, SP = 85.00%) respectively. Conclusions: Thyromental distance is the most important sensitive factors predicting difficult intubation on the basis of cormack lehane grading in indian patients.

Factors Predicting Difficult Endotracheal Intubation.

Factors Predicting Difficult Endotracheal Intubation.

Airway assessment factors as a predictive marker of difficult direct laryngoscopy: A prospective study

Background and objectives: Preoperative evaluation of anatomical landmarks help identify potentially difficult laryngoscopies; however, predictive reliability is unclear. Thus, this study was undertaken to identify and compare the most reliable variables, in prediction of difficult direct laryngoscopy. Methodology: Pre-operative assessment of ten parameters using clinical and goniometric measurement were taken and consecutively predictors of difficult intubation were identified. On the day of surgery, after premedication and induction, laryngoscopy was performed. The glottic views were graded according to the Cormack and Lehane classification. Patients of Cormack Lehane class II B and above were considered as difficult to intubate.Results: 15.4% of the cases were identified as difficult intubation. Cormack and Lehane classification had the highest diagnostic accuracy followed by Thyromental distance, sternomental distance and Modified Mallampati classification in that order. Mandibular-hyoid distance had highest sensitivity. when all ten parameters were taken into consideration, 95.7% of cases were classified correctly. Also, it classified correctly 97.7% of easy and 84.2% difficult intubation. Conclusion: Apart from Cormack-Lehane grading, upon comparison of all the parameters, a combination of Thyromental distance, Atlanto-occipital joint extension and Modified Mallampati classification were able to correctly classify 95.3% of difficult intubation.

Neck circumference to thyromental distance ratio: Is a reliable predictor of difficult intubation in obese patients?

Neck circumference to thyromental distance ratio: Is a reliable predictor of difficult intubation in obese patients? - IJCA- Print ISSN No: - 2394-4781 Online ISSN No:- 2394-4994 Article DOI No:- 10.18231/2394-4994.2019.0029, Indian Journal of Clinical Anaesthesia-Indian J Clin Anaesth

Incidence, impact and indicators of difficult intubations in the neonatal intensive care unit: a report from the National Emergency Airway Registry for Neonates

ObjectiveTo determine the incidence, indicators and clinical impact of difficult tracheal intubations in the neonatal intensive care unit (NICU).DesignRetrospective review of prospectively collected data on intubations performed in the NICU...

A comparative study of effectiveness of video laryngoscopy and direct laryngoscopy in patients with at least three predictors of difficult intubation

A comparative study of effectiveness of video laryngoscopy and direct laryngoscopy in patients with at least three predictors of difficult intubation

Prediction of Difficult Intubation in Apparently Normal Patients by Combining Modified Mallampatti Test and Thyromental Distance

Prediction of Difficult Intubation in Apparently Normal Patients by Combining Modified Mallampatti Test and Thyromental Distance

A prospective observational study of predictors of difficult intubation in Indian patients.

Background and Aims:During routine preoperative assessment of patients one of the commonest practices is predicting difficulty of intubation. The present study was undertaken to evaluate parameters associated with difficult intubation and to test on new set of patients. At the end, to form simple predictive rule to decreased the number of false alarms.Material and Methods:In initial series of 483 Indian population patients we measured age, sex, weight, height, interincisor gap, mandibular length, neck movement, neck circumference, subluxation of mandible, sternocricoid distance, and identified factors associated with difficult intubation. These were applied on next 480 patients of prospective series and simple predictive rule in form of risk sum score was developed.Results:After analyzing initial series data we found that weight (P = 0.033), height (P = 0.034), interincisor gap (P = 0.005), subluxation (P < 0.001), neck movement (P < 0.001), and sternocricoid distance (P = 0.020) were significantly associated with difficult intubation. These six factors were applied on next set of 480 patients to found accuracy of predicting difficult intubation of weight (51.7%), height (83.8%), interincisor gap (80.2%), subluxation (77.7%), neck movement (82.7%), and sternocricoid distance (79.2). Total score greater than 2 predicted 92.8% of difficult laryngoscopies correctly as against 33.9% would be falsely labeled as difficult.Conclusion:Interincisor gap and sternocricoid distance are the two most sensitive factors predicting difficult intubation in Indian patients. However, risk sum score of more than 6 may lead to better anticipation of truly difficult intubations.