Morbid obesity is defined as a BMI of at least 40 kg m−2 or a BMI of at least 35 kg m−2 with concomitant obesity-related disease.1 An increasing number of morbidly obese patients undergo elective and emergency operations daily. Difficult airways, characterised by both difficult mask ventilation and difficult intubation, have become increasingly common among obese and morbidly obese patients.2–4
There has been conflicting evidence within the literature regarding the association between obesity and difficult tracheal intubation. Multiple studies have shown an association between obesity or morbid obesity and difficult intubation.5–7 A meta-analysis reported the odds of difficult intubation to be three times higher in morbidly obese patients than in patients of normal weight.4 Conversely, Neligan et al.8 and Brodsky et al.9 found no association between morbid obesity and difficult intubation. There is limited literature and conflicting evidence on the association between neck circumference and difficult intubation.6,9 Two studies have shown an association between neck circumference and difficult intubation. Neck circumference greater than 43 cm has been found to indicate an increased risk of difficult intubation,6 although Neligan et al.8 recently found no predictive association between neck circumference and difficult intubation.
In the literature, there are little data regarding the relationship between neck circumference and difficult mask ventilation. There is also conflicting evidence regarding the association between obesity and difficult mask ventilation.7,10–13
The primary objective of this study was to determine whether neck circumference and obesity were independent predictors of difficult tracheal intubation. The secondary objective was to determine whether neck circumference and obesity were independent predictors of difficult mask ventilation.
Materials and methods
Study approval (REB #11-0647-AE) was obtained from the University Health Network Research Ethics Board of the University Health Network, Canada (Co-chairperson Dr A. Gagliardi) on 1 May 2012 and informed consent was obtained from patients. Morbidly obese patients with a BMI of at least 40 kg m−2 undergoing elective surgery requiring tracheal intubation were enrolled. Patients with a cervical spine abnormality, emergency procedures, known difficult intubation or upper airway disease, and planned awake intubation were excluded from the study.
Patients were seen preoperatively by an anaesthesiologist or resident. A complete medical history was obtained, including sex, American Society of Anesthesiologists’ (ASA) physical status, history of snoring, obstructive sleep apnoea and use of continuous positive airway pressure (CPAP). Physical examination of patients included height, weight and the presence of a beard. BMI was calculated using the height and weight measurements. Waist circumference was measured in a horizontal plane at a point midway between the lowest rib margin and the iliac crest using a flexible measuring tape with centimetre markings. Similarly, hip circumference, defined as the maximum circumference over the buttocks, was measured. Airway parameters, including Mallampati score and mandibular protrusion, as well as mouth opening, thyromental distance, sternomental distance and hyomental distance measured using a ruler with cm markings, were recorded. Using a flexible measuring tape, neck circumference at the level of the thyroid cartilage was also measured.6,9 Preoperative investigations were as per the standard practice of the institution. One trained research assistant recorded all preoperative and intraoperative patient data.
Pre-induction, patients were positioned with a Troop elevation pillow (Trudell Medical Inc, London, Ontario, Canada) to align the suprasternal notch with the external auditory meatus to facilitate tracheal intubation. Patients were monitored using electrocardiogram, pulse oximetry and noninvasive arterial blood pressure measurement. Preoxygenation was carried out for a minimum of 3 min to achieve an end-tidal oxygen concentration more than 85%. Anaesthesia was induced using fentanyl 2 to 3 μg kg−1, midazolam 1 to 2 mg kg−1, a ‘sleep dose’ of propofol (2 to 3 mg kg−1), and rocuronium 0.6 mg kg−1 was administered to facilitate tracheal intubation and artificial ventilation of the lungs. Mask ventilation was attempted using an appropriately sized facemask as determined by the attending anaesthetist.
Mask ventilation was graded according to the method described by Han et al.14 Grade 0 was given when no ventilation was attempted. Grade 1 was defined as normal effective mask ventilation, Grade 2 as mask ventilation with an oral airway or other adjuvant use and Grade 3 as difficult, in which ventilation of the lungs was insufficient to maintain oxygenation, or mask ventilation was unstable, or mask ventilation required two providers. Finally, Grade 4 was defined as impossible intubation, after which the use of a supraglottic airway or other device was required to maintain ventilation and oxygenation. In the present study, difficult mask ventilation was defined as mask ventilation Grade 3 or Grade 4. Anaesthesiologists involved in each case had at least 1 year of clinical experience in airway management.
After adequate relaxation had been achieved, tracheal intubation was attempted using direct laryngoscopy with an appropriately sized Macintosh blade. If difficulties were encountered during intubation, the use of adjuncts or alternatives was permitted. An Intubation Difficultly Scale (IDS) was determined on the basis of seven variables:15 number of intubation attempts, additional operators, alternative intubation techniques, vocal cord position, Cormack and Lehane's classification for laryngoscopic view,16 need for external laryngeal pressure and the use of a lifting force with the laryngoscope. All seven variables were assessed and the IDS calculated. In this study, difficult intubation was defined as an IDS of at least 5; any value below 5 was considered easy.5,17 Once tracheal intubation had been completed successfully as evidenced by a proper capnograph waveform, further management of the patient and postanaesthesia care unit (PACU) care was at the discretion of the attending anaesthesiologist.
Univariate analyses were performed to determine clinical factors associated with difficult intubation. Unpaired t-test analysis was used for continuous variables, while χ2 analysis was used for categorical or binary variables. Univariate analyses were performed to determine clinical factors associated with the secondary outcome, difficult mask ventilation. A Bonferroni correction was applied to the univariate analyses.
For each continuous variable, a Receiver operating curve (ROC) analysis was performed to determine the cut-off point resulting in the best combination of sensitivity and specificity. Multiple logistic regression analysis was subsequently performed to determine independent predictors of difficult intubation. All variables with P value less than 0.10 from the univariate analyses and variables significantly associated with difficult intubation from the literature were entered into a backward logistic model. In the final model, all variables with P value less than 0.05 along with their respective odds ratio and 95% confidence intervals were obtained. Multiple logistic regression analysis was also undertaken to determine independent predictors of difficult mask ventilation.
Statistical analysis was performed using the Statistical Package for Social Sciences (SPSS, version 20 for IBM SPSS Statistics for Macintosh, Version 20.0. Armonk, NY, USA). All comparisons were conducted at the significance level with P value less than 0.05.
In order to derive two or more covariates in a logistic regression model, 20 events are needed. Previous studies demonstrated an incidence of difficult visualisation or intubation of 20% in obese surgical patients.10 Therefore, a sample size of 100 patients and approximately 20 events were needed to minimise the risk of overfitting of the logistics model.
One hundred and four morbidly obese patients consented to participate in the study. Patient demographics are summarised in Table 1. Eighty-eight individuals were women and 16 were men. Airway assessment parameters are also summarised in Table 1.
Difficult tracheal intubation was present in 13 (13%) of the morbidly obese patients. The variables associated with difficult intubation by univariate analysis (Table 2) were male sex, BMI more than 50 kg m−2, waist circumference, neck circumference more than 42 cm and ASA physical status. Independent predictors of difficult intubation by multiple logistic regression were neck circumference more than 42 cm and BMI more than 50 kg m−2 (Table 3). Morbidly obese patients with a neck circumference more than 42 cm, as well as patients with BMI more than 50 kg m−2 were five times more likely to experience difficult intubation. In participants with a neck circumference less than 42 cm (62), there were two cases of difficult intubation. Of the participants with a neck circumference greater than 42 cm (42), 11 experienced difficult intubation. The area under the ROC curve for the final model was 0.79.
Eleven patients (11%) experienced difficult mask ventilation and no patient experienced impossible mask ventilation. Clinical variables associated with difficult mask ventilation by univariate analysis were BMI more than 50 kg m−2, male sex, neck circumference, thyromental distance and history of obstructive sleep apnoea (Table 4). Independent predictors of difficult mask ventilation by multiple logistic regression analysis included male sex and a BMI more than 50 kg m−2 (Table 3). Sleep apnoea showed a trend towards a predictive relationship for difficult intubation. The odds of difficult mask ventilation were 8.5 times higher in men than in women. Patients with a BMI more than 50 kg m−2 were at a five times greater risk of difficult intubation. The area under the ROC curve for the final model was 0.83.
The probability of difficult intubation in relation to neck circumference and the probability of difficult mask ventilation in relation to neck circumference are shown in Fig. 1.
In our study, the incidences of difficult intubation and difficult mask ventilation among morbidly obese patients were found to be 13 and 11%, respectively. BMI more than 50 kg m−2 and neck circumference more than 42 cm were independent predictors of difficult intubation. BMI more than 50 kg m−2 and male sex were independent predictors of difficult mask ventilation.
Neck circumference is an increasingly common predictive measure for difficult airway management. Our study found neck circumference more than 42 cm to be an independent predictor of difficult intubation. In patients with a neck circumference greater than 42 cm, the risk of difficult intubation increased five-fold. Brodsky et al.9 previously identified neck circumference as an independent predictor of difficult intubation, with the risk of difficult intubation increasing seven-fold as neck circumference increased from 40 to 60 cm. In that study, however, a patient was defined as difficult to intubate if the product of laryngoscopic view and number of intubation attempts was greater than a predetermined value. This definition of difficult intubation is not commonly used and may affect the ability to generalise or to compare results among studies. Gonzalez et al.6 studied difficult intubation (defined as an IDS score of more than 5) prospectively, in 131 lean and obese patients. Neck circumference greater than 43 cm was an independent predictor of difficult intubation and increased the risk of difficult intubation by 37%. Conversely, Neligan et al.8 studied the predictive value of neck circumference in 180 obese patients, finding no association between neck circumference and difficult intubation. In that study, difficult intubation was defined using the number of intubation attempts and laryngoscopic view. Our finding of neck circumference as an independent predictor of difficult intubation is consistent with the findings of Brodsky et al.9 and Gonzalez et al.6 The finding of Neligan et al.8 that neck circumference was not predictive may be explained by the differences in definition of difficult intubation.
Our study found that a high BMI was an independent predictor of difficult intubation. In patients with a BMI more than 50 kg m−2, the odds of difficult intubation increased five-fold. Lundstrom et al.18 found a BMI of more than 35 kg m−2 to be an independent predictor of difficult and failed intubation after observing a cohort of 91 332 consecutive patients. Juvin et al.5 compared 263 lean and obese individuals and found a higher incidence of difficult intubation in the obese individuals. Conversely, Brodsky et al.9 conducted a study of 100 morbidly obese patients and did not find obesity to be an independent predictor of difficult intubation. Our findings confirmed the commoner view that morbid obesity is an independent predictor of difficult tracheal intubation.4,5,17,18
At present, there are no studies in the literature showing an association between neck circumference and difficult mask ventilation. In our study, univariate analysis showed a significant association between a neck circumference of more than 42 cm and difficult mask ventilation. Figure 1 demonstrates a direct relationship between the probability of difficult ventilation and neck circumference. However, logistic regression analysis did not show that neck circumference was an independent predictor of difficult mask ventilation.
This study found a BMI of more than 50 kg m−2 to be an independent predictor of both difficult mask ventilation and difficult tracheal intubation. In patients with a BMI of more than 50 kg m−2, the odds of difficult mask ventilation increased five-fold. In the literature, there is conflicting evidence about an association between a high BMI and difficult mask ventilation. In a prospective study, Langeron et al.11 studied difficult mask ventilation in 1502 diverse surgical patients. A BMI more than 26 kg m−2 was determined to be an independent predictor of difficult mask ventilation. Similarly, Kheterpal et al.10 conducted a population study of 22 600 individuals and found a BMI of more than 30 kg m−2 to be among the independent predictors. A literature review conducted by El-Orbany and Woehlck13 identified BMI as an independent predictor. Conversely, Shah and Sundaram7 found no association between obesity and difficult mask ventilation. However, their study used a broader definition of difficult mask ventilation than commonly used in the literature. Our findings are consistent with the majority of the literature.10,11,13 Our cut-off value, a BMI of more than 50 kg m−2 was significantly greater than the cut-off values in the existing literature, differing by approximately 20 kg m−2. However, the discrepancy may be attributed to differences in sample demographics.
Our study has several limitations. First, we defined difficult intubation in terms of an IDS score. Difficult intubation is more commonly defined using the Cormack and Lehane view classification.4 However, the Cormack and Lehane classification is based only on visualisation of the larynx and does not consider the ease of intubation, which is incorporated into the IDS score. Second, difficult intubation was defined as an IDS score of at least 5. This value differs slightly in the literature, with a number of studies5,6,19 defining difficult intubation as an IDS score of more than 5. Third, the present study used Troop pillows to put patients in the ramp intubating position. The positioning of patients varies within the literature, with several placing patients in the ‘sniff’ position.5–7,11,17 This should be considered in any comparisons within the literature, as previous research has shown the use of a Troop pillow and positioning to decrease the incidence of difficult airway.20 Fourth, the number of outcome events – patients with difficult intubation (n = 13) and difficult mask ventilation (n = 11) – were relatively low. However, the final logistic regression models for difficult intubation and difficult mask ventilation showed reasonably good fit with areas under the ROC curves of 0.79 and 0.83, respectively.
In summary, in morbidly obese patients undergoing general anaesthesia, we found that a BMI of more than 50 kg m−2 and neck circumference more than 42 cm were independent predictors of difficult intubation. A BMI more than 50 kg m−2 and male sex were independent predictors of difficult mask ventilation. Morbidly obese patients with such predictors should alert clinicians to the possibility of difficult airway management and appropriate backup plans and equipment should be made available.
Acknowledgements relating to this article
Assistance with the study: none.
Financial support and sponsorship: this study was funded by the Department of Anaesthesia, Toronto Western Hospital, University Health Network, University of Toronto.
Conflicts of interest: none.
Presentation: preliminary data were presented at the Society of Airway Management Annual Meeting in Seattle, Washington, USA, on 19 September 2014.
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