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Obstructive Sleep Apnea Is Not a Risk Factor for Difficult Intubation in Morbidly Obese Patients

Neligan, Patrick J. MA, MB, FFARSCI*; Porter, Steven MD; Max, Bryan MD; Malhotra, Guarav MD; Greenblatt, Eric P. MD*; Ochroch, E Andrew MD, MSCE*

doi: 10.1213/ane.0b013e3181b12a0c
Patient Safety: Medical Intelligence Article
Chinese Language Editions

BACKGROUND: Morbid obesity (MO), obstructive sleep apnea (OSA), and neck circumference (NC) are widely believed to be independent risk factors for difficult tracheal intubation. In this study, we sought to determine whether these factors were associated with increased risk of difficult intubation in patients undergoing bariatric surgery. The predictive factors tested were OSA and its severity, as determined by apnea-hypopnea index (AHI), gender, NC, and body mass index (BMI).

METHODS: All sequentially enrolled MO patients underwent preoperative polysomnography. Severity of OSA was quantified using AHI and the American Society of Anesthesiologists’ OSA severity scale. All patients had a standardized anesthetic that included positioning in the “ramped position” for direct laryngoscopy.

RESULTS: One hundred eighty consecutive patients were recruited, 140 women and 40 men. The incidence of OSA was 68%. The mean BMI was 49.4 kg/m2. The mean AHI was 31.3 (range, 0-135). All the patients’ tracheas were intubated successfully without the aid of rescue airways by anesthesiology residents. Six patients required three or more intubation attempts, a difficult intubation rate of 3.3%. There was an 8.3% incidence of difficult laryngoscopy, defined as a Cormack and Lehane Grade 3 or 4 view. There was no relationship between NC and difficult intubation (odds ratio 1.02, 95% confidence interval 0.93-1.1), between the diagnosis of OSA and difficult intubation (P = 0.09), or between BMI and difficult intubation (odds ratio 0.99, 95% confidence interval 0.92-1.06, P = 0.8). There was no relationship between number of intubation attempts and BMI (P = 0.8), AHI (P = 0.82), or NC (P = 0.3). Mallampati Grade III or more predicted difficult intubation (P = 0.02), as did male gender (P = 0.02). Finally, there was no relationship between Cormack and Lehane grade and BMI (P = 0.88), AHI (P = 0.93), or OSA (P = 0.6). Increasing NC was associated with difficult laryngoscopy but not difficult intubation (P = 0.02).

CONCLUSIONS: In MO patients undergoing bariatric surgery in the “ramped position,” there was no relationship between the presence and severity of OSA, BMI, or NC and difficulty of intubation or laryngoscopy grade. Only a Mallampati score of 3 or 4 or male gender predicted difficult intubation.

From the *Department of Anesthesiology and Critical Care, Hospital of University of Pennsylvania; and †University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania.

Accepted for publication May 9, 2009.

Address correspondence and reprint requests to Patrick J. Neligan, MA, MB, FFARSCI, Department of Anaesthesia and Critical Care, National University of Ireland, Galway (NUIG), Newcastle Rd., Galway, Ireland. Address e-mail to

Morbid obesity (MO) and obstructive sleep apnea hypopnea syndrome (OSA) are widely believed to be independent risk factors for difficulty in tracheal intubation.1–3 A series of studies have demonstrated that Mallampati score and neck circumference (NC) may predict difficult intubation in obese patients undergoing bariatric surgery2,4–6; the published difficult intubation rate for all MO patients is 10%-16.7%.2,5,6 In addition, investigators have demonstrated in chart reviews and case-matched studies that the diagnosis of OSA is associated with increased risk of difficult intubation.6–8 Because there has been a dramatic increase in the prevalence of OSA associated with the obesity epidemic, these data suggest that clinicians may encounter difficult intubation with increased frequency. However, these studies were retrospective analyses in different patient populations and could have involved selection bias.

The purpose of this study was to prospectively determine whether OSA (and its severity), NC, and body mass index (BMI) are independent risk factors for difficult intubation in a cohort of MO patients undergoing weight loss surgery.

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This was a prospective observational study. The research protocol was reviewed by the Institutional Review Board and a waiver of consent was approved. Patients were enrolled sequentially. Each patient underwent preoperative polysomnography. The degree of sleep apnea was quantified in terms of apnea-hypopnea index (AHI—the number of episodes of apnea or hypopnea in 1 h) and stratified using the American Society of Anesthesiologists’ (ASA) OSA severity scale. This scale quantifies severity of sleep apnea based on the AHI (0-5 no sleep apnea, 6-20 mild sleep apnea, 21-40 moderate sleep apnea, >40 severe sleep apnea). For the purpose of this study, we used a grading system based on the following scale: Grade 0 = no sleep apnea, Grade 1 = mild sleep apnea, Grade 2 = moderate sleep apnea, and Grade 3 = severe sleep apnea. Patients were weighed and measured preoperatively and their BMI calculated. All patients were scheduled to undergo bariatric surgery, such as hand-assisted laparoscopic Roux-en-Y gastric bypass, laparoscopic gastric banding, or robot-assisted gastric banding.

A trained observer (SP/BM/GM) visited each patient in the preoperative holding area and determined Mallampati score, thyromental distance, mouth opening, and NC. To optimize the laryngoscopic view, all patients were placed in the “ramped” position in the operating room. This was achieved by stacking blankets behind the patient’s back, resulting in elevation of the head, upper body, and shoulders significantly above the chest.9 As a result, the external auditory meatus was horizontal with the sternal notch, and this was checked by a trained observer in each patient. Similar positioning can be accomplished using a commercially available elevation pillow.10 All patients underwent a standardized induction of anesthesia with propofol (1-2 mg/kg) and fentanyl (1-2 μg/kg), and neuromuscular blockade achieved with vecuronium (0.1 mg/kg) or succinylcholine (1 mg/kg). The first three laryngoscopic attempts were to be performed by an anesthesiology resident. Subsequent attempts were to be performed by an attending anesthesiologist, in keeping with clinical practice at our institution. At the time of intubation, the observer recorded the Cormack and Lehane (CL) view on first laryngoscopy without external laryngeal manipulation. Additional attempts at intubation were quantified.

The primary outcome measure was correlation between AHI and difficult intubation, as quantified by the number of intubation attempts, the CL view, the need for rescue airway management, or case cancellation for failed tracheal intubation. Secondary outcome measures were correlation between intubation attempts and CL grade with BMI, NC, Mallampati score, and gender.

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Statistical Analysis

Proportional data were examined by χ2 analysis or Fisher’s exact test where cell values are below 5. Nonnormally distributed (nonparametric) data were analyzed with the Mann-Whitney U-test. Normally distributed data were analyzed using Student’s t-test for pairwise comparison. Both the number of intubation attempts and CL grade were considered ordinal data, and ordinal logistic regression models were developed to determine the relationship between them and the a priori identified potential predictors: gender, age, BMI, OSA, AHI, ASA OSA, Mallampati score, NC, and year of anesthesia resident training. These predictors were first tested as univariate predictors and then models were developed to evaluate confounding among these predictors.

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One hundred eighty consecutive patients were enrolled into the study, 140 women and 40 men (Table 1). Of these, 124 patients (68%) had OSA: 65% of the women and 85% of the men (Table 2). The mean BMI was 49.4 kg/m2 (range, 36-77). The mean AHI was 31.3 (range, 0-135), and the median ASA OSA grade was 2 (1st quartile = 1, 3rd quartile = 3). The median Mallampati score was 2 (1st quartile = 1, 3rd quartile = 3). The mean NC was 43.9 cm (range, 28-66 cm). All the patients’ tracheas were intubated successfully by anesthesiology residents, the majority of whom were in their first year of training. The median number of intubation attempts was 1 (range, 1-4; 1st quartile = 1, 3rd quartile = 2) and the mean CL grade was 1 (range, 1-4; 1st quartile = 1, 3rd quartile = 3, Fig. 1). There were no failed intubations and no rescue airway maneuvers were required. Six patients required three or more intubation attempts, resulting in a difficult intubation rate of 3.3%. There was an 8.3% incidence of difficult laryngoscopy as defined as a CL Grade 3 or Grade 4 view (Fig. 2).

Table 1

Table 1

Table 2

Table 2

Figure 1

Figure 1

Figure 2

Figure 2

Univariate analysis indicated that there was no relationship between the diagnosis (P = 0.09) or severity of OSA, as measured by AHI or ASA grade and difficulty in laryngoscopy or difficult intubation (P = 0.82). There was no relationship between NC (per cm) and difficult intubation (odds ratio 1.02, 95% confidence interval 0.93-1.1) or BMI and difficult intubation (odds ratio 0.99, 95% confidence interval 0.92-1.06, P = 0.8). There was no relationship between number of intubation attempts and BMI (P = 0.8) or NC (P = 0.3). Mallampati Grade III or IV predicted difficult intubation (P = 0.02), as did male gender (P = 0.02). Finally, there was no relationship between CL grade and BMI (P = 0.88), AHI (P = 0.93), or OSA (P = 0.6). Increasing NC was associated with worse CL grade (P = 0.02), but not difficult intubation (P = 0.23).

Multivariate analysis confirmed the univariate findings and did not reveal any confounding between Mallampati and male gender.

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This study refutes the widely held belief that the severity of OSA predicts difficult intubation.3 The study confirms previous reports that Mallampati Grade 3 or 4 predicts difficult intubation. In addition, men were more likely difficult to intubate, independent of BMI or the diagnosis of OSA. NC predicted difficulty in laryngoscopy, but not difficult intubation.

OSA occurs in up to 70% of MO patients undergoing bariatric surgery.11 OSA is associated with narrowing of the upper airway because of fat in the pharyngeal wall (at the level of the soft palate and submental area) and with loss of pharyngeal dilator activity during sleep.3 Not all patients who have OSA are obese, and not all morbidly obese patients have OSA. There is no direct relationship between OSA and BMI,12 although there is a known correlation with central obesity.

Currently, the ASA OSA severity scale defines no OSA as an AHI <5, mild OSA an AHI of 6-20, moderate OSA as an AHI of 21-40, and severe OSA as an AHI >40.13 The increasing severity presumes that this translates into increasing perioperative risk with regard to airway management, postoperative airway obstruction, hypoventilation, and apnea.14,15

Obese patients, and particularly those with OSA, are widely believed to be more difficult to intubate compared with the general population.16 However, there are few published data and much of the current focus on this issue relates to anecdotal information and sentinel events within individual institutions.17,18 A study of 764 consecutive patients without airway pathology undergoing general anesthesia failed to show a correlation between BMI and difficult laryngoscopy.18 Juvin et al.,2 using a previously published difficult intubation score,19 claimed a difficult intubation rate of 15.5% in patients with BMI >35 kg/m2 compared with 2.2% of controls (BMI <30), an absolute risk increase of 13.3%. However, the rate of difficult laryngoscopy was similar for both groups (10.4% vs 10.1%). No patients’ tracheas proved impossible to intubate.

Similar to our study, Brodsky et al.5 investigated difficult intubation in 100 patients with a BMI >40 kg/m2. There was one failed intubation and 12 problematic intubations. There was no association between difficult intubation and increased BMI.20 In the logistic regression analysis, only NC, male gender, and Mallampati score predicted difficult intubation.5

Gonzalez et al.21 compared intubation difficulty in 70 obese (mean BMI 44 ± 8) and 61 nonobese (mean BMI 24 ± 3) patients. They were placed in the “sniffing” position, with towels or pillows under their shoulders, and the head elevated and neck extended. The incidence of difficult intubation was 14.3% in obese patients versus 3% in nonobese patients (P = 0.03). There were no impossible intubations. Only 13% of the obese patients had a diagnosis of OSA, although it is unclear how this diagnosis was made and whether the true incidence was underreported.

There is good reason to suspect that the presence of OSA may increase the risk of difficult intubation. Obese patients who are difficult to intubate may have more paratracheal soft tissue.1 In a case-matched study of 15 patients who had proven difficult intubation, there was a significant relationship with AHI and OSA.7 Siyam and Benhamou8 performed a retrospective case-control analysis of difficult intubation in the setting of OSA and reported an incidence of difficult intubation of 21.9% in patients with OSA versus 2.6% in controls (P = 0.05). There was no relationship between the severity of AHI and difficulty of intubation. Kim and Lee6 undertook a retrospective case-matched study of 90 patients with OSA undergoing uvulopalatopharyngoplasty (UPPP) surgery, versus 90 controls. The difficult intubation rate was 16.7% in the UPPP patients versus 3.3% (P = 0.003) in controls. An AHI >40 was associated with a significant increase in the risk of difficult intubation.

Why do the data in our study conflict with previous studies in patients with obesity and sleep apnea? Ours was a prospective study of patients undergoing bariatric surgery managed by a group of anesthesiologists using a standardized approach. All patients were placed in the “ramped” position for administration of oxygen and tracheal intubation. In this patient population, the ramped position significantly improves laryngoscopic view.22 We believe that this positioning, with the head, shoulders, and upper body elevated above the chest, is a key component in our relatively low rate of difficult intubation. One previous study that used the conventional “sniffing” position reported a 33% incidence of CL Grade 1 view.23 In this study, Grade 1 views were observed in 66% of the patients. Despite finding an absolute risk increase of 11.3% for difficult intubation, Gonzalez et al.21 reported a 66% incidence of Grade 1 view in their obese cohort. In normal weight patients, the “sniff” position is achieved by raising the occiput 7 cm, resulting in 35° of neck flexion on the chest and 85-90° of extension of the head at the atlanto-occipital joint.24 However, it is usually not possible to achieve this position in obese patients, because of fat deposition in the suboccipital and nuchal area. Repositioning such patients with a ramp, or equivalent, realigns the three axes and facilitates intubation.25

Mallampati score and male gender predicted difficulty with laryngoscopy and intubation; this is consistent with the previous work by Brodsky et al.5 and Voyagis et al.26 Increasing NC predicted difficulty with laryngoscopy, but not difficulty with intubation. Although there is a known association between NC and OSA, the presence of OSA and its severity did not predict difficult laryngoscopy or difficult intubation. This is a significant finding because the association between sleep apnea and difficult intubation is widely reported.6–8 Our data conflict with those published by Hiremath et al.,7 Siyam and Benhamou,8 and Kim and Lee6 The former study was a retrospective analysis of patients who had been proven difficult to intubate.7 Consequently, selection bias is likely, and the study included only 15 patients. The study by Siyam and Benhamou8 included only 36 patients with OSA, 92% of whom were men. The patients had an average BMI of 29.7 (±4.4); 57% of patients had a Mallampati score of 3 or 4. Although the majority of patients in our study were women, owing to the demographic of patients undergoing bariatric surgery, more men were included in our study than in that of Siyam and Benhamou. In our study, 22.5% of men had a Mallampati score of 3 or 4, mean BMI was 51.9, and the mean AHI was 46. Although male gender predicted difficult intubation, this was not related to a diagnosis of OSA. The study by Kim and Lee6 was a retrospective airway analysis of patients who were undergoing UPPP for OSA, and the average BMI of their OSA patients was 25.3 (±3.3) and for the control group it was 27.9 (±2.9). Consequently, their cohort who had a mean BMI of 49.44 (range, 36-77.5) is probably not comparable to ours. This suggests that patients with primary OSA may have a different risk profile for difficult intubation than those with OSA secondary to obesity.

Our study was not designed to address the incidence of an absolute inability to intubate because that would require an enormous number of patients. For example, el-Ghanzori et al.27 failed to reach significance in a study with more than 10,000 subjects. We chose the number of intubation attempts and CL grade as intermediate markers of difficulty of intubation and potentially impossible intubation. These intermediate markers are widely accepted in the anesthesiology community and have been used in previous studies.5,21 Our approach provides support for the use of IV induction and direct laryngoscopy with patients in the “ramped” position as an alternative to awake intubation and all of its discomforts and possible complications.

All of the patients enrolled in this study were intubated by anesthesiology residents, the majority of whom were in their first year of training. Each was supervised by an attending anesthesiologist who specialized in bariatric anesthesia. It is likely that the meticulous approach to airway assessment and positioning by the attending anesthesiologists impacted on the ability of the residents to secure the airway. However, it is not possible to quantify this effect.

Finally, what are the implications of this study for clinicians involved in airway management? Although the incidence of difficult airway was low in this study, and there were no failed intubations, the study was performed in laboratory-like conditions in the operating room. Many studies have demonstrated that MO patients may be difficult to intubate in conditions that are suboptimal and positioning is poor. This may occur, for example, in the emergency department or during cardiac arrests, when patients are frequently laid flat for cardiopulmonary resuscitation, and the pillow is removed. In addition to difficulty assessing the patient’s airway,28 time for preparation and positioning is often minimal.29 Hence, we strongly recommend placing such patients in the head-elevated ear to sternum (“ramped”) position for intubation.

In summary, this was a prospective observational study of difficulty of intubation in consecutive MO patients undergoing bariatric surgery who were positioned in the “ramped” position. There was no relationship between the presence and severity of OSA and difficult intubation. Only a Mallampati score of 3 or 4 or male gender predicted difficult intubation.

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