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Critical Care, Trauma, and Resuscitation: Research Reports

The Association Between Obesity and Difficult Prehospital Tracheal Intubation

Holmberg, Timothy J. MD*; Bowman, Stephen M. PhD; Warner, Keir J. BS; Vavilala, Monica S. MD*; Bulger, Eileen M. MD; Copass, Michael K. MD§; Sharar, Sam R. MD*

Author Information
doi: 10.1213/ANE.0b013e31820effcc

The sharply increasing prevalence of obesity in the United States over the last 2 decades14 is associated with increases in health care utilization,5,6 health care costs,5,7 and mortality.811 This increased prevalence of obesity has affected multiple medical disciplines, and presents unique challenges for equipment, techniques, and protocols in acute care settings including intensive care,12 trauma nursing,13,14 diagnostic radiology,15 perioperative trauma care,1618 and emergency medical services (EMS).1921

Emergency tracheal intubation (TI) is an important EMS and perioperative patient care procedure that offers potential life-saving benefit, but also presents significant potential risk to critically ill patients. In obese patients, TI can be particularly challenging because of increased risk of difficult mask ventilation,22,23 impaired respiratory mechanics,24,25 and poor tolerance of apnea.2628 Moreover, anatomical changes related to obesity (e.g., limited neck extension and mouth opening, crowding and distortion of oropharygeal anatomy) are widely thought to contribute to difficult laryngoscopy and TI.12 In the anesthesiology literature, obesity has been found to be associated with increased rates of difficult laryngoscopy or TI in some studies,2931 and others have failed to demonstrate such associations;3234 thus, the role of obesity as a risk factor for difficult TI has remained controversial.

There are obvious and important differences between the elective operating room and the emergent prehospital settings with respect to airway management, including provider factors (e.g., training, experience, equipment, medications, time) and patient factors (e.g., cooperation with airway assessment, positioning limitations, gastric contents, acute airway injury, unknown concomitant medical issues). However, prehospital advanced life support (ALS) providers often receive airway management education and experience in the elective operating room setting. Thus, knowledge of prehospital airway management issues is important to both ALS educators and anesthesiology providers so that airway management training can be placed into proper perspective.

Investigations of the effects of obesity on TI have largely been conducted in the operating room setting, and less often in the EMS setting. In the prehospital setting, a limited number of studies assessing risk factors for difficult TI have suggested (but not confirmed) an association between obesity and difficult TI.3538 Because obesity can be a readily identifiable risk factor for difficult intubation by prehospital providers, confirmation of such an association would be valuable so that ALS provider education, training, and clinical practice could be most appropriately designed and delivered. The goal of the current study was to assess whether difficult TI in the prehospital setting is more common in obese patients than in patients of normal weight, using airway management data collected prospectively by ALS providers experienced in TI, coupled with patient height and weight data and calculated body mass index (BMI).


We combined (a) prospectively collected airway management and outcome data on all adult patients who underwent attempted prehospital oral TI by ALS providers of Seattle Medic One over a 4-year period [a full description of this database can be found in Wamer et al.38], and (b) collected height and weight data as recorded in the medical records for the subset of these patients who were transported to Harborview Medical Center (HMC), the local county hospital and regional level 1 trauma center. The study protocol was reviewed and approved by the University of Washington Human Subjects Division, and the requirement for written informed consent was waived. Subjects were eligible if they were evaluated by Seattle Medic One ALS providers and underwent attempted TI in the prehospital setting, were ≥15 years in age, and were subsequently transported by medic unit or ambulance to the emergency department at HMC for evaluation.

The Seattle Medic One program uses a tiered medical response triaged by EMS dispatchers on the basis of the severity of illness or injury. Average response time is 3 minutes for basic life support units and 5 minutes for ALS units. Basic life support providers are certified at the Emergency Medical Technician—Basic level and trained in bag-mask ventilation (including oral/nasal–pharyngeal airways). Eighty-six ALS providers serve a metropolitan region with a daytime population of 720,000. Seven ALS units are available within the city, each staffed by 2 certified ALS providers from the Seattle Fire Department Paramedic Training program. This program requires 2500 hours of didactic, laboratory, and field experience before initial ALS certification. New ALS students receive airway management training from both senior ALS providers and anesthesiologists in the University of Washington's Department of Anesthesiology at HMC. Initial training consists of didactic lectures and intensive mannequin training, progressing to a minimum of 2 full days of airway management (including rapid sequence TI and alternative airway use) in the operating room, which is supervised by attending faculty anesthesiologists. After successful completion of this training, ALS students provide airway management in the prehospital setting under the strict supervision of senior ALS providers and under orders by medical control. As was recently reported,39 ALS students perform a median of 29 TIs on patients during their training year (interquartile range 25 to 33). In addition, all certified ALS providers are required to recertify their airway management skills every 2 years in an advanced airway workshop consisting of didactic lectures and airway laboratory sessions, as well as perform a minimum of 12 uncomplicated TIs each year in the clinical setting (prehospital or in the operating room).

The airway management protocol in place during the study consisted of multiple techniques, including bag-mask ventilation with oral/nasal–pharyngeal airways, TI under direct vision, Eschmann stylet–assisted TI, retrograde TI, transtracheal jet ventilation, and surgical cricothyroidotomy, as has been described in a published algorithm.38 Supraglottic alternative airways were not available at the time of the study; however, laryngeal mask airways are now being introduced and retrograde TI is no longer used. Rapid sequence TI (including succinylcholine administration) is performed in critically ill patients with intact airway reflexes who are not in cardiac or respiratory arrest, but only under direct order from medical control. If the ALS providers anticipate or experience difficulty with TI (see “difficult airway” definition below), they may continue with bag-mask ventilation and expeditious transport, or proceed to one of the alternative airway techniques listed above, at their discretion and in consultation with medical control. Proper positioning of tracheal tube placement during the study was confirmed by breath sounds and either colorimetric end-tidal carbon dioxide (available for all patients) or continuous capnography (not available for all patients).

In the Seattle Medic One system, each medic unit is staffed by 2 ALS providers, and by protocol, each is allowed up to 2 attempts at TI (attempted TI was defined as laryngoscope placement in the mouth and direct laryngoscopy) before either proceeding to more advanced airway techniques or transporting with bag-mask ventilation. Airway management data were collected for every patient using a standard-procedure, predefined airway management questionnaire completed immediately after conclusion of the patient's prehospital care by the ALS provider who performed the airway management. This questionnaire detailed the number and sequence of intubation attempts, the use of any advanced airway procedures, and the final airway outcome. A difficult airway was defined as ≥4 attempts at oral TI or the use of an alternative airway procedure before 4 TI attempts. The airway management questionnaire data were self-reported and reviewed on a daily basis by the medical director for consistency and accuracy, both as a quality improvement activity and as a teaching tool for ALS students present at the scene during the provision of clinical care. The questionnaire data were not independently verified.

These prospectively collected airway management data were then merged with the electronic database maintained by Seattle Medic One (Seattle Fire Department) that includes more comprehensive and detailed prehospital evaluation and treatment data for all patient encounters. Merging of these 2 complementary datasets assured that a prehospital airway questionnaire was completed for every patient who underwent attempted TI. The electronic database also provided additional detail regarding mechanism of injury or illness, use of neuromuscular blocking drugs (succinylcholine) to facilitate TI, and vital signs.

All eligible patients transported to HMC subsequently underwent retrospective review of their hospital medical records for height and weight data. For patients admitted to an intensive care unit from the emergency room, these data were routinely collected upon arrival in the unit, with bed scales used for weight, and supine length measurements used for height. For patients admitted to the wards, weight measurements were routinely made with either a bed scale or a standing scale, and height data were routinely obtained either by standing measurement or by history elicited from the patient (or family). For patients who had no height/weight measurements that could be obtained by any of the methods above, a review of the hospital medical record was performed for such data recorded during any outpatient encounters in the 2 months before admission. No height or weight data were available for study patients discharged from the emergency room or who died before obtaining height and weight data; these patients were excluded before data analysis. The BMI was calculated as the weight in kilograms divided by the square of the height in meters. In accordance with guidelines from the National Heart, Lung, and Blood Institute of the National Institutes of Health, and recommendations by the World Health Organization, BMIs were categorized as follows: BMI of 18.4 to 24.9 kg/m2 = normal weight; BMI of 25 to 29.9 kg/m2 = overweight; BMI of 30 to 34.9 kg/m2 = obesity class I; BMI of 35 to 39.9 kg/m2 = obesity class II; and BMI equal to or >40 kg/m2 = obesity class III.4042

Univariate and bivariate analyses were completed using t tests for continuous variables and χ2 testing for categorical variables. To identify significant associations between patient or clinical characteristics and presence of a difficult TI, we used multivariable logistic regression with difficult airway as the dependent variable, and the independent and control variables described above. Explanatory variables are presented as odds ratios, with P values and 95% confidence intervals (CIs). All analyses and statistical comparisons were performed using Stata 9.2 (StataCorp LP, College Station, Texas).


During the 4-year study period there were 80,501 ALS patient contacts, of which 4114 (5.1%) underwent attempted oral TI. Of these 4114, 1912 (46.5%) were transported to HMC for care, with the remainder transported to other hospitals in the Seattle metropolitan area and were excluded. Of these 1912 HMC patients, 29 were younger than 15 years old and were excluded. The prehospital records for 309 patients could not be linked to the hospital's inpatient medical record system and were excluded. An additional 699 had insufficient height and weight data in their hospital charts and were also excluded. Fifty-two patients were excluded because of height and weight data obtained >2 months before admission or after discharge, leaving 823 in the final cohort (Fig. 1). Demographic data for the final cohort are summarized in Table 1.

Figure 1:
Patient selection flowchart showing subject numbers for the initial subject pool, the various subject group exclusions, and the final study cohort. ALS = advanced life support.
Table 1:
Study Population Demographics (n = 823)

The overall success rate of oral TI in this study cohort was 98.5% (811 of 823) and similar to the success for the entire prehospital TI population (98.3%). Of the 823 patients in whom TI was attempted, 767 (93.1%) required 3 or fewer attempts. The remaining 56 (6.8%) patients met the predetermined definition of “difficult TI” (≥4 attempts at TI or use of an advanced airway technique). Of these 56 difficult TI patients, 42 required ≥4 attempts for eventual oral TI success, and 2 TIs were successfully performed using the retrograde technique. Of the 12 patients whose various attempts at TI proved unsuccessful, 10 underwent bag-mask ventilation during transport to the hospital and 2 were found to have esophageal placement of the tracheal tube upon arrival at the hospital (Table 2). No surgical airways were performed in the study cohort.

Table 2:
Airway Management Outcomes

In the univariate analysis, there were no significant associations between difficult intubation and diagnostic category (trauma vs. nontrauma diagnosis), age, use of succinylcholine, or gender. The results of the logistic regression model are summarized in Table 3. Using diagnostic category, age, and gender as control variables, we found that patients with class III obesity (BMI ≥ 40 kg/m2) had a significant association with difficult TI in comparison with their leaner (BMI <30 kg/m2) counterparts (odds ratio 3.68, 95% CI 1.27 to 10.59). However, patients with class I or II obesity (BMI ≥30 kg/m2 and <40 kg/m2) had no such association (odds ratio 0.98, CI 0.46 to 2.07) (Table 3).

Table 3:
Logistic Regression Analysis for Difficult Tracheal Intubation

The specific frequencies of (a) difficult airway and (b) TI success, as a function of BMI, are shown in Table 4. The frequencies of difficult airway were not statistically different between BMI groups; however, the frequency of difficult airway in those with class III obesity (16.7%) was almost 3 times the frequency observed in the lean and class I/II obesity groups (6%). Nonetheless, the frequency of TI success was similar in all 3 BMI groups (96%–98%). The percentage of patients with class III obesity who were successfully intubated on the first attempt (79.2%) was not different (P = 0.86) from the percentage of lean and less obese patients who were successfully intubated on the first attempt (74.8%).

Table 4:
Effect of Obesity on Airway Difficulty and Tracheal Intubation (TI) Success


Definitive control of the airway using TI is a central component in the resuscitative care of critically ill and injured patients. Unexpected difficulty in establishing airway control can be a significant source of morbidity and mortality. The ability to anticipate difficulty in TI is often limited in the prehospital setting because of a number of factors, including urgency of care, limited patient cooperation with airway assessment, head/neck anatomy that is distorted by injury, and concomitant medical conditions or injuries that are incompletely characterized. Obese patients present a number of additional challenges to TI, including altered airway anatomy and increased sensitivity to the adverse effects of apnea and hypoxia during prolonged attempts at TI. As a readily apparent physical characteristic, therefore, obesity has the potential to be a simple and identifiable risk factor for difficult TI performed by prehospital providers. Evaluating this association is the first step in identifying and implementing specific training activities or system modifications (e.g., protocols, equipment) for prehospital airway management in obese patients. In the current study, we found that in patients who underwent attempted TI in the prehospital setting by ALS personnel experienced in the procedure, morbid obesity (class III obesity, BMI ≥40 kg/m2) was 3 to 4 times more likely to be associated with difficult TI in comparison with a leaner population (BMI <30 kg/m2). However, moderate obesity (class I and II obesity, BMI 30 to 39.9 kg/m2) was not associated with increased TI difficulty.

The assessment of obesity as a risk factor for difficult laryngoscopy and TI in the elective operating room setting has received considerable attention in the anesthesiology literature.2934 These reports are inconsistent, but generally support the view that obesity can make laryngoscopy and TI more difficult. One exception is that of Brodsky et al.,41 who reported that high Mallampati score and large neck circumference, but not absolute body weight or BMI, predicted difficult intubation in morbidly obese (BMI ≥ 40 kg/m2) patients undergoing elective surgery. Unlike the prehospital setting in which patient preparation and airway assessment are severely limited, these elective patients underwent a complete head/neck physical examination of the airway, received standard aspiration prophylaxis (sodium citrate and metoclopramide), were “ramp” positioned on pillows and towels, were fully preoxygenated before rapid sequence induction and TI, and were intubated by experienced anesthesiology residents or attendings. Despite these differences in clinical setting in comparison with the prehospital environment of the current study, the TI success rate (100%) and frequency of difficult airway (12%) were similar to those we observed for the same level of obesity (96% and 17%, respectively).

Few studies have evaluated obesity as a specific risk factor for difficult TI in the prehospital setting. From an analysis of prospective observational data of 783 prehospital TIs performed by ALS personnel,37 Wang et al. reported that increased body weight was associated with an increased frequency of TI failure (odds ratio = 1.55, CI 1.24 to 1.95). However, this study was limited because of the use of body weight data only (not BMI) and the infrequent use of neuromuscular blocking drugs in their EMS system (the authors excluded all patients who received such drugs). In addition, their primary outcome was failed TI (defined as failure to achieve TI on the last out-of-hospital attempt, as determined by the out-of-hospital rescuer), as opposed to the more rigorous definition of difficult TI used in the current study. Finally, the overall rate of failed TI reported by Wang et al. was 13.4%, significantly higher than the 1.5% failed TI rate observed in the current study.

In 2006, Combes et al. reported a similar study of difficult prehospital airway management using prospective data from 1442 patients who underwent attempted prehospital TI in France.35 All TIs were performed either by physicians or specialized nurses experienced in TI, rather than by experienced ALS providers as in the current study. Using the Intubation Difficulty Scale36 as the primary outcome, the overall frequency of difficult TI was 7.4% and similar to that observed in the current study (6.8%). Although the authors found obesity to be one of several independent risk factors for difficult TI, the strength of this association was statistically small and of questionable clinical significance (odds ratio 1.0, CI 1.0 to 1.1, for each increase of 1 kg/m2 in BMI).

The current study is the first to specifically investigate the relationship between obesity and difficult TI in a prehospital EMS system consisting of skilled and experienced ALS providers who (a) undergo their initial clinical airway management training in the operating room under the supervision of anesthesiology faculty in an academic department, and (b) have high documented success rates (>96%) with prehospital TI.3840 Although the low frequencies of difficult (6.8%) or failed (1.5%) TI in the current study argue against the initiation of complicated or expensive changes to existing training or clinical protocols, our finding that morbidly obese patients have an increased frequency of difficult TI in the prehospital setting does suggest that this subset of patients may benefit from such efforts. First, with respect to laryngoscopy and TI skills, operating room training of such ALS providers might include use of head/neck elevation (i.e., ramp) to facilitate laryngoscopy.42 Similarly, clinical management protocols might be modified to direct ALS providers to use portable head/neck elevation devices43 in morbidly obese patients who do not require cervical stabilization, or to minimize the number of TI attempts before proceeding to either transport with bag-mask ventilation or use of an alternative airway management technique. Second, our findings highlight the importance of education and training in the use of alternative airway techniques that may obviate the need for laryngoscopy and TI in selected cases. Such techniques include bag-mask ventilation, or use of the laryngeal mask airway as a “rescue” airway in morbidly obese patients who cannot undergo successful TI and who also cannot be successfully managed with bag-mask ventilation.44

In the larger context of prehospital airway management (including TI) by nonphysician ALS providers, the current results demonstrate that certified ALS providers who (a) are comprehensively trained by senior ALS providers and anesthesiology faculty and (b) achieve reasonable skill maintenance (i.e., >12 TIs annually) can have high TI success rates (≥96%) in both lean and obese patient groups (Table 4) that minimize the need for use of alternative or rescue airways. Furthermore, as in previous reports of prehospital airway management in patients with traumatic brain injury,45 TI success is equivalent in patients requiring succinylcholine to those who do not. In contrast, national prehospital TI success rates vary significantly (33%–100% success),46 including a mere 69% success rate in a recently reported large population of patients admitted to a busy level 1 trauma center.47 Proposed reasons for this variability include disparities in ALS provider education, training, skill maintenance, and experience. By comparison, in-hospital success rates for emergent TI performed by anesthesiologists in similar trauma patients are very high, exceeding 99% in a recent report of 6088 acutely injured patients over a 10-year period at a busy level 1 trauma center.48 Future efforts should focus on the identification and improvement of factors that contribute to poor TI success in the resource-challenged prehospital environment, including appropriate ALS provider education and use of alternative airways, so that airway management contributes positively to patient outcomes.

This study has several limitations. First, in the absence of a uniformly accepted definition for “difficult TI” in the prehospital setting, our definition for difficult TI is somewhat arbitrary. Several definitions of “difficult TI” have been used by other investigators, such as the grade of direct laryngoscopic view,31 grade of laryngoscopic view coupled with number of intubation attempts,29 and more elaborate tools such as the Intubation Difficulty Scale.30,35 For our purposes, we chose to define “difficult TI” as ≥4 laryngoscopic attempts on the basis of the established clinical and quality assurance protocol of the local ALS provider agency. After 4 unsuccessful attempts (or sooner, at the provider's discretion), the protocol allows the ALS providers to choose alternative techniques. Second, as a result of self-imposed data quality requirements, our cohort did not include all patients in whom prehospital TI was attempted by local ALS providers (Fig. 1), thereby introducing a possible selection bias. We excluded all but the oldest pediatric patients from our analysis, because of the relative infrequency of pediatric patients in our system and the unique characteristics of pediatric airway management. In addition, we analyzed only a subset of all patients who underwent attempted prehospital TI, the 46.5% who were transported to a single, large medical center in the Seattle metropolitan area (HMC), because our ability to obtain accurate height/weight data was limited to this institution. We also lacked BMI data on patients who died immediately upon hospital arrival (in the emergency department or operating room), and do not know what contribution, if any, obesity played in their poor outcome. As a result of the various exclusions, only 20% of the population who underwent attempted prehospital TI were included in the final cohort. Third, in previous studies, the use of neuromuscular blocking drugs has been associated with increased TI success in the prehospital setting.40 In our system, the use of succinylcholine to facilitate TI is very common (73.5% of cases in our final cohort). However, we did not find an association between succinylcholine use and the frequency of difficult TI in our data, and thus opted to exclude succinylcholine use from our final multivariate model. This also means that our results are likely not generalizeable to prehospital ALS agencies that do not routinely use neuromuscular blocking drugs for TI. Fourth, patients had to be admitted to the hospital to obtain complete height/weight data for BMI calculations, thus excluding all patients who died before or during their stay in the emergency department. However, additional data analysis from a published report of 4091 attempted prehospital TIs by the same prehospital ALS provider group (Seattle Medic One)38 demonstrates that the mortality rate before hospital admission (i.e., in the field or in the emergency room) is identical (22%) in patients who undergo a “difficult” TI compared with a “routine” TI. Similarly, the rates of difficult prehospital TI are identical (5.2%) in patients who die before hospital admission and those who die after hospital admission. Furthermore, the prevalence of obesity in our study population is similar to that reported in other regions of the United States. Last, we were unable to assess the frequency of peri-intubation hypoxemia (a potential indirect indicator of difficult TI), because oxyhemoglobin saturation data were infrequently recorded in the prehospital setting during the study period.

In summary, safely managing the airway in the austere and resource-limited prehospital setting in relation to the well-equipped, well-staffed, and well-lit comforts of the emergency department and operating room is a challenging endeavor that requires comprehensive training, skill maintenance, and individualized clinical management by nonphysician ALS providers. In a prehospital system of ALS providers skilled and experienced in TI, morbid obesity (BMI ≥ 40 kg/m2) was associated with difficult TI in adults undergoing emergent airway management, yet resulted in a TI success rate similar to that in lean and less-obese patients. This observation should be integrated into both the training of prehospital ALS providers and their clinical airway management algorithms with respect to TI and alternative airway techniques.


Name: Timothy J. Holmberg, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: This author has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Stephen M. Bowman, PhD.

Contribution: This author helped analyze the data and write the manuscript.

Attestation: This author has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Keir J. Warner, BS.

Contribution: This author helped conduct the study and write the manuscript.

Attestation: This author has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Monica S. Vavilala, MD.

Contribution: This author helped design the study and write the manuscript.

Attestation: This author reviewed the analysis of the data and approved the final manuscript.

Name: Eileen M. Bulger, MD.

Contribution: This author helped design the study, conduct the study, and write the manuscript.

Attestation: This author has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Michael K. Copass, MD.

Contribution: This author helped write the manuscript.

Attestation: This author has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Sam R. Sharar, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: This author has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.


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