Associations of obesity with tracheal intubation success on first attempt and adverse events in the emergency department: An analysis of the multicenter prospective observational study in Japan
Associations of obesity with tracheal intubation success on first attempt and adverse events in the emergency department: An analysis of the multicenter prospective observational study in Japan
Hiromasa Yakushiji 1 2
Tadahiro Goto 2
Wataru Shirasaka 1 2
Yusuke Hagiwara 0 2
Hiroko Watase 2
Hiroshi Okamoto 2
Kohei Hasegawa 2 3
0 Department of Pediatric Emergency and Critical Care Medicine, Tokyo Metropolitan Children's Medical Center, Tokyo, Japan, 4 Department of Surgery, University of Washington, Seattle, Washington, United States of America, 5 Center for Clinical Epidemiology, St. Luke's International University , Tokyo , Japan
1 Department of Emergency Medicine, Kishiwada Tokushukai Hospital , Osaka , Japan , 2 Department of Emergency Medicine, Massachusetts General Hospital , Boston, Massachusetts , United States of America
2 Editor: David Meyre, McMaster University , CANADA
3 Harvard Medical School , Boston, Massachusetts , United States of America
Data Availability Statement: There are restrictions
on the availability of data due to the signed consent
agreements for data security, which allow access
only to external researchers for research
monitoring purposes. The study data cannot be
made publicly available because it contains
protected health information of the participants and
violates the ethical agreement with the IRBs that
approved the study. The study was approved by
the IRB of Fukui University Hospital, Fukui
Obesity is deemed to increase the risk of difficult tracheal intubation. However, there is a
dearth of research that examines the relationship of obesity with intubation success and
adverse events in the emergency department (ED). We analyzed the data from a
prospective, observational, multicenter studyÐthe Japanese Emergency Airway Network (JEAN) 2
study from 2012 through 2016. We included all adults (aged 18 years) who underwent
tracheal intubation in the ED. Patients were categorized into three groups according to their
body mass index (BMI): lean (<25.0 kg/m), overweight (25.0±29.9 kg/m), and obesity
( 30.0 kg/m). Outcomes of interest were intubation success on the first attempt and
intubation-related adverse events. Of 6,889 patients who are eligible for the analysis, 5,370
patients (77%) were lean, 1,177 (17%) were overweight, and 342 (4%) were obese.
Compared to the lean patients, the intubation success rates were significantly lower in the
overweight and obese patients (70.9% in lean, 66.4% in overweight, and 59.3% in obese
patients; P<0.001). In the multivariable analysis, compared to the lean patients, overweight
(adjusted odds ratio [OR], 0.85; 95%CI, 0.74±0.98) and obese (adjusted OR, 0.62; 95%CI,
0.49±0.79) patients had a significantly lower success rate on the first attempt. Additionally,
obesity was significantly associated with a higher risk of adverse events (adjusted OR, 1.62;
95%CI, 1.23±2.13). Based on the data from a multicenter prospectively study, obesity was
associated with a lower success rate on the first intubation attempt and a higher risk of
adverse event in the ED.
Prefectural Hospital, Kameda Medical Center,
Kurashiki Central Hospital, Nagoya Ekisaikai
Hospital, Nigata City General Hospital, Okinawa
Chubu Prefectural Hospital, Otowa Hospital,
Shonankamakura General Hospital, St Marianna
University School of Medicine Hospital, Tokyo Bay
Urayasu Ichikawa Medical Center, University
Hospital, Kyoto Prefectural University of Medicine,
Yokohama Rosai Hospital, and Kishiwada
Tokushukai Hospitals. The relevant data may be
accessed upon request. Data requests from
qualified investigators performing research in
emergency airway management should be made to
the Japanese Emergency Medicine Network
Coordinating Center (Email: jemnetoffice@jemnet.
Funding: This study was supported by St. Luke's
Life Science Institute (https://cce.luke.ac.jp/
subsidy/history/2014.html). YH received the
funding. The grant number is not available. The
funders had no role in study design, data collection
and analysis, decision to publish, or preparation of
Competing interests: The authors have declared
that no competing interests exist.
Emerging evidence indicates that difficult intubation and repeated intubation attempts are
related to a higher risk of intubation-related adverse events in the emergency department (ED)
[1±4]. Thus, early recognition of difficult airway with an optimal preparation and use of
alternative methods is critical. The anesthesia literature has identified the factors that predicts
difficult intubationsÐe.g., short thyromental distance, large neck circumference, and obesity [
Despite the differences in patient population and available resources from the anesthesia
settings, there is insufficient existing evidence of the association between obesity and
intubation outcomes in the ED. The limited emergency medicine literatureÐwhich is based on
retrospective studies [
]Ðis conflicting with the intubation success rates in obese patients to be
no different from  or higher than [
] those in non-obese patients in the ED. As obesity is a
common comorbid condition in the ED population [
], further clarification of its impact on
the intubation outcomes will inform the strategies to guide optimal emergency airway
management in the ED.
To address the knowledge gap in the literature, we aimed to investigate the association of
obesity with intubation success and adverse event rates in the ED, by using the data from a large
prospective multicenter study. We hypothesized that obesity is associated with a lower success
rate of first intubation attempt and a higher rate of adverse events in the ED.
Materials and methods
Study design and setting
We analyzed the data from a prospective, observational, multicenter studyÐthe Japanese
Emergency Airway Network (JEAN) 2 studyÐfrom February 2012 through November 2016.
The study setting, methods of measurement, and measured variables have been reported
previously [3,4,10±14]. In short, the JEAN 2Ða consortium of 14 academic and community medical
centers from different geographic regions across JapanÐprospectively enrolled all pediatric
and adult patients who underwent emergency tracheal intubation in one of the participating
EDs. All 14 EDs were staffed by emergency attending physicians and had affiliations with
emergency medicine residency training programs. The participating institutions included 11
Critical Medical Care Centers and had an average ED census of 31,000 patient visits per year
(range 14,000 to 66,000). In this observational study, each ED maintained individual protocols
about the procedures and policy for ED airway management. Intubations were performed by
attending physicians, or by resident physicians at the discretion of attending physicians. The
study was approved by the Institutional Review Board of each participating center, including
the Institutional Review Board of Kishiwada Tokushukai Hospital, with waiver of informed
consent prior to data collection.
Selection of participants
For the present analysis, we included all adult patients (aged 18 years) who underwent
intubation in one of the participating EDs during a 58-month period (from February 2012 through
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The JEAN 2 study prospectively collected the data for consecutive patients. After each
intubation, the intubatorÐphysician performing each intubationÐcompleted a standardized data
collection form that included the patient's age, sex, estimated weight and height, primary
indication for intubation, methods and medications of airway management, devices used to
facilitate the intubation, level of training and specialty of the intubator, number of attempts, success
or failure at each attempt, vital signs, and intubation-related adverse events. We monitored
compliance with data form completion. Where the data collection form was missing, we
interviewed the involved physicians and reviewed medical records to ascertain the airway
management details. These post-hoc interviews occurred within two weeks of the patient encounter.
The primary exposure of interest was the patient obesity status, according to patient's body
mass index (BMI): lean (<25.0 kg/m), overweight (25.0±29.9 kg/m), and obesity ( 30.0 kg/m)
]. BMI was calculated based on the weight and height that are estimated by the intubator at
the intubation in the ED. The literature has indicated that the physician-estimated weight,
height, and BMI category are relatively accurate [
The outcomes of interest were intubation success on the first attempt and intubation-related
adverse events. An ªintubation attemptº was defined as a single insertion of the device (direct
or video laryngoscope, regardless of video channeling or use of adjunct devices) past the teeth.
An attempt was successful if it resulted in an endotracheal tube being placed through the vocal
cords. Intubation-related adverse events included cardiac arrest, post-intubation hypoxemia
(pulse oximetry saturation <90%), hypotension (systolic blood pressure <90 mmHg),
dysrhythmia, esophageal intubation with delayed recognition, mainstem bronchial intubation,
airway trauma, dental or lip trauma, regurgitation, and allergic reaction [3,4,10±14].
Esophageal intubation was diagnosed with physical examination, ultrasonography, end-tidal CO2
monitor, chest x-ray, or any combination of these methods.
First, we compared the patient characteristics between the BMI categories by using χ2 or
Kruskal-Wallis tests as appropriate. Next, to determine the association of BMI category with each
of the intubation outcomes, we constructed multivariable random-effects models with binary
response to account for patient clustering within the EDs. The models adjusted for potential
confounders, including age, sex, primary indication for intubation (medical cardiac arrest,
traumatic cardiac arrest, medical non-cardiac-arrest, and traumatic non-cardiac-arrest),
methods of intubation (no medication, rapid sequence intubation [RSI], sedative only, and others),
intubation devices (direct laryngoscope, video laryngoscope, and others), and training level
and specialty of intubator (transitional-year residents [post-graduate year 1 and 2 physicians],
emergency medicine residents, emergency physician, and others). In the sensitivity analysis,
we repeated the analyses with stratification by cardiac arrest as the primary indication. We also
repeated the subgroup analysis in the patients who underwent RSI. P-values of <0.05 were
regarded as statistically significant. All statistical analyses were performed using STATA 14.1
(StataCorp, College Station, TX).
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Baseline characteristics of patients
We recorded 7,657 patients (capture rate, 97%; S1 Fig) who underwent emergency airway
management during the 58-month study period. We excluded 252 pediatric patients (aged
<18 years) or without the information on age, 457 patients without the information on
BMI, and 59 patients who underwent surgical intubation. Of 6,889 patients eligible for the
analysis, 5,370 patients (77%) were lean, 1,177 (17%) were overweight, and 342 (4%) were
obese. Baseline characteristics of these groups are shown in Table 1. Compared to lean
patients, overweight and obese patients were younger and more likely to be intubated for
medical indication, intubated with RSI, and intubated with a video laryngoscope (all P<0.001).
Associations between BMI category and first-pass intubation success rate
Overall, there was a negative relationship between BMI and success rate on the first intubation
attempt (Fig 1). The success rates were 70.9% (95%CI, 69.7%-72.1%) in lean patients, 66.4%
(95%CI, 63.7%-69.1%) in overweight patients, and 59.3% (95%CI, 53.9%-64.6%) in obese
Abbreviation: IQR, interquartile range. Data were presented as number (percentage) of patients unless otherwise indicated.
Others include intubations using paralytics or analgesics only
²Others include intubation using a bougie or fiberoptic scope
³Defined as post-graduate years 1 and 2
§Others include intbuations by surgeon and anesthesiologist
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Fig 1. Association of body mass index with the success rate on the first intubation attempt. The fitted line represents locally weighted scatterplot
smoothed (LOWESS) curve. There was a negative relationship between body mass index and success rate on the first intubation attempt.
patients (Table 2). In the unadjusted model, compared to lean patients, overweight and obesity
patients had a significantly lower success rate on the first intubation attempt (unadjusted OR
for overweight 0.85 [95%CI 0.74±0.97] P = 0.02; unadjusted OR for obesity 0.62 [95%CI 0.49±
0.78] P<0.001). In the multivariable model adjusting for age, sex, primary indication for
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intubation, methods of intubation, devices for intubation, and training level and specialty of
the intubator, these associations remained significant (adjusted OR for overweight 0.85 [95%
CI 0.74±0.98] P = 0.03; adjusted OR for obesity 0.62 [95%CI 0.49±0.79] P<0.001). While there
were some differences in the success rates between the training level and specialty groups (S1
Table), the associations remained significant (Table 2).
Associations between BMI category and intubation-related adverse event
There was a positive relationship between BMI and adverse event rates (Fig 2). Table 3
describes the unadjusted and adjusted associations between BMI category and adverse event
Fig 2. Association of body mass index with the adverse event rates. The fitted line represents locally weighted scatterplot smoothed (LOWESS) curve.
There was a positive relationship between body mass index and adverse event rates.
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Abbreviations: OR, odds ratio; CI, confidence interval
Adjusted for age, sex, primary indication for intubation, methods of intubation, devices for intubation, and training level and specialty of the intubator.
rates. The adverse event rates were 15.8% (95%CI, 14.9%-16.9%) in lean patients, 18.1% (95%
CI, 15.9%-20.4%) in overweight patients, and 24.2% (95%CI, 19.8%-29.2%) in obese patients.
In the both unadjusted and adjusted models, obesity was significantly associated with a higher
risk of adverse events (unadjusted OR 1.61 [95%CI 1.24±2.10] P<0.001; adjusted OR 1.62
[95%CI 1.23±2.13] P<0.001).
To assess the robustness of the results, we performed a series of sensitivity analyses. In the
stratified analysis, in both cardiac arrest and non-cardiac-arrest patients, obese patients had a
significantly lower success rate (adjusted OR 0.59 [95%CI 0.39±0.90] P = 0.01 in cardiac arrest;
adjusted OR 0.64 [95%CI 0.47±0.86] P = 0.003 in non-cardiac-arrest; S2 Table) compared to
lean patients. Additionally, obesity was associated with a significantly higher risk of adverse
events (adjusted OR 1.62 [95%CI 1.00±2.93] P = 0.04 in cardiac arrest; adjusted OR 1.52 [95%
CI 1.10±2.08] P = 0.009 in non-cardiac-arrest; S3 Table). Among the patients who underwent
RSI, obesity was associated with a significantly lower success rate (adjusted OR 0.59 [95%CI
0.39±0.90] P = 0.03; S4 Table). With the limited statistical power in this subgroup analysis,
obese patients also had a non-significantly higher rate of adverse event (adjusted OR 1.40 [95%
CI 0.88±2.22] P = 0.15; S5 Table). An exploratory analysis of the relationship between obesity
and esophageal intubations did not demonstrate significant associations (all P>0.05; S6
In this large multicenter prospective study of 6,889 patients who underwent emergency airway
management in Japan, we found that overweight and obesity were significantly associated with
a lower success rate on the first intubation attempt in the ED even with adjustment for
potential confounders. In addition, obesity was also associated with a higher rate of adverse events
in the ED. These significant associations persisted across different patient subgroups.
Our findings are consistent with the previous literature that demonstrated the association
between obesity and decreased intubation success rates in the operating room and ED
]. For example, in a prospective study of intubations in the operating room,
overweight and obesity were associated with a higher risk of failure on the first and
second intubation attempts [
]. Another retrospective study of 1,053 intubations at a
single academic ED reported that, compared with lean and overweight patients, obese
patients were more likely to require multiple intubation attempts [
]. However, few
other studies reported inconsistent findingsÐno significant association between obesity
and intubation success rate [
]. The apparent inconsistency across the studies may be
attributable to the differences in the study design, setting (e.g., single center study),
population, and outcomes (e.g., intubation success within three or more attempts, intubation
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difficulty scale scores, or Cormack score) [
]. Instead, given the emerging evidence
on the importance of first-pass intubation success (e.g., its contribution to the decreased
rate of adverse events) [1±4], the current study has focused on this clinically important
outcome in the ED.
We also found the significant association between obesity and a higher risk of
intubationrelated adverse events. The sparse literature has investigated this association in the ED setting
]. For example, a retrospective single-center study of 1,435 ED intubations reported a
statistically significant but clinically non-significant higher rate of early respiratory complications
. Another retrospective single-center study of 1,053 ED intubations reported that, compared
to lean patients, overweight and obese patients are more likely to have immediate
post-intubation complications [
]. Our multicenter studyÐwith a sample size that are many times larger
than any other prior ED studies on this topic [
]±builds on their findings and extends them
by demonstrating the robust association of obesity with adverse event rates in addition to that
with first-pass success rates, clinically important outcomes in the ED [1±4].
The underlying mechanisms of the observed associations are likely multifactorial. For
example, the link between obesity and lower intubation success rates may be explained by
suboptimal medication dosing (sedatives and neuromuscular blockades), altered upper
airway anatomy, reduced glottic visualization, or any combination of these factors. Indeed, the
literature has documented that obese individuals have excessive soft tissues in the velopalate,
retropharynx, and submandibular regions and that these excessive tissues contribute to
difficulty in intubation [
]. Furthermore, the link between obesity and higher adverse event
rates may be attributable to the higher likelihood of repeated intubation attempts [1±4],
reduced tidal and expiratory reserve volumes , lower functional residual capacity [
excess soft tissues in the airway [
], and changes in the upper airway and fat mass on the
chest wall leading to difficult mask ventilation [
]. Further investigation into the underlying
mechanisms would inform the development of optimal airway management strategies in this
We acknowledge that this study has several potential limitations. First, in the current study
population, only 17% were overweight and 4% were obese (BMI 30.0 kg/m2), which are
lower than the previous reports in other industrialized countries [
]. Second, the BMI was
calculated based on the weight and height estimated by the intubator, and hence
misclassification of the BMI category is possible. However, prior study indicated that the
physician-estimated weight and height as well as BMI are relatively accurate [
]. Furthermore, in the
emergency setting, the exact patient's height and weight are often unknown and the use of
objective measurements with calibrated instruments are not feasible. Therefore, our study
reflects the ED airway management in the real-world setting and has implications on the
clinical decision-making in the ED. Third, our study did not have the information of
pre-intubation techniques (e.g., positioning, passive oxygenation, sedation for first look prior to
paralysis). Fourth, as with any observational studies, the associations of obesity with intubation
outcomes does not necessarily prove causality and might be confounded by unmeasured
factors (e.g., individual intubator experience). Finally, our study sample chiefly consisted of
academic EDs in Japan. While it is tempting to dismiss the generalizability of these inferences, the
observed associations between obesity and intubation-related outcomes persisted across
several analytical assumptions. Furthermore, multiple studies arrived at similar conclusions
despite the different patient populations (e.g., operating room populations [
healthcare setting .
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Based on the data from a large, prospective, multicenter study of 6,889 ED intubations, we
found that obesity were significantly associated with a lower success rate on the first intubation
attempt. In addition, obesity was also associated with a higher risk of intubation-related
adverse events. For clinicians, our data underscore the importance of early recognitions of
markers for difficult intubation (e.g., obesity) and optimization (e.g., optimal positioning).
Lastly, for researchers, our findings should facilitate further investigation into the development
of effective airway management measures in this high-risk population.
S1 Fig. Patients receiving emergency airway management in the emergency department.
S1 Table. Success rate on the first intubation attempt according to the training level and
specialty of intubator.
S2 Table. Unadjusted and adjusted associations between body mass index and success
rates on the first intubation attempt with stratification by cardiac arrest as the primary
S3 Table. Unadjusted and adjusted associations between body mass index and
intubationrelated adverse events with stratification by cardiac arrest as the primary indication.
S4 Table. Unadjusted and adjusted associations between body mass index and success rates
on the first intubation attempt in patients who underwent rapid sequence intubation.
S5 Table. Unadjusted and adjusted associations between body mass index and
intubationrelated adverse event in patients who underwent rapid sequence intubation.
S6 Table. Unadjusted and adjusted associations between body mass index and esophageal
The authors acknowledge the following research personnel at the study hospitals for their
assistance with this project: Fukui University Hospital (Yohei Kamikawa, MD; Hideya Nagai,
MD; Hiroshi Morita, MD), Fukui Prefectural Hospital (Yusuke Miyoshi, MD; Yukinori Kato,
MD; Hidenori Higashi, MD), Kameda Medical Center (Sho Segawa, MD; Kitai Yuya, MD;
Kenzo Tanaka, MD), Kishiwada Tokushukai Hospital (Hiromasa Yakushiji, MD), Kurashiki
Central Hospital (Hiroshi Okamoto, MD), Nagoya Ekisaikai Hospital (Yukari Goto, MD;
Shigeki Tsuboi, MD), Nigata City General Hospital (Nobuhiro Sato, MD), Okinawa Chubu
Prefectural Hospital (Masashi Okubo, MD; Yukiko Nakayama, MD), Otowa Hospital (Nobuhiro
Miyamae, MD), Shonankamakura General Hospital (Hirose Kaoru, MD; Taichi Imamura,
MD; Azusa Wendan, MD), St Marianna University School of Medicine Hospital (Yasuaki
Koyama, MD), Tokyo Bay Urayasu Ichikawa Medical Center (Hiroshi Kamura, MD;
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NakashimaYoshiyuki, MD), University Hospital, Kyoto Prefectural University of Medicine
(Jin Irie, MD), and Yokohama Rosai Hospital (Seiro Oya, MD), and our many emergency
physicians and residents for their perseverance in pursuing new knowledge about this vital
Conceptualization: Kohei Hasegawa.
Data curation: Yusuke Hagiwara, Hiroko Watase, Hiroshi Okamoto.
Formal analysis: Tadahiro Goto.
Investigation: Wataru Shirasaka, Yusuke Hagiwara, Hiroko Watase, Hiroshi Okamoto.
Supervision: Kohei Hasegawa.
Writing ± original draft: Hiromasa Yakushiji.
Writing ± review & editing: Tadahiro Goto, Kohei Hasegawa.
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