AIRWAY/ORIGINAL RESEARCH

Use of End Tidal Oxygen Monitoring to Assess

Preoxygenation During Rapid Sequence Intubation
in the Emergency Department
Nicholas D. Caputo, MD, MSc*; Matthew Oliver, MBBS; Jason R. West, MD; Robert Hackett, MBBS; John C. Sakles, MD
*Corresponding Author. E-mail: ncaputo.md@gmail.com, Twitter: @NickMD1980.

Study objective: Preoxygenation is important to prevent oxygen desaturation during emergency airway management. The
purpose of this study is to describe the use of end tidal oxygen (ETO2) during rapid sequence intubation in the emergency
department.

Methods: This study was carried out in 2 academic centers in Sydney, Australia, and New York City. We included patients
undergoing rapid sequence intubation in the emergency department. A standard gas analyzer was used to measure ETO2.
Preoxygenation methods included nonrebreather mask and bag-valve-mask ventilation. We measured ETO2 before preoxygenation
and at administration of rapid sequence intubation medications. We also characterized peri-intubation SpO2, identifying instances
of SpO2 less than 90%.

Results: We included 100 patients during a 6-month period. Median ETO2 level before and after preoxygenation was 53%
(interquartile range [IQR] 43% to 65%) and 78% (IQR 64% to 86%), respectively. One fourth of patients achieved an ETO2 level
greater than 85%. Median ETO2 level achieved varied with preoxygenation method, ranging from 80% (IQR 60% to 87%) for the
nonrebreather mask group to 77% (IQR 65% to 86%) for the bag-valve-mask group. The method with the highest median ETO2
level was nonrebreather mask at flush rate (86%; IQR 80% to 90%) and the lowest median ETO2 level was nonrebreather mask at
15 L/min (57%; IQR 53% to 60%). Eighteen patients (18%) experienced oxygen desaturation (SpO2 <90%); of these, 14 (78%) did
not reach an ETO2 level greater than 85% at induction.

Conclusion: ETO2 varied with different preoxygenation techniques employed in the emergency department. Most patients
undergoing rapid sequence intubation did not achieve maximal preoxygenation. Measuring ETO2 in the emergency department
may be a valuable adjunct for optimizing preoxygenation during emergency airway management. [Ann Emerg Med. 2019;-:1-6.]

Please see page XX for the Editor’s Capsule Summary of this article.

0196-0644/$-see front matter

Copyright © 2019 by the American College of Emergency Physicians.
https://doi.org/10.1016/j.annemergmed.2019.01.038

INTRODUCTION Difficult Airway Society suggests that when critically ill

Background patients are intubated, preoxygenation should be
Rapid sequence intubation is the most common method performed until an ETO2 level of greater than or equal to
of airway management in the emergency department 85% is attained.5,8 ETO2 level has not been widely used in
(ED).1 Use of an induction agent and a neuromuscular- clinical practice in the ED.
blocking agent results in transient apnea during the
intubation attempt. If prolonged, apnea and resulting Goals of This Investigation
oxygen desaturation may lead to serious adverse events such We sought to describe our preliminary use of ETO2
as dysrhythmias, hypotension, and cardiac arrest.2 to assess preoxygenation during rapid sequence intubation
Although practice guidelines advocate preoxygenation to in the ED.
prevent oxygen desaturation, the adequacy of
preoxygenation is not typically assessed in the ED.3-5 Study Design and Setting
We conducted a prospective observational cohort study
at 2 urban, academic EDs in Sydney, Australia, and New
Importance York City. This study was approved with waiver of
In the operating room, anesthesiologists have used gas consent by the institutional review board and ethics boards
analyzers to quantify and optimize preoxygenation.6,7 The at each institution.

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End Tidal Oxygen Monitoring to Assess Preoxygenation During Rapid Sequence Intubation Caputo et al

Editor’s Capsule Summary supplemental nasal cannula oxygen was chosen by the
attending emergency physician and ranged from 15 L/min
What is already known on this topic to flush rate.
Preoxygenation is recommended before rapid
sequence intubation. Selection of Participants
We included all adult patients (
18 years) undergoing
What question this study addressed
rapid sequence intubation during October 2017 to
Can end tidal expired oxygen levels (ETO2) February 2018. We excluded patients in cardiac arrest,
characterize preoxygenation before rapid sequence receiving noninvasive ventilation before intubation,
intubation? intubated in the out-of-hospital setting, or who underwent
What this study adds to our knowledge awake intubation.
In this series of 100 rapid sequence intubations,
median ETO2 levels before and after preoxygenation Methods of Measurement
were 53% (interquartile range 43% to 65%) and To quantify ETO2 level achieved with current
78% (interquartile range 64% to 86%). Of the 18 preoxygenation practices, emergency physicians were
patients experiencing oxygen desaturation (SaO2 blinded to the ETO2 data collected during the procedure.
<90%), 14 (78%) had an ETO2 level less than 85% Independent observers (research assistants, nurses, and
before induction. residents) collected all ETO2 measurements. The observers
underwent training for the study and collected data in real
How this is relevant to clinical practice time, using a standardized data collection tool.
ETO2 level may be useful for guiding preoxygenation Vital signs were obtained from the cardiac monitor
during rapid sequence intubation in the ED. (Philips IntelliVue; Philips, Andover, MA) in real time.
However, formal validation is necessary before ETO2 level was measured by Phillips G5 Gas Analyzer
clinical implementation. (Philips) at the New York site and by a Philips G7 Gas
Analyzer (Philips) at the Sydney site. The newer-generation
G7 module is a more compact version of the G5 and
The annual census of each ED was 175,000 patients facilitates similar ETO2 measurements. Although not the
(New York) and 80,000 patients (Sydney). The focus of this study, the devices also measured FiO2. The
departments perform greater than 900 intubations a year analyzers use a single flow sensor to make measurements for
combined (approximately 300 at the Sydney site and 600 at FiO2 supplied and ETO2 exhaled by gas sampling. SpO2 was
the New York site). Each department has emergency measured through standard finger oximeters (Covidien
medicine trainees. The majority of intubations are Oximax; Covidien). Hypoxemia was defined as an SpO2
performed by emergency medicine trainees under the direct level less than 90%.
supervision of an emergency medicine attending physician. For patients undergoing bag-valve-mask preoxygenation,
Both sites practice rapid sequence intubation, with the ETO2 level was measured by side-stream gas sampling
specific technique used chosen by the emergency medicine connected between the bag-valve-mask device and the
attending physician. mask. For patients receiving nonrebreather mask
Clinical airway management practices at each site called preoxygenation, side-stream sampling was used by means of
for a minimum of 3 minutes of preoxygenation with either a nasal prong gas sampler (AirLife ETCO2 Nasal Cannula at
a bag-valve-mask device or a nonrebreather mask before the New York site and CapnoEZY at the Sydney site). For
rapid sequence intubation. If a bag-valve-mask device was patients receiving preoxygenation by nonrebreather mask
used, the mask seal was maintained by the operator, and plus nasal cannula oxygen, the New York site used a
assisted breaths were given at the discretion of the attending separate standard nasal cannula. The Sydney site used
physician. Positive end-expiratory pressure could be CapnoEZY nasal prongs. Waveform capnography was used
provided if clinically warranted by means of a positive end- to verify that ventilation was occurring during ETO2
expiratory pressure valve connected to the bag-valve-mask measurement recordings.
device, and at pressures ranging from 1 to 20 cm H2O. For To validate the use of single-breath versus continuous
nonrebreather mask, the oxygen flow rate was selected by ETO2 measurements, we conducted a preliminary study
the emergency medicine attending physician and was set at using 4 healthy volunteers (attaining 20 measurements at
either 15 L/min or flush rate (50 to 70 L/min at the New each site). We compared continuous and single-breath ETO2
York site and 19 L/min at the Sydney site). The use of measurements for both bag-valve-mask device and

2 Annals of Emergency Medicine Volume -, no. - : - 2019

Caputo et al End Tidal Oxygen Monitoring to Assess Preoxygenation During Rapid Sequence Intubation

nonrebreather mask (n¼10 measurements for each group was 70% (95% CI 62% to 78%) for continuous
method). Each volunteer was preoxygenated for 3 minutes ETO2, and single-breath ETO2 was 75% (95% CI 68% to
with bag-valve-mask device and nonrebreather mask. The 82%); the mean difference was 5% (95% CI 3.5% to
ETO2 was recorded at the end of 3 minutes of continuous 6.5%) for the nonrebreather mask group. Agreement was
analysis. At the conclusion of the 3 minutes, the subject good between the measures with Pearson's correlation
held a single breath for 10 seconds and then exhaled. This coefficient 0.81 (95% CI 0.59 to 0.92) for the bag-valve-
single-breath measurement was then compared with the mask device and 0.75 ( 95% CI 0.45 to 0.90) for
final measurement during the continuous ETO2 nonrebreather mask (Bland-Altman plot depicted in
measurement. Figure E1, available online at http://www.annemergmed.
Preoxygenation interval was defined as the elapsed time com).
from emergency medicine attending physician decision to Of 154 patients undergoing rapid sequence intubation,
intubate to the point of induction. Apnea time was defined we included 100, including 75 at the New York site and 25
as the elapsed time from induction to confirmation of tube at the Sydney site. Reasons for exclusion included the
placement by waveform capnography. All other data on following: noninvasive ventilation before intubation
baseline characteristics, pre- and postlaryngoscopy (n¼18), intubation in the field (n¼11), use of “awake”
management, and clinical outcomes were collected from intubation (n¼13), and traumatic cardiac arrest (n¼3).
the medical record by study personnel. We tested interrater Data were not available for 9 patients. The cohort was
agreement for ETO2 and SpO2 by using the first 5 cases. composed of older patients, greater than half of whom were
men, with a primary indication for intubation that was
Outcome Measures pulmonary at the New York site and neurologic at the
The primary outcome was ETO2 level, measured before Sydney site (Table). Video laryngoscopy was used for 55%
preoxygenation and at induction. The secondary outcome of patients at the New York site (data not collected at the
was SpO2, measured before and immediately after Sydney site), and senior trainees intubated 65% of the
intubation. patients. The first-pass success was 90% and all patients
were intubated within 2 attempts.
For the first 5 consecutively included patients, interrater
Primary Data Analysis
agreement was high for ETO2 (percentage agreement¼80%)
To validate the use of single-breath versus continuous
and SpO2 (percentage agreement¼100%) before
ETO2 in healthy volunteers, we used Pearson’s correlation
preoxygenation and at rapid sequence intubation
coefficient and a Bland-Altman plot. For the clinical series,
induction.
we analyzed the data with descriptive techniques. We
The median ETO2 level at initiation of preoxygenation
determined median ETO2 level before preoxygenation and at
was 53% (interquartile range [IQR] 43% to 65%) and at
induction, and SpO2 before and after intubation,
induction was 78% (IQR 64% to 86%). The median ETO2
examining the full cohort, as well as stratifying by
levels for the nonrebreather mask and the bag-valve-mask
preoxygenation technique. We plotted ETO2 and SpO2
device subgroups were similar at induction (80%, IQR
measurements for each patient. We determined the
59% to 87%; and 77%, IQR 65% to 86%, respectively).
proportion of patients achieving ETO2 level greater than
The method with the highest median ETO2 level was
85%. We also determined the proportion of patients who
nonrebreather mask at flush rate (86%; IQR 80% to 90%),
experienced oxygen desaturation (SpO2 <90%). For the
and the lowest median ETO2 level was nonrebreather mask
initial 5 subjects, we assessed percentage agreement
at 15 L/min (57%; IQR 53% to 60%). The median FiO2
between raters. All analyses were performed with XLStat
at induction was 90% (IQR 78% to 94%). The mean
(version 2018.7; Addinsoft, New York, NY).
SpO2 at induction was 97% (95% CI 93% to 99%).
Oxygen saturations were higher during rapid sequence
RESULTS intubation when higher ETO2 levels were achieved by
In the validation of continuous versus single-breath ETO2 induction (Figure). Only approximately a quarter of the
measurements using human volunteers, continuous ETO2 patients achieved an ETO2 level greater than 85% (n¼26).
level for the bag-valve-mask group was 82% (95% Of 100 patients, 36 (36%) were able to be preoxygenated
confidence interval [CI] 80% to 86%) and single-breath to an ETO2 level of 70% to 85%, and 27 (27%) to an ETO2
ETO2 was 80% (95% CI 74% to 86%); the mean difference level of 50% to 69%. A total of 11 patients (11%) did not
was 2.2% (95% CI 1.4% to 3%) for the bag-valve-mask achieve an ETO2 level greater than 50%; the majority were
group. Continuous ETO2 level for the nonrebreather mask in the bag-valve-mask group.

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End Tidal Oxygen Monitoring to Assess Preoxygenation During Rapid Sequence Intubation Caputo et al

Table. Baseline characteristics of the overall cohort. issue existed with current preoxygenation strategies or
Site 1 Site 2 Total Cohort techniques in the ED by means of measuring ETO2.
Characteristic (n[75) (n[25) (N[100) Although our validation of continuous monitoring
Age, median (IQR), y 57 41 53 demonstrated results similar to those of a single breath
Male sex, No. (%) 50 60 56 technique, they were not identical, so caution needs to be
Indication, No. (%) taken. The type of monitoring technique may have
Pulmonary 36 1 37 (37) limitations on accuracy because there is a potential for a
Neurologic 18 7 25 (25) difference between single-breath and continuous ETO2
Trauma 14 5 19 (19) measurements. The ETO2 results of continuous measuring
Infections (not including 1 6 7 (7)
may be slightly higher than single-breath measurements in
pulmonary) critically ill patients because of oxygen mixing from the
Other 6 6 12 (12) source in the mask when the patient is not ventilating
Preoxygenation adequately, which does not occur during a single-breath
method, No. (%) measurement. In critically ill patients, it is difficult to control
BVM at 15 L/min 7 6 12 (12) for tidal volume breathing before securing an airway, and
BVM at FR 17 0 17 (17) therefore mixing may take place at times.
BVMþPEEP at 15 L/min 4 8 13 (13) ETO2 represents only a measure of the concentration of
BVMþPEEP at FR 13 0 13 (13) oxygen in the functional reserve capacity. Critically ill
NRB at 15 L/min 9 3 12 (12) patients may achieve a very high ETO2 but have a short safe
NRB at FR 17 0 17 (17) apnea time because of a markedly reduced functional reserve
NRBþNC at 15 L/min 4 8 12 (12) capacity, increased oxygen consumption, or both.9 ETO2
NRBþNC at FR 4 0 4 (4) measurements can be inaccurate if proper technique is not
Timing used. For example, in a patient with severe hypoventilation
Preoxygenation time, 12 (11 to 15) 11 (8 to 14) 12 (10 to14) the oxygen analyzer would essentially measure the FiO2
median (IQR), min being delivered to the patient.10 The concurrent use of
Operator level of waveform analysis (of both carbon dioxide and oxygen) is
training, No. (%) vital to determine alveolar gas exchange.
PGY Although our gas analyzer devices were able to measure
1 11 0 8 FiO2, we limited inferences on this measure because of
2 36 0 27 inherent technical limitations with this technique; for

3 53 100 65 example, if a patient receiving nonrebreather mask is not
ventilating well, the supplemental oxygen may concentrate
BVM, Bag-valve-mask; PEEP, positive end-expiratory pressure; NRB, nonrebreather
mask; FR, flush rate (ie, 50 to 70 L/min); NC, nasal cannula; PGY, postgraduate year. in the mask, causing falsely elevated FiO2. If validated,
measured FiO2 could offer another tool for monitoring and
enhancing preoxygenation.
A total of 18 patients (18%) desaturated below 90%,
and 2 (2%) desaturated below 80%. Of these 18 patients,
2 (11%) had an ETO2 level greater than 85% at rapid DISCUSSION
sequence intubation induction; the remaining cases had In this observational study, we describe our experience
ETO2 level less than 85% at induction. with using ETO2 as a means of quantifying the adequacy of
current preoxygenation strategies in the ED. ETO2 use must
be formally validated before its widespread use in the ED
LIMITATIONS setting. However, this technology could provide an
There are several limitations to this study. The study additional tool to guide clinical care during emergency
centers were academic EDs with training programs, and thus airway management.
the results may not generalize to nonacademic centers. Denitrogenation of the functional reserve capacity with
Because of the preliminary nature of this study, we did not oxygen is a vital step in preoxygenation before intubation to
compare differing techniques nor associations with patient create an intrapulmonary reservoir of oxygen that prevents
outcomes or adverse events (dysrhythmia, hypotension, and hypoxemia during the apneic phase of intubation. Gas
cardiac arrest). Although this was a limitation, we believed it analysis monitoring of ETO2 results has been used in the
was necessary to determine the foundation of whether an operating room for decades to help guide clinicians on the

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Caputo et al End Tidal Oxygen Monitoring to Assess Preoxygenation During Rapid Sequence Intubation

100%

90%

80%

70%

60%

50%

40%

30%

20%
BVM at FR BVM+PEEP BVM at 15 BVM+PEEP NRB at FR NRB+NC NRB at 15 NRB+NC
at FR LPM at 15 LPM at FR LPM at 15 LPM
ETO2 at Induction SpO2 at induction Lowest SpO2 During RSI

Figure. Induction ETO2, induction SpO2, and lowest SpO2 measurement for each rapid sequence intubation patient. LPM, Liters per
minute; RSI, rapid sequence intubation.

effectiveness of denitrogenation before a patient undergoes adequate preoxygenation, defined as ETO2 level greater than
anesthesia for operative intervention.10,11 These analyzers or equal to 90%. Given that this study was conducted in
provide useful information that can be used to adjust in real the controlled environment of the operating room on
time the method of preoxygenation being used, adjust what elective surgical patients, it perhaps is not surprising that
flow rate is being used, determine whether there is good the majority of patients who achieved optimal
mask seal, and determine whether these adjustments make preoxygenation in our study was so low. Compared with
a difference. Historically, EDs have not routinely used gas studies on healthy volunteers undergoing preoxygenation,
analyzers for ETO2, likely because of the lack of access to the our results appear to be fairly similar.14-17 This suggests
technology, high costs, and unfamiliarity with the device that measuring ETO2 in critically ill patients is feasible and
and technique. Although rapid sequence intubation in the may be useful to help optimize preoxygenation.
ED includes the preoxygenation phase of patients, because In summary, in this preliminary study we observed
the procedure is generally considered to be deemed considerable variation in ETO2 level during preoxygenation
emergency, this may reflect less than adequate focus being for ED rapid sequence intubation. ETO2 may be a useful
placed on spending time and resources on optimizing adjunct to optimize preoxygenation during emergency
patients, who are already at high risk for desaturation by the airway management.
nature of their critical illness.3 Many articles in the
emergency medicine literature have emphasized the The authors acknowledge Scott Weingart, MD, and
importance of the preoxygenation of patients, and Nicholas Chrimes, MBBS, for advice and guidance throughout
subsequently clinical practices have changed to optimize the article preparation phase of the study.
patients before intubation.3,12 This is where the
implementation of ETO2 may have use.
Supervising editor: Henry E. Wang, MD, MS. Specific detailed
We found that only 26% of patients were able to be
information about possible conflict of interest for individual editors
preoxygenated to the recommended goal of an ETO2 level is available at https://www.annemergmed.com/editors.
greater than or equal to 85%.5,8 There are a number of
Author affiliations: From the Department of Emergency Medicine,
potential causes for low ETO2 measurements. Low FiO2
Lincoln Medical Center, Bronx, NY (Caputo, West); the Department
may result from inadequate oxygen flow or leaks in the face of Emergency Medicine (Oliver) and Department of Anaesthesia
mask seal. Also, the time required for denitrogenation (Hackett), Royal Prince Alfred Hospital, University of Sydney,
varies between patients; 3 minutes may not be sufficient for Sydney, Australia; and the Department of Emergency Medicine,
some patients. In the operating room, Baillard et al13 found University of Arizona College of Medicine, Tucson, AZ (Sakles).
that of 1,050 patients preoxygenated for a set time of Author contributions: NDC, MO, JRW, RH, and JCS contributed to
3 minutes with an anesthetic circuit, only 44% achieved the research design and methodology, initial drafting, and editing

Volume -, no. - : - 2019 Annals of Emergency Medicine 5

End Tidal Oxygen Monitoring to Assess Preoxygenation During Rapid Sequence Intubation Caputo et al

of the manuscript. NDC, JRW, and MO supervised the data 5. Frerk C, Mitchell VS, McNarry AF, et al. Difficult Airway Society
collection. NDC performed the data analysis. NDC takes Intubation Guidelines Working Group. Difficult Airway Society 2015
responsibility for the paper as a whole. guidelines for management of unanticipated difficult intubation in
adults. Br J Anaesth. 2015;115:827-848.
All authors attest to meeting the four ICMJE.org authorship criteria: 6. Benumof JL. Preoxygenation: best method for both efficacy and
(1) Substantial contributions to the conception or design of the efficiency. Anesthesiology. 1999;91:603-605.
work; or the acquisition, analysis, or interpretation of data for the 7. Pourmand A, Robinson C, Dorwart K, et al. Pre-oxygenation:
work; AND (2) Drafting the work or revising it critically for important implications in emergency airway management. Am J Emerg Med.
intellectual content; AND (3) Final approval of the version to be 2017;35:1177-1183.
8. Higgs A, McGrath BA, Goddard C, et al. Difficult Airway Society;
published; AND (4) Agreement to be accountable for all aspects of
Intensive Care Society; Faculty of Intensive Care Medicine; Royal
the work in ensuring that questions related to the accuracy or College of Anaesthetists. Guidelines for the management of
integrity of any part of the work are appropriately investigated and tracheal intubation in critically ill adults. Br J Anaesth. 2018;120:
resolved. 323-352.
9. Benumof JL, Herway ST. High end-tidal oxygen concentration can be a
Funding and support: By Annals policy, all authors are required to
misleading sole indicator of the completeness of preoxygenation.
disclose any and all commercial, financial, and other relationships Anesth Analg. 2017;124:2093.
in any way related to the subject of this article as per ICMJE conflict 10. American Society of Anesthesiologists. Standards for basic anesthetic
of interest guidelines (see www.icmje.org). The authors have stated monitoring, October 15, 2003. Available at: http://www.asahq.org/
that no such relationships exist. publicationsAndServices/standards/02.pdf#2. Accessed August 2,
2018.
Publication dates: Received for publication December 12, 2018. 11. Australia and New Zealand College of Anaesthetists.
Revisions received January 6, 2019, and January 21, 2019. Recommendations on monitoring during anaesthesia, October 2000.
Accepted for publication January 22, 2019. Available at: http://www.medeserv.com.au/anzca/publications/
profdocs/profstandards/PS18_2000.htm. Accessed September 26,
Presented at the American College of Emergency Physicians
2004.
national Scientific Assembly, October 2018, San Diego, CA; and 12. Sakles JC. Maintenance of oxygenation during rapid sequence
the New York Chapter of American College of Emergency intubation in the emergency department. Acad Emerg Med.
Physicians regional Scientific Assembly, July 2018, Lake George, 2017;24:1395-1404.
NY. 13. Baillard C, Depret F, Levy V, et al. Incidence and prediction of
inadequate preoxygenation before induction of anaesthesia. Ann Fr
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