Abstract
Objective
Preoxygenation decreases morbidity for patients requiring endotracheal intubation. However, rigorous means for determining adequate pre-oxygenation are limited in the emergency department (ED). End-tidal oxygenation (EtO2) monitoring could potentially improve preoxygenation in the ED. The accuracy of nasal cannula (NC) EtO2 in patients receiving supplemental oxygen is unknown. Our study examined the correlation between NC EtO2 and single-breath (SB) EtO2.
Methods
Healthy volunteers were randomized to receive supplemental oxygen via a nonrebreather mask (NRBM) or a noninvasive ventilation mask (NIV). Participants underwent 3-minute trials at 3 different settings: NRBM at 15 liters per minute (LPM), 35 LPM, and 55 LPM, or NIV at 40% fraction of inspired oxygen (FiO2), 70% FiO2, and 100% FiO2. NC EtO2 and SB EtO2 were obtained at the end of each trial.
Results
Complete data were obtained for 104 participants. Beta regression analysis revealed a strong correlation between NC EtO2 and SB EtO2 in the NIV group (pR2 = 0.7) and a moderate correlation (pR2 = 0.4) in the NRBM group. Mean differences in the NRBM arm were 13.1% (15 LPM), 18.0% (35 LPM), 17.1% (55 LPM), and 1.8% (40% FiO2), 5.1% (70% FiO2), and 10.1% (100% FiO2) in the NIV arm.
Conclusions
Supplemental oxygen led to an overestimation of NC EtO2 across both groups, with NRBM more than NIV. The correlation between SB EtO2 and NC EtO2 suggests NC EtO2 may be useful in assessing preoxygenation in real-time. Further study is needed to examine its clinical efficacy in preventing desaturation events during endotracheal intubation in the ED.
Keywords: endotracheal intubation, preoxygenation, pre-oxygenation, end-tidal oxygen, supplemental oxygen
The Bottom Line.
End-tidal oxygen monitoring has the potential to better assess the preoxygenation of patients before intubation and, therefore, minimize risk to our patients. This study aimed to determine whether supplemental oxygen delivery would affect end-tidal oxygen measurements in healthy volunteers. In general, the addition of supplemental oxygen resulted in falsely elevated end-tidal oxygen readings in the range of 2% to 18%, depending on the oxygen delivery method. Our findings will help to better inform preintubation decisions, as emergency department physicians can better estimate the patient’s true end-tidal oxygen levels while pre-oxygenation is in progress.
1. Introduction
1.1. Background
Preoxygenation is a well-established practice that denitrogenates the lungs and prolongs safe apnea time during endotracheal intubation,1 as hypoxia during intubation can lead to adverse patient outcomes.2, 3, 4, 5 In the emergency department (ED), 3 minutes of full tidal breaths with high flows of oxygen via either a nonrebreather mask (NRBM) or noninvasive ventilation mask (NIV)6,7 is generally used as a surrogate for adequate preoxygenation.8, 9, 10, 11 Pulse oximetry readings can also be used as an adjunct for the determination of adequate preoxygenation; however, this device does not directly measure the percentage of oxygen present in the lungs.10
1.2. Importance
In-line end-tidal oxygen (EtO2) measurements have been well studied in operating room settings and directly correlate with the prevention of oxygen desaturation events when the EtO2 (the percentage of oxygen in an exhaled breath) is greater than 90% before the induction of apnea.1,10,12,13 In contrast to the operating room setting, the use of bag-valve masks to preoxygenate patients is generally avoided in the ED setting,13 limiting the feasibility of in-line EtO2 detection. Single-breath (SB) EtO2 measurement is the gold standard for preoxygenation and has been well-validated.7,14, 15, 16 Unfortunately, this method is also impractical in patients with respiratory distress. The most pragmatic EtO2 monitoring method in the ED setting is via real-time oxygenation sensors attached to a nasal cannula (NC); however, limited research exists examining the accuracy of real-time EtO2 measurements in patients receiving supplemental oxygen.
1.3. Goals of This Investigation
We hypothesized that the supplemental oxygen flowing through the EtO2 sensor would falsely elevate the readings, but that this error created by supplemental oxygen could be quantified. Our primary outcome was to estimate the concordance of EtO2 between real-time NC and SB in healthy volunteers receiving supplemental oxygen for 3 minutes. Our secondary outcome was to evaluate the difference between SB EtO2 at the time when the NC device reached maximum EtO2 (TmaxNC) compared with SB EtO2 at 3 minutes.
2. Methods
2.1. Study Setting and Selection of Subjects
Volunteers were recruited from the Grady Memorial Hospital staff, the Emory University School of Medicine, and the Morehouse School of Medicine and enrolled between September 2019 and October 2023. Enrollment was paused from February 2020 until July 2023 due to the COVID-19 pandemic. A total of 105 participants began the study, with 1 withdrawing prior to any data collection due to discomfort from NIV. The study was approved by our institutional review board (IRB) and registered at ClinicalTrials.gov (NCT03840486). We obtained written consent from all participants and allowed them to withdraw from the study at any time.
Inclusion criteria included the following: age >18 years, self-identified as being in good health, grossly normal dentition as determined by study investigators, no self-reported symptoms of upper respiratory infection or other infectious process, no history of severe pulmonary disease or asthma requiring daily use of an inhaler, and for female participants, self-report of not being pregnant. Study participants were excluded if they did not agree to study enrollment or were unable to tolerate the entire course of supplemental oxygen required to complete the study.
2.2. Study Design and Interventions
Following screening and informed consent, participants were randomized to either the (1) NRBM or (2) NIV arms of the study, along with providing study investigators with their self-reported age, gender, height, and weight (Fig 1). The NC device (NomoLine LH Adult Nasal/Oral CO2 Cannula; Masimo) was placed on each participant, and investigators confirmed adequate morphology of EtO2 waveforms on the gas analyzer monitor (ISA OR+ module and Root monitor). SB EtO2 measurements were obtained using a gas analyzer (Handi+, model R218P12; Maxtec) attached to a piece of standard corrugated ventilator tubing.
Figure 1.
Study flow diagram for the real-time end-tidal oxygen (ETO2) measurement study.
FiO2, fraction of inspired oxygen; LPM, liters per minute; NC, nasal cannula; NIV, noninvasive ventilation mask; NRBM, nonrebreather mask; PEEP, positive end-expiratory pressure; SB, single-breath.
Before delivering supplemental oxygen, an SB EtO2 measurement was obtained to establish a baseline. Then, the NRBM (1059 adult nonrebreather mask with safety vent; Hudson RCI) or NIV mask (AF531 Oro-Nasal Mask, size medium on V60 ventilator; Philips Respironics) was placed over the NC device. Each subject participated in 3 3-minute trials of variable oxygen delivery. For the NRBM arm, this entailed trials at 15 LPM, 35 LPM, and 55 LPM. In the NIV arm, trials consisted of a 40% fraction of inspired oxygen (FiO2), 70% FiO2, and 100% FiO2, all at positive end-expiratory pressure = 10 cm H20. For the NRBM study arm, an oxygen flow meter capable of measuring up to 70 LPM (Oxygen Chrome Flowmeter 1MFA8001EC; Precision Medical) was used to achieve accurate flow rates. Study participants were randomized regarding the order in which they received the varied flow and FiO2 rates.
After each trial, study investigators noted the NC EtO2 reading and requested that the study participants hold their breath. With the assistance of the study investigators, the study participants removed the NRBM or NIV mask and breathed into the SB EtO2 gas analyzer. At the end of each trial and following several minutes of breathing room air, SB EtO2 readings were obtained to ensure that the study participant’s SB EtO2 readings returned to their room air baseline. This process was repeated twice at a different LPM or FiO2 (depending on group allocation).
To determine the time required to reach the maximum EtO2 on the NC device (TmaxNC), a fourth trial was performed at 55 LPM or 100% FiO2 (depending on group allocation), following the NC level continuously until it reached its previous maximum. We then recorded the time in seconds and measured an SB EtO2 at that time (TmaxNC), allowing for a comparison of SB EtO2 at TmaxNC versus SB EtO2 at a time of 3 minutes.
2.3. Outcomes
The primary outcome was the degree of oxygenation measured at each flow rate. The study was sufficiently powered to detect a 5% difference in EtO2 between the NC and SB measurements. Five percent was chosen because it equates to approximately 30 extra seconds of safe apnea time in an 80-kg male (5% × 2400 mL/O2 consumption at 250 mL/min). Based on previous studies with similar designs, we estimated that the standard deviation of EtO2 would be approximately 6.8%.7,15 These values were used to estimate the sample size required to achieve 80% power at an alpha of 0.0085 (Sidak-corrected P value for 6 tests). This analysis returned a total sample size of 104 (52 per arm).10 The secondary outcome was the time taken to reach the maximum EtO2 measurements.
2.4. Data Analysis
Categorical variables were described using frequencies and percentages. Continuous/scale variables were described using medians with interquartile ranges and means with standard deviations. The degree of association between NC EtO2 and SB EtO2 across the oxygen delivery levels was assessed in 2 ways. First, because EtO2 is a percentage, this was evaluated using mixed-effects regression with a beta distribution and logit link. The mixed-effects model was used to account for multiple measurements from the same participant. To assess the degree of association, we present (1) the pseudo-R2 (ie, the square of the correlation between the model-predicted and observed values) and (2) the root mean squared error (RMSE; ie, the square root of the mean of the squared errors). Second, we computed the paired sample mean difference and 95% confidence interval for that difference between SB EtO2 and NC EtO2. Confidence intervals were computed using bias-corrected and accelerated bootstrap resampling (100,000 resamples were used to generate stable estimates). For our secondary outcome, we compared the NIV and NRBM groups for the time taken (TmaxNC) to reach the maximum EtO2 measurements. Mean and median times were estimated using the Kaplan-Meier method. Kaplan-Meier curves were compared using the log-rank test. Analyses were conducted using R (v. 4.3.2; R Core Team, 2023). SB EtO2 at TmaxNC and SB EtO2 at 3 minutes were compared by calculating the paired sample mean difference and 95% confidence interval for that difference.
3. Results
A total of 104 participants completed the study. The majority identified as female (64.4%) and the median age was 28 (IQR, 26-33.5) years. The median baseline body mass index was 24.8 (IQR, 21.9-28.3) and the median SB baseline EtO2 was 16.8% (IQR, 16.1-17.5). Table 1 presents the baseline characteristics stratified by the study arm.
Table 1.
Baseline characteristics of study participants in the real-time end-tidal oxygen (ETO2) measurement study, September 2019-October 2023.
| Characteristic | NIV | NRBM | Total |
|---|---|---|---|
| Age (y), median (IQR) | 28 (26-34) | 27 (25-34.5) | 28 (26-33.5) |
| Gender, n (%) | |||
| Female | 30 (57.7) | 37 (71.2) | 67 (64.4) |
| Male | 22 (42.3) | 15 (29.8) | 37 (35.6) |
| BMI (kg/m2), median (IQR) | 24.7 (23-28.2) | 24.9 (21.5-28.3) | 24.8 (21.9-28.3) |
| Baseline EtO2, median (IQR) | 16.8% (16.1%-17.3%) | 16.8% (16%-17.8%) | 16.8% (16.1%-17.5%) |
BMI, body mass index; IQR, interquartile range NIV, noninvasive ventilation; NRBM, nonrebreather mask.
EtO2 measurements between NC and SB were significantly correlated in both arms of the study, although this correlation was considerably stronger in the NIV arm (Fig 2, Table 2). For NIV participants, the pseudo-R2 was 0.72 (95% CI, 0.56-0.83), and the model RMSE indicated that the predicted SB value differed from the observed SB value by an average of approximately 8.5% EtO2. For the NRBM arm, the pseudo-R2 was 0.43 (95% CI, 0.30-0.55), and the model RMSE indicated that the predicted SB value differed from the observed SB value by an average of approximately 11.7% EtO2. The formulas for determining SB EtO2 based on the NC EtO2 readings for the NIV and NRBM groups can be found in Supplementary Appendix 1.
Figure 2.
Scatterplot describing the association between real-time end-tidal oxygen nasal cannula measurements and single-breath end-tidal oxygen measurements in the noninvasive ventilation (top) and nonrebreather mask (bottom) study arms. The solid black line depicts the line of best fit from the beta regression; the dashed lines depict the 95% confidence interval for the line of best fit.
FiO2, fraction of inspired oxygen; LPM, liters per minute; NIV, Noninvasive ventilation mask; NRBM, Nonrebreather mask.
Table 2.
Regression table describing the association between real-time end-tidal oxygen nasal cannula measurements and single-breath end-tidal oxygen measurements in the noninvasive ventilation (NIV) and nonrebreather mask (NRBM) study arms.
| Group | pR2 (95% CI) | r | RMSE | P |
|---|---|---|---|---|
| NIV | 0.72 (0.56-0.83) | 0.85 | 8.46 | < .001 |
| NRBM | 0.43 (0.30-0.55) | 0.66 | 11.68 | < .001 |
CI, confidence interval; pR2, pseudo R-squared; r, linear correlation; RMSE, root-mean-square error.
The mean differences between the NC EtO2 and SB EtO2 measurements following 3 minutes in the NIV group were 1.8% (1.1%-2.7%), 5.1% (3.5%-7.1%), and 10.0% (7.3%-14.3%), at 40% FiO2, 70% FiO2, and 100% FiO2, respectively (Fig 3 top and Table 3). In the NRBM arm, mean differences between NC EtO2 and SB EtO2 at 3 minutes were 13.1% (10.3%-16.8%) at 15 LPM, 18.0% (14.9%-21.4%) at 35 LPM, and 17.1% (13.9%-21.0%) at 55 LPM (Fig 3 bottom and Table 3). All 6 comparisons were statistically significant (P < .0085).
Figure 3.
Mean absolute differences between the single-breath (SB) end-tidal oxygen measurements (ETO2) and real-time nasal cannula (NC) ETO2 stratified by flow rates/oxygen concentrations for the noninvasive ventilation (NIV, top) and nonrebreather mask (NRBM, bottom) groups. The bottom and top of the boxes depict the 25th and 75th percentiles, respectively. The line contained within the boxes depicts the median. The diamonds depict the mean.
FiO2, fraction of inspired oxygen; LPM, liters per minute.
Table 3.
Nasal cannula (NC) real-time end-tidal oxygen (ETO2) measurements and single-breath (SB) ETO2 measurements across increasing oxygen concentrations and flow rates for the noninvasive ventilation (NIV) and nonrebreather mask (NRBM) study arms stratified by flow rate/concentration and study arm.
| Parameters | NC | SB | Difference (95% CI) |
|---|---|---|---|
| NIV | |||
| 40% FiO2 | |||
| Mean (±SD) | 34.3% (4.8) | 32.5% (4.7) | 1.8% (1.1-2.7) |
| Median (IQR) | 34% (33- 35.8) | 33.1% (29.9-34.6) | |
| 70% FiO2 | |||
| Mean (±SD) | 61.6% (4.4) | 56.6% (7.7) | 5.1% (3.5-7.1) |
| Median (IQR) | 63% (6164) | 58.4% (52.2-61.5) | |
| 100% FiO2 | |||
| Mean (±SD) | 90.1% (5.0) | 80.1% (13.3) | 10.0% (7.3-14.3) |
| Median (IQR) | 92% (91-93) | 84.6% (73.7-88.1) | |
| NRBM | |||
| 15 LPM | |||
| Mean (±SD) | 57.8% (11.6) | 44.8% (6.9) | 13.1% (10.3-16.8) |
| Median (IQR) | 58.5% (48.3-66) | 44.9% (38.7-50.1) | |
| 35 LPM | |||
| Mean (±SD) | 78.2% (8.9) | 60.2% (12.2) | 18.0% (14.9-21.4) |
| Median (IQR) | 79% (72.3-84.8) | 60.8% (51.4-69.4) | |
| 55 LPM | |||
| Mean (±SD) | 86.4% (5.7) | 69.3% (14.8) | 17.1% (13.9-21.0) |
| Median (IQR) | 87.5% (83-91) | 70.5% (60.3-82.1) |
FiO2, fraction of inspired oxygen; IQR, interquartile range; LPM, liters per minute; SD, standard deviation.
The mean time to reach a maximum reading on NC EtO2 (TmaxNC) was 130 seconds (118 - 140 seconds) in the NIV arm and 116 seconds (95-138 seconds) in the NRBM arm (Table 4). There was no significant difference between the two groups (P = .14). The mean difference between SB EtO2 at TmaxNC and SB EtO2 at 3 minutes was −1.5% (−4.7% to 1.7%) and 1.1% (−2.0% to 4.6%) in the NIV and NRBM groups, respectively. Appendix 2 contains the Kaplan-Meier curves depicting the time it takes to reach TmaxNC.
Table 4.
Time required to reach the 3-minute nasal cannula (NC) end-tidal oxygen (EtO2) reading and the mean difference between the single-breath (SB) EtO2 measurement at TmaxNC and 3 minutes for the noninvasive ventilation and nonrebreather mask arms.
| Parameter | Mean time to TmaxNC (95% CI) | Median time to TmaxNC (95% CI) | SB EtO2 at 3 minutes (95% CI) | SB EtO2 at TmaxNC (95% CI) | Mean Difference (95% CI) |
|---|---|---|---|---|---|
| NIV | 129.8 s (117.5-142) | 126 (107.2-144.8) | 80.1% (75.9-83.2) | 78.5% (75.0-81.7) | −1.5% (−4.7-1.7) |
| NRBM | 116.2 s (94.7-137.4) | 100 (88.5-111.5) | 69.3% (65.1-73.1) | 70.4% (66.3-73.8) | 1.1% (−2.0-4.6) |
NIV, noninvasive ventilation study arm; NRBM, nonrebreather mask study arm.
4. Limitations
This study was limited to healthy volunteers. While this inclusion criterion allowed for safely evaluating the correlation between NC EtO2 and SB EtO2, this study population does not reflect the target population of those in the ED who are critically ill and who often have underlying lung pathology and inadequate ventilation. This correlation would be less reliable in patients with hypoventilation, as the NC sensor might theoretically report a higher EtO2 reading due to a higher ratio of ambient supplemental oxygen to expired gas. It is also possible critically ill patients with hypoventilation could be found to have falsely decreased time to first reach maximum NC EtO2 for the same reason.13,17 Additionally, in having our participants hold their breath before breathing into the SB sensor, it is possible that the larger last breath may have spuriously elevated the EtO2 reading. From a practical standpoint, this is unavoidable and reflects the methodology of other, similar studies.15,18 To address these limitations, future directions would include evaluating real-time NC ETO2 measurements in a large, randomized controlled trial enrolling ED patients presenting with respiratory failure.
5. Discussion
In our study of healthy volunteers, we compared the NC EtO2 device with the gold standard SB EtO2 device in participants receiving NIV at different FiO2 levels and NRBM at different flow rates and saw a significant correlation between NC EtO2 and SB EtO2 in each arm at 3 minutes. In general, increasing O2 delivery introduced more EtO2 discordance between the devices.
Previous studies have demonstrated that hypoxic events are common during ED ETI19 and higher EtO2 readings directly correlate with longer safe apnea times,20 yet EtO2 is rarely used in the ED setting.21 In the most practical application of EtO2 tracking in the ED setting to date, Oliver et al22 determined that monitoring EtO2 periintubation significantly increased the quality of preoxygenation, with 67% of monitored patients reaching an EtO2 > 85% compared with 26% without monitoring.18 This was associated with a decrease in hypoxic events from 18% to 8%. One limitation of this study, as pointed out by the authors, is that they did not account for the potential error created by the supplemental oxygen being delivered, and a smaller proportion of patients may be reaching an EtO2 > 85%. Our study addresses this gap by quantifying the effects of supplemental oxygen blowing by the device to better inform the appropriate implementation of NC EtO2 monitoring in the ED.
For our primary outcome, we set an a priori mean difference of 5% between SB EtO2 and NC EtO2 as a surrogate for 30 seconds of apnea time. Most of the NC EtO2 readings were not within our clinically significant 5% of SB EtO2 readings but demonstrate the better performance of the NC EtO2 device at lower O2 delivery levels (ie, 15 LPM and 40% FiO2). These data confirm our hypothesis that a portion of administered oxygen was detected using the NC EtO2 sensor across all groups, particularly in the NRBM arm. This may potentially confound the clinician’s ability to discern the patient’s actual (ie, SB) EtO2. Based on the mean differences between NC EtO2 and SB EtO2 in our study, one would expect the SB EtO2 to be, on average, 13% to 17% lower than that displayed on the NC EtO2 reading in a healthy person on an NRBM and, on average, 2% to 10% lower in a healthy person breathing NIV. This result is to be expected as the NIV system is a closed circuit (except for an intentional air leak built into the system), whereas the NRBM is a semi-open system that allows active entrainment of room air, which would be expected to affect the real-time EtO2 readings.
While the mean differences above provide a quick estimation of the patient’s true EtO2 at the bedside, the correlations between the NC and SB devices allow for a more accurate representation of the patient’s EtO2 (Supplementary Appendix 1). Our data show that, in healthy volunteers, EtO2 estimated from the NC device is highly correlated with actual SB EtO2 in the NIV group and moderately correlated with actual SB EtO2 in the NRBM group. These findings suggest that NC EtO2 can be used as a proxy for SB EtO2 when the latter is not available.
The EtO2 values obtained in our study were lower than those seen in previous studies evaluating preoxygenation with various devices. Specifically, our mean SB EtO2 reading for NRBM at 15 LPM was 44.8% at 3 minutes, which is lower than the 52% to 74% range as reported in other studies.7,14, 15, 16,23 This may have been related to increased mask leak/entrainment of room air at this lower flow rate. In contrast, the mean SB EtO2 of 69.3% at 55 LPM was closer to the 74% to 86% seen in other studies, suggesting that the additional flow limited the mixing of room air.7,24 The only other comparable study for NIV found similar SB EtO2 readings as ours at 100% and positive end-expiratory pressure of 10 cc H20, (78.6% and 80.1%, respectively).25 These similarities at higher flow rates between our study and others increase the reliability of our findings.
For our secondary outcome, the time required to reach maximum NC EtO2 was shorter than the traditionally accepted 3 minutes required for adequate preoxygenation,10 and the accuracy of this was confirmed using SB EtO2 readings. The small, nonsignificant differences observed between SB EtO2 at TmaxNC and 3 minutes suggest that the supplemental oxygen was not “overwhelming” the NC EtO2 device, ie, displaying an EtO2 value higher than was in the lungs, at the earlier time point. In the clinical setting, this may translate into a simple method for earlier detection of adequate preoxygenation and therefore an earlier time to safely initiate endotracheal intubation.
Our study demonstrated that estimates of a healthy volunteer’s EtO2 via an NC sensor can be made with a reasonable degree of accuracy, even in the setting of supplemental oxygen delivery. This real-time data could allow improved decision-making during ED endotracheal intubation by more accurately identifying the preoxygenation plateau and more reliably estimating preoxygenation adequacy. This could result in a quicker decision to proceed with intubation if EtO2 measurements are deemed sufficient. Conversely, if EtO2 measurements are not considered adequate, different ETI techniques other than rapid sequence intubation (such as delayed sequence intubation or awake intubation) or different preoxygenation strategies can be employed. This knowledge of the patient’s preoxygenation status could have a direct effect on decreasing severe desaturations during ED endotracheal intubation.
Author Contributions
SC and TM conceived the study, designed the protocol, and obtained research funding. SL and SC supervised the conduct of the trial and data collection. SL, KM, NJ, JD, JA, AM, RM, JS, and SC undertook the recruitment of participating centers and patients and managed the data. TM provided statistical advice on study design and analyzed the data. SL drafted the manuscript and all authors contributed substantially to its revision. SL and SC take responsibility for the paper in its entirety.
Funding and Support
This study was funded by the Emory Medical Care Foundation internal grant.
Conflict of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Footnotes
Presented at the Society for Academic Emergency Medicine Lightning Oral Abstract Presentation. May 16, 2024. Phoenix, Arizona.
Presented at the Society for Academic Emergency Medicine Lightning Oral Abstract Presentation. May 17, 2024. Phoenix, Arizona.
Clinical trial registration number: NCT03840486
Supervising Editor: Nicholas Caputo, MD, MSc
Supplementary material associated with this article can be found in the online version at https://doi.org/10.1016/j.acepjo.2025.100079
Supplementary Materials
References
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