Anesthesiology

Comparison of saline versus air for identifying endotracheal intubation with ultrasound

a b s t r a c t

Introduction: After intubation has been performed, it is important to rapidly confirm the correct location of the endotracheal tube . Multiple techniques have been described, each with different limitations. Ultrasound has been increasingly recognized as an alternate modality for identifying the ETT location. However, it can be challenging to visualize the air-filled ETT cuff. Saline insufflation of the ETT cuff has been suggested to improve visualization of the ETT but data are limited. Our study sought to compare the diagnostic accuracy of air versus saline ETT cuff inflation on the diagnostic accuracy of intubation.

Methods: This was a randomized trial comparing air versus saline cuff inflation using a cadaver model. Adult ca- davers were intubated in a random sequence with respect to both the location of intubation (i.e., tracheal vs esophageal) and air versus saline. Blinded sonographers assessed the location of the ETT using the static tech- nique. Outcomes included accuracy of sonographer identification, Time to identification, and operator confidence. Results: 480 total assessments were performed. When using air, ultrasound was 95.8% sensitive (95% CI 90.5% to 98.6%) and 100% specific (95% CI 97.0% to 100%) with a mean time to confirmation of 8.5 s (95% CI 7.6 s to 9.4 s) and a mean operator confidence of 4.32/5.0 (95% CI 4.21 to 4.42). When using saline, ultrasound was 100% sen- sitive (95% CI 97.0% to 100%) and 100% specific (95% CI 97.0% to 100%) with a mean time to confirmation of 6.3 s (95% CI 5.9 s to 6.8 s) and a mean operator confidence of 4.52/5.0 (95% CI 4.44 to 4.60).

Conclusion: There was no statistically significant difference between air versus saline for Intubation confirmation. However, saline was associated with fewer false negatives. Additionally, time to confirmation was faster and op- erator confidence was higher with the saline group. Further studies should determine if the outcomes would change with more novice sonographers or in specific patient populations.

(C) 2022

  1. Introduction

Endotracheal intubation is a common procedure in Emergency Med- icine. Literature has suggested that the First pass success rate is only 83% [1]. Numerous techniques have been described for the confirmation of Endotracheal tube position, though each have unique limitations in specific situations [2-4]. Moreover, while quantitative capnography is often considered the gold standard, it can be less accurate in low flow states such as cardiac arrest [3].

* Corresponding author at: 1750 West Harrison Street, Suite 108 Kellogg, Chicago, IL 60612, United States of America.

E-mail address: [email protected] (M. Gottlieb).

Recent research has supported the role of point-of-care ultrasound to confirm the ETT location [5,6]. This has even led to the 2015 American Heart Association Advanced Cardiac Life Support Guidelines including ultrasound as a reasonable alternative to capnography among patients in cardiac arrest [7]. Despite this, uptake of ultrasound for this application has been variable. One potential reason for this is that it can be more challenging to visualize the ETT in the trachea due to the air-filled cuff potentially mimicking the empty trachea.

Recently, researchers have instilled saline instead of air to better vi-

sualize the ETT cuff when assessing depth of placement among pediatric patients [8-10]. Another very small study evaluated saline for confirm- ing ETT location and demonstrated good accuracy, but did not compare this against air insufflation [11]. However, no study to date has evalu- ated the role of saline versus air for identifying tracheal versus

https://doi.org/10.1016/j.ajem.2022.05.053

0735-6757/(C) 2022

Esophageal intubation. Our study sought to evaluate the diagnostic ac- curacy, time to confirmation, and operator confidence with using saline versus air for ETT cuff insufflation.

  1. Methods

This was a blinded, randomized trial performed in the Cadaver lab at Rush University Medical Center. Two cadavers with markedly different neck sizes were utilized to simulate the variation in patient populations. Institutional review board approval was obtained from Rush University Medical Center for this study with waiver of informed consent.

Prior to the study, a random number generator was used to random- ize whether the ETT would be placed in the trachea or esophagus with a goal of having equal numbers of tracheal and esophageal intubations in order to best determine the test characteristics. An attending Emergency Medicine physician with extensive intubation experience performed all intubations using video laryngoscopy prior to the sonog- rapher entering the room. The intubating physician insufflated the ETT cuffs with 10 cc of either air or saline, alternating between air or saline for each round. After the intubation was performed, the intubator exited the room to avoid any potential reaction that could bias the sonogra- phers. After the intubation, each sonographer would sequentially enter the room and assess the location of the ETT using the static tech- nique. Only one sonographer was present in the room at a time.

Six ultrasound fellowship-trained attending physician sonographers with prior experience in the use of ultrasound for ETT confirmation in- dependently performed the assessments. A Zonare ZS-3 ultrasound ma- chine with an 14-5 MHz linear transducer was utilized for all assessments. To assess ETT location, the sonographer would place the ultrasound transducer across the neck at the suprasternal level and vi- sualize the trachea. The ETT would then be twisted with the non- dominant hand to visualize the presence of motion artifacts within the trachea. If the ETT or motion artifact were visualized within the trachea, it was deemed a tracheal intubation (Figs. 1 and 2). The transducer was also slid laterally in each direction to identify the esophagus. The pres- ence of the ETT within the esophagus was considered diagnostic of an esophageal intubation (Figs. 3 and 4).

For each assessment, a research assistant recorded the sonographer prediction of the ETT location, time to ETT prediction, and operator con- fidence. Operator confidence was assessed utilizing a Likert scale rang- ing from 1 to 5 with 1 signifying “not confident at all” and 5 signifying “very confident”. Comparison between the predicted and actual location was performed after study completion.

Analyses were performed using Statistical Package for the Social Sci- ences (SPSS, Inc., Armonk, NY), version 23.0. A priori, it was determined 472 total assessments were needed using the following assumptions: alpha 5%, beta 20%, estimated correct identification of 95% with saline

Image of Fig. 2

Fig. 2. Tracheal intubation with saline-insufflated endotracheal tube cuff.

or air, and a non-inferiority limit of 5%. Operator confidence and time to ETT prediction were assessed using descriptive statistics including means and standard deviations. Diagnostic accuracy was assessed by sensitivity and specificity. Data were calculated with 95% confidence in- tervals (95% CIs).

  1. Results

A total of 480 intubation assessments were performed with 240 tra- cheal intubations and 240 esophageal intubations. When the ETT cuff was filled with saline (n = 240), ultrasound was 100% sensitive (95% CI 97.0% to 100%) and 100% specific (95% CI 97.0% to 100%). When the ETT cuff was filled with air (n = 240), ultrasound was 95.8% sensitive (95% CI 90.5% to 98.6%) and 100% specific (95% CI 97.0% to 100%). The difference was not statistically significant but all five false negatives were in the air cohort.

When comparing mean time confirmation, the saline group was faster (6.3 s [95% CI 5.9 s to 6.8 s] vs 8.5 s [95% CI 7.6 s to 9.4 s]). The op- erators were also more confident in the saline cohort (4.52/5.0 [95% CI 4.44 to 4.60] vs 4.32/5.0 [95% CI 4.21 to 4.42]).

  1. Discussion

It is important to rapidly and accurately confirm ETT placement in the Emergency Department setting. Prior research has demonstrated that ultrasound is accurate for the confirmation of ETT location [6]. This has been consistent regardless of ETT size, transducer type, or tech- nique [12-16]. However, most literature has relied upon assessing an air-filled ETT cuff, which can be harder to visualize in the trachea as both are air-filled, circular structures. Saline offers the benefit of better sound wave transmission, which may improve visualization compared

Image of Fig. 1 Image of Fig. 3

Fig. 1. Tracheal intubation with air-insufflated endotracheal tube cuff. Fig. 3. Esophageal intubation with air-insufflated endotracheal tube cuff.

Image of Fig. 4

Fig. 4. Esophageal intubation with saline-insufflated endotracheal tube cuff.

with air alone. No study has directly compared the accuracy of these two techniques.

This was the first study to directly compare the accuracy of ultra- sound for ETT confirmation using an air-filled versus saline-filled ETT cuff. While we anticipated a much larger difference, we noted only a small improvement in accuracy with saline. This may be due to the high baseline accuracy with both techniques. However, it is worth not- ing that all five false negatives (i.e., tracheal intubation which was incor- rectly identified as esophageal) were in the air group suggesting that saline may be more beneficial for identifying tracheal intubations than esophageal intubations.

Additionally, we found that the time to confirm was faster with sa- line. We further found that operator confidence was higher with the sa- line group. This may be due to enhanced visibility with saline despite acceptable visibility with air in most instances. While these were statis- tically significant, it is unclear the clinical significance of this difference. While the present study does not support the routine use of saline for ETT cuff insufflation when using ultrasound, this may still be valu- able when images are more limited or if the initial view was unable to confirm the location with an air-filled cuff. However, it is important to balance this with the risk of aspiration should the balloon rupture after instilling saline. Therefore, it is important to assess the ETT balloon prior to the intubation. Once the ETT location has been confirmed with saline and the ETT is secured, we recommend removing the saline and

replacing the ETT cuff with air.

  1. Limitations

This study has several important limitations. First, as a cadaver study, this may not fully reflect the appearance and characteristics of a live patient. However, cadaver models have been extensively used when assessing the accuracy of ultrasound for ETT confirmation and have demonstrated similar test characteristics to Live patients for this technique [11-18]. While the cadavers were intentionally selected to ensure differences in neck size, the limited number of cadavers may not fully reflect the broader population. Finally, we used experienced sonographers for the assessments and it is unclear if more novice sonog- raphers may have a more marked difference. Future research will be needed to evaluate this group.

  1. Conclusion

There was no statistically significant difference between air versus saline for intubation confirmation. However, saline was associated with fewer false negatives. Additionally, time to confirmation was faster and operator confidence was higher with the saline group. Further

studies should determine if the outcomes would change with more nov- ice sonographers or in specific patient populations.

Disclosures/funding

This research did not receive any specific grant from funding agen- cies in the public, commercial, or not-for-profit sectors. We have no dis- closures to declare.

Meetings

None.

IRB

Approved.

CRediT authorship contribution statement

Michael Gottlieb: Writing – review & editing, Writing – original draft, Supervision, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Daven Patel: Writing – review & editing, Writing – original draft, Project administration, Investigation, Data curation, Conceptualization. Christine Jung: Writing – review & editing, Writing – original draft, Project administration, Investigation, Data curation, Conceptualization. David Murray: Writing – review & editing, Writing – original draft, Project administration, Investigation, Data curation, Conceptualization. Molly Hartrich: Writing – review & editing, Writing – original draft, Project administration, Investigation, Data curation, Conceptualization. Evelyn Schraft: Writing – review & editing, Writing – original draft, Project administration, Investigation, Data curation, Conceptualization. Jasmine Ginn: Writing – review & editing, Writing – original draft, Project administration, Investigation, Data curation, Conceptualization. Gary D. Peksa: Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Data curation, Conceptualization. Tina Sundaram: Writing – review & editing, Writing – original draft, Project administration, Investigation, Data curation, Conceptualization.

Declaration of Competing Interest

We have no conflicts of interest nor financial support to disclose.

Acknowledgements

We would like to thank Louis Hondros MD, James Williams PhD, Da- vid Karwo, the Rush Anatomy Lab, and the Rush Simulation Lab for their assistance with this study.

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