In operation theatre settings, endotracheal intubation is considered a fundamental procedure to preserve the airway during general anesthesia. Assurance of proper endotracheal tube (ETT) placement is crucial immediately after intubation as undiagnosed esophageal intubation can be catastrophic. Screening of anesthesia mortality discovered that 69% of the deaths were related to airway management [1].

Direct visualization of the ETT passing through the vocal cords is often used in practice, but it is not possible at all times, especially if laryngoscopy is difficult. The alternative methods of confirmation include observation of chest rising after intubation and auscultation of both lungs are varying in their degree of accuracy.

Moreover, 55% of one lung intubations may pass undiagnosed by chest auscultation [2,3]. Wave form capnography has been considered as the most reliable procedure for confirmation of proper ETT placement with (1.00) sensitivity and (1.00) specificity so that waveform capnography can be approved as the typical technique for the fundamental realization of ETT position [4]. Recent guidelines recommended continuous waveform CO₂ monitoring in addition to clinical evaluation as the most accurate measures to approve the accurate position of an endotracheal tube (ETT). However, End-tidal carbon dioxide (ETCO₂) measurement requires a minimum of 5 “breathing cycles” for validation, which can lead to gastric insufflation and numerous complications if the ETT was falsely set in the oesophagus, also capnography can ’ t discriminate between main tracheal and endobronchial placement of endotracheal tube [5].

Tracheal ultrasonography (TUS) has proven utility as a non-invasive tool for confirmation of ETT position that occurs in real time and provide appropriated results in patients undergoing general anesthesia. TUS discovers oesophageal intubation even before ventilating the patient, which prevents unnecessary forced ventilation to the stomach and its associated complications [6,7].

Unrecognized one lung intubation may lead to numerous complications. Ultrasonography of the pleura can provide through anatomic data of lung expansion and therefore of ventilation. In normal efficient, ventilated lungs, the 2 layers of pleura (parietal and visceral pleura) can be viewed by ultrasound as distinctive bright borders or echogenic lines. With ventilation, these two layers glide upon each other, this is commonly referred to as the sliding lung sign (SLS) [8], and this should specify endotracheal intubation so the physicians can be able to discriminate between right main stem bronchus and appropriate endotracheal intubation by observing for the lung sliding sign in both sides of chest, as a right main stem bronchus intubation induces absent lung sliding sign on the left lung [9,10].

The primary purpose for this study was to determine the diagnostic power of the integrated tracheal and sonographic estimation of pleural sliding for the rapid verification of endotracheal tube location during general anesthesia with reference to CO₂ monitors (capnography) (the typical technique), also to evaluate the capability of SLS to discriminate between main tracheal and right main stem bronchial intubation. A secondary outcome was to determine the elapsed time before validation of endotracheal tube position.

After approval of the ethical committee, this prospective observational study was carried out in the operative theatre (OT) in Aswan University Hospital between September 2018 and March 2019 on 80 patients who required intubation for general anesthesia during elective surgeries. Patients with age less than 18 years, history of difficult intubation, anatomic neck distortion, abnormal airway anatomy or lung pathologies that would affect the study technique were excluded from the study. On admission to OT, monitors were connected and baseline heart rate, systolic, diastolic and mean arterial blood pressure, pulse oximetry, and ECG were collected.

Intubations were done by the anesthesia residents and modified Cormack-Lehane score obtained by direct laryngoscopy based on the structures seen was documented for every intubation trial. Combined bedside ultra-sonography was carried out immediately after intubation by the examiner who was not concerned in the patients' care and not informed about the capnography results. A SonoScape ultrasound (Model: A5, SonoScape (CHINA) CO, LTD) equipped with a 9-12 MHz linear transducer was used for tracheal and lung sliding ultrasonography.

Immediately after intubation, the transducer was placed transversely on the anterior neck just above the suprasternal notch and then shifted to the left to scan whether the oesophagus was empty or distended by ETT (Figure 1). The ETT was defined as “endotracheal” if only a one hyperechoic air-mucosa (A-M) interface with a comet-tail artifact was observed or “intraesophageal” if a second A-M interface, imitating an additional airway (double-tract sign) was seen (Figure 2).

Figure 1: Tracheal ultrasonography shows one air-mucosa interface with comet-tail artifact in tracheal intubation.

Figure 2: Tracheal ultrasonography shows two air-mucosa interfaces with double-tract sign in oesophageal intubation.

To detect SLS over both lungs, the probe was placed horizontally on both sides; Images were taken within the third to fifth intercostal spaces in the midclavicular line and a positive SLS signified lung expansion with ventilation. On the basis of the presence or absence of the sliding lung sign on both sides of the chest, a discrimination of endotracheal tube location was made, presence on both sides of the chest was presumed to imply tracheal intubation. SLS presence on the right but absence on the left was assumed to specify right main stem bronchial intubation. Finally, absence of SLS on either side was presumed to denote oesophageal intubation (Figure 3).

Figure 3: SLS in ultrasonography.

Subsequently, the correct position of the endotracheal tube was established by the anesthesia resident via the use a capnometer (Scio Four, Drager, Germany), a positive result of capnography was defined as the finding of exhaled CO₂ (>4 mm Hg) after 5 breathing cycles with a distinctive CO₂ waveform.

An anesthesia technician using a stopwatch calculated the durations from the end of the endotracheal tube insertion to the time when the sonographer had interpreted the sonographic results and to the time of obtaining the results of capnography. A data collection sheet was written down to record the patients' age, gender, BMI, modified Cormack-Lehane score, capnography results, ultrasonography results and the time passed preceding tube position affirmation.

Statistical analysis

Date analysis were done using SPSS (Statistical Package for Social Science) software program version 19.0 (SPSS Inc., Chicago, IL). Data were presented as number, percentage, mean and standard deviation. Pearson correlation was done to measure correlation between quantitative variables. ROC Curve was used to assess the accuracy of TUS, SLS and the combined TUS and SLS against wave form capnography as the standard criterion. P-value was considered statistically significant if <0.05.

Major catastrophes during general anesthesia result from inability to intubate the trachea following administration of a muscle relaxant and failure to detect it in time [11]. The role of Tracheal Ultrasonography (TUS) in confirming appropriate endotracheal tube placement during general anesthesia has gained significance not only in the operating room but also in the intensive care unit and emergency department.

A systematic review and meta-analysis by Chou et al. [12] involving 12 eligible studies together with another one done by Das et al.[13] included eleven studies and 969 intubations, supported that tracheal ultrasonography has high significance for approving endotracheal intubation and recognizing oesophageal intubation with a reasonable degree of sensitivity and specificity. Moreover, Osman and colleagues concluded that airway ultrasonography can replace waveform capnography in confirming ETT placement in centers without capnography with an overall accuracy 98.1% [14].

We found that TUS detected five oesophageal intubations from six cases detected by continuous wave form capnography and one detected as endotracheal (false positive) which explained as the esophagus was detected immediately posterior to the trachea in this patient rendering judgment difficult. Moreover, it did not detect three out of 74 tracheal intubations (false negatives), which may explained by that the accuracy of static assessments may possibly less than that of the dynamic ones because better to visualize the motion artifact with the last technique [15]. In the current study, TUS achieved high sensitivity and PPV (95.95% and 98.6%) with 95.00% accuracy and AUC 0.896 for confirming ETT placement and the consumed time was faster than ETCO₂ time (P<0.0001), these results are in concordance with those obtained by previous studies [16-19].

Unfortunately, TUS and capnography cannot detect adequate endotracheal intubation which means that the tip of the ETT is placed in the trachea and does not proceed beyond the carina into the right main stem bronchus [20,21]. For this reason, TUS may be a good option for primary confirmation of ETT placement but one-lung intubation may be misdiagnosed. As a result, in our study, we integrated ultasonographic lung sliding sign for secondary confirmation of ETT placement and to exclude right main stem intubation. In the present study, SLS has higher specificity, PPV (100% both) and AUC 0.966 compared to TUS but has lower sensitivity, NPP (93.24%, 54.5% respectively) and overall accuracy 93.75. We suggest that SLS accurately identified oesophageal intubation, but was less accurate in identifying tracheal intubation compared to capnography. On combining TUS to SLS, specificity and negative predictive value raised to 100%, the accuracy was highly precise (98.75%).

In harmony with our results, Sim et al. [22] in their study have described that the overall accuracy of ultasonographic SLS for confirming ETT placement during emergency intubation was 88.7% with the positive predictive value was 94.7% in the non-cardiac-arrest group and 100% in the cardiac-arrest group. Additionally, in a previous study [23] on 30 patients admitted due to severe airway problems; the Sensitivity, specificity, PPV and NPV for confirming proper endotracheal intubation by transcricothyroid membrane ultrasonography were 96.3%, 100%, 100% and 75%, respectively and after verification by ultrasonographic SLS sensitivity, specificity, PPV and NPV were each 100%. Also, SLS efficacy in ensuring precise placement of the left-sided double lumen tubes is a useful tool as discussed in previous studies [24-26].

In the current study, the mean time elapsed for the confirmation of ETT location by SLS was 10.63 seconds which significantly less than capnography time (P<0.0001), moreover the total ultrasonography time was significantly less than that of capnography (P<0.001) and the mean difference was 5 seconds in favor of ultrasound. In concordance with our results, Pfeiffer et al. noted that the time consumed for performing bilateral lung ultrasound for confirming endotracheal intubation was faster than the standard method of auscultation and capnography in elective surgical patients [27].

Major catastrophes during general anesthesia result from inability to intubate the trachea following administration of a muscle relaxant and failure to detect it in time [11]. The role of Tracheal Ultrasonography (TUS) in confirming appropriate endotracheal tube placement during general anesthesia has gained significance not only in the operating room but also in the intensive care unit and emergency department.

A systematic review and meta-analysis by Chou et al. [12] involving 12 eligible studies together with another one done by Das et al.[13] included eleven studies and 969 intubations, supported that tracheal ultrasonography has high significance for approving endotracheal intubation and recognizing oesophageal intubation with a reasonable degree of sensitivity and specificity. Moreover, Osman and colleagues concluded that airway ultrasonography can replace waveform capnography in confirming ETT placement in centers without capnography with an overall accuracy 98.1% [14].

We found that TUS detected five oesophageal intubations from six cases detected by continuous wave form capnography and one detected as endotracheal (false positive) which explained as the esophagus was detected immediately posterior to the trachea in this patient rendering judgment difficult. Moreover, it did not detect three out of 74 tracheal intubations (false negatives), which may explained by that the accuracy of static assessments may possibly less than that of the dynamic ones because better to visualize the motion artifact with the last technique [15]. In the current study, TUS achieved high sensitivity and PPV (95.95% and 98.6%) with 95.00% accuracy and AUC 0.896 for confirming ETT placement and the consumed time was faster than ETCO₂ time (P<0.001), these results are in concordance with those obtained by previous studies [16-19].

Unfortunately, TUS and capnography cannot detect adequate endotracheal intubation which means that the tip of the ETT is placed in the trachea and does not proceed beyond the carina into the right main stem bronchus [20,21]. For this reason, TUS may be a good option for primary confirmation of ETT placement but one-lung intubation may be misdiagnosed. As a result, in our study, we integrated ultasonographic lung sliding sign for secondary confirmation of ETT placement and to exclude right main stem intubation. In the present study, SLS has higher specificity, PPV (100% both) and AUC 0.966 compared to TUS but has lower sensitivity, NPP (93.24%, 54.5% respectively) and overall accuracy 93.75. We suggest that SLS accurately identified oesophageal intubation, but was less accurate in identifying tracheal intubation compared to capnography. On combining TUS to SLS, specificity and negative predictive value raised to 100%, the accuracy was highly precise (98.75%).

In harmony with our results, Sim et al. [22] in their study have described that the overall accuracy of ultasonographic SLS for confirming ETT placement during emergency intubation was 88.7% with the positive predictive value was 94.7% in the non-cardiac-arrest group and 100% in the cardiac-arrest group. Additionally, in a previous study [23] on 30 patients admitted due to severe airway problems; the Sensitivity, specificity, PPV and NPV for confirming proper endotracheal intubation by transcricothyroid membrane ultrasonography were 96.3%, 100%, 100% and 75%, respectively and after verification by ultrasonographic SLS sensitivity, specificity, PPV and NPV were each 100%. Also, SLS efficacy in ensuring precise placement of the left-sided double lumen tubes is a useful tool as discussed in previous studies [24-26].

In the current study, the mean time elapsed for the confirmation of ETT location by SLS was 10.63 seconds which significantly less than capnography time (P<0.0001), moreover the total ultrasonography time was significantly less than that of capnography (P<0.001) and the mean difference was 5 seconds in favor of ultrasound. In concordance with our results, Pfeiffer et al. noted that the time consumed for performing bilateral lung ultrasound for confirming endotracheal intubation was faster than the standard method of auscultation and capnography in elective surgical patients [27].

First, this study involved patients undergoing intubation for general anesthesia during elective surgeries, this limit the generalization of our results to the resuscitation and intensive care situations. Second, the number of patients is relatively small, so the authentication of our results with greater numbers of participants to decrease selection bias. Third, Ultra-sonography is identified to be operator dependable technique. To our knowledge, no study to date has investigated the role of sonographer experience in accurate documentation of ETT location. Finally, TUS may not be practical for each intubation attempt. The presence of a huge neck masses, abnormality in the upper airway anatomy, soft tissue air, or substantial neck edema can make visualization more difficult to detect ETT position.

The integrated tracheal and lung sliding ultrasonography could be performed in a stepwise fashion to provide a more comprehensive affirmation of proper ETT position after general anesthesia. Moreover, we revealed a significant time advantage of ultra-sonography over capnography in authorizing appropriate endotracheal intubation.

The author appreciated the anesthesia residents and the anesthesia technicians for their help in this research.

No fund was paid by any institution.

There were no conflicts of interest.