Outcome studies have repeatedly shown that unrecognized esophageal intubation is a leading cause of death or brain damage in anesthetic practice [1-4]. Esophageal intubation as a complication in emergency airway management in critically ill patients occurs in 8% of the attempts . Contributing factors to this high incidence include intubation under unfavorable conditions, unavailability of monitoring equipment, violation of the standard technique of auscultation of lung fields and epigastrium, and intubations performed by inexperienced personnel. Thus, reliable methods to confirm correct tube position are necessary.
The methods most commonly used for confirming tube position are clinical evaluation by auscultation of the lungs and the epigastrium; capnographic determination of ETCO2; the esophageal detection method (EDM) using a self-inflating bulb (SIB) or syringe; and the tracheal transillumination technique using a lighted stylet. Although various studies have demonstrated the reliability of each of these methods, no study has directly compared them. In addition, the influence of the examiner's experience on the use and interpretation of these various detection methods has not been assessed. We therefore conducted a study to compare the reliability of four different methods for immediate detection of the tube position and to assess the influence of the examiner experience.
The study was performed at the Medical Intensive Care Unit of the University Hospital of Vienna after approval of the local ethical review board and in conformance with Austrian law.
Thirty-eight consecutive, orotracheally intubated, adult patients requiring prolonged controlled mechanical ventilation were eligible for study inclusion. The correct tube position was verified by a chest radiograph. Hemodynamic and respiratory stability, as well as adequate sedation and analgesia with midazolam and sufentanil, were prerequisites for study entry. Exclusion criteria were severe respiratory failure (fraction of inspired oxygen [FIO2] >0.5, positive end-expiratory pressure >10 cm H2 O, and/or inspiratory to expiratory ratio >0.5), need for vasopressors, gastric reflux via the feeding tube of >200 mL 4 h before inclusion, pregnancy, hemorrhagic diathesis, gastroesophageal diseases, and tracheostomy.
A second tube (7.5-9.0 mm inner diameter) was inserted into the esophagus under laryngoscopic control. This tube was identical to the tracheal airway and was always inserted by the same investigator. The cuffs of both tubes were inflated with 8 +/- 3 mL of air. The patient's head was covered with a sheet so that only the ends of both tubes were visible. The tracheal tube was then disconnected from the ventilator, and the first of two examiners was allowed to enter the room. Examiner 1 was one of two physicians with at least 4 yr experience in critical care; Examiner 2 was one of two medical students in the last year of training with little or no experience in critical care. Thirty seconds was allowed for determination of the tube position using one of the four methods described below. During the study period, patients were ventilated manually by the investigator with an approximate tidal volume of 10 mL/kg body weight at a frequency of 12 breaths/min. In cases in which the esophageal tube was ventilated, the stomach was decompressed afterward. The selection of the tube (tracheal or esophageal) to be tested and the detection method was chosen according to a randomization list. After the experienced examiner left the room, the inexperienced examiner evaluated the same tube using the same method of evaluation. This procedure was repeated until all detection methods were tested by both examiners in a particular patient.
During the study, heart rate, blood pressure, and oxygen saturation were monitored continuously. Before and immediately after the last examination, arterial blood gases were checked. In case of respiratory or hemodynamic deterioration, the study was discontinued.
Auscultation was performed with patients in the supine position over both lungs in the infraclavicular fossa and the fifth intercostal space in the midaxillary line and over the epigastrium.
ETCO2 was determined using a spectrographic/infrared capnograph (Capnosat[trade mark sign]; Drager, Lubeck, Germany). The examiners looked for the characteristic CO2 waveform.
EDM was performed by using the SIB (Tube Chek[trade mark sign]; Ambu, Glostrup, Denmark; capacity 60 mL), which was compressed before connection to the tube. Rapid reinflation (<4 s) of the SIB was indicative of tracheal positioning, whereas delayed (>4 s) or no reinflation was considered indicative of esophageal placement.
For transillumination, a lighted stylet (Trachlight[trade mark sign]; Laerdal, Armonk, NY) was inserted into the tube. If a bright, circumscribed glow was seen below the cricothyroid membrane, the tube position was identified as tracheal. If no light or only a very weak light was detectable, the tube was considered to be in an esophageal position. The room was darkened before examination.
Continuous variables are presented as means +/- SD, nominal variables are presented as percentages.
For every detection technique, an indicator variable was created (1 = the examiner was right in the particular patient, 0 = the examiner was wrong). Differences in the indicator variable between the two examiners per detection technique were analyzed by using the sign test. For an overall comparison between the examiners, the differences in the indicator variables were summed over the detection techniques. The Wilcoxon signed-rank test was used to compare the examiners.
To compare the four methods, the indicator variables were summed over the examiners. All pairwise differences of these aggregated indicators among the four methods were analyzed again using the Wilcoxon signed-rank test.
All confidence limits were calculated according to the method of Pearson and Clopper . The significance level was set to alpha = 0.05. A Bonferroni-Shaffer correction for multiplicity of testing for the differences among the detection methods was performed . Sensitivity and specificity were calculated for each method pooled over the examiners.
Thirty-eight patients (62 +/- 11 yr; 22 male, 16 female; body mass index 27 +/- 5 kg/m2) were included in the study. The main reasons for admittance to the intensive care unit were cardiopulmonary resuscitation (n = 16), heart surgery (n = 6), and respiratory insufficiency due to various disorders (e.g., pulmonary embolism, pneumonia, pulmonary edema) (n = 16). All patients were in a stable respiratory and hemodynamic state at study entry (FIO2 0.4 +/- 0.1, positive end-expiratory pressure 5 +/- 2 cm H2 O, oxygen saturation 96% +/- 3%, heart rate 89 +/- 27 bpm, mean arterial blood pressure 84 +/- 17 mm Hg). Blood pressure, oxygen saturation, and heart rate remained stable in all study patients. Arterial blood gas checks did not show major changes before and after the study. In none of the cases did the study have to be discontinued prematurely because of complications.
The two examiners performed 152 examinations. Of the tubes tested, 74 were in the tracheal position, and 78 were in the esophageal position. In 130 of 152 examinations, both examiners correctly diagnosed the position of the tube. Four times a wrong result was obtained by both. Four times only the experienced examiner was wrong, and 14 times only the inexperienced examiner was wrong. The indicator variables aggregated over techniques in the 38 patients were significantly different between the examiners (P = 0.0129).
Using auscultation, the experienced examiner diagnosed the proper tube position in all cases, whereas the inexperienced examiner was correct in only 68% of cases (P = 0.0005) (Figure 1). Ten (83%) of these incorrect results were false-positive (esophageal tube identified as tracheal), and only two (17%) were false-negative (tracheal tube identified as esophageal
Using ETCO2, both examiners were correct in all cases (Figure 1).
Using EDM, in one case, the experienced examiner determined a wrong tube position; in another case, both examiners determined a wrong tube position (Figure 1). All incorrect results were false-negatives (tube positioned in trachea, but SIB failed to reinflate).
Using transillumination, the experienced examiner was incorrect in six cases (16%; twice false-positive, four times false-negative), and the inexperienced examiner was wrong in five cases (13%; twice false-positive, three times false-negative) (Figure 1). Three times both examiners were incorrect in the same patients.
Comparing the four methods for both examiners, a statistical difference was found between ETCO2 and auscultation (P = 0.0005) and ETCO2 and Trachlight[trade mark sign] (P = 0.0078). The difference between EDM and auscultation (P = 0.0225) failed to reach significance when applying the critical limit 0.05/3 after adjustment for multiple testing. The calculated differences between ETCO2 and EDM, EDM and Trachlight[trade mark sign], and auscultation and Trachlight[trade mark sign] were not statistically significant.
Sensitivity and specificity of the four methods pooled for the two examiners are shown in Table 1.
Many methods have been used to distinguish tracheal from esophageal tube placement, and almost all can fail [8,9]. One of the most reliable signs of correct tracheal placement is visualization of the tube entering the larynx [8,10]. However, complete visualization is sometimes impossible, particularly under emergency conditions. Furthermore, the tube may slip out of the larynx after initial successful tracheal placement.
In our study, the use of capnography led to correct diagnoses of tube positioning in all patients tested, once more confirming its reliability in noncardiac arrest patients. However, during cardiac arrest and cardiopulmonary resuscitation, when low cardiac output leads to low expired carbon dioxide concentrations, false-negative results have often been observed. Bozeman et al.  reported an error rate of approximately 30% in ETCO2 determination in cardiac arrest victims, whereas the EDM correctly indicated tracheal tube placement in all of these patients. The EDM relies on the anatomic differences between the trachea and the esophagus. Application of negative pressure leads to collapse of the esophagus because of its fibromuscular structure, whereas the trachea is held open by its rigid cartilaginous rings. Because this anatomic difference persists regardless of the metabolic and perfusion state of the patient, the SIB technique seems superior to capnography in cardiac arrest patients [11,12]. Therefore, some authors recommend the EDM for initial assessment of tube position in the preclinical setting, eventually followed by capnometry [13,14]. Nevertheless, both methods may lead to false-negative results in the case of tracheobronchial obstruction or pulmonary edema, which may be encountered during emergency intubations [9,15].1,2
(1) Czinn EA, Wafai Y, Salem MR, et al. Efficacy of the self-inflating bulb for confirmation of emergency tracheal intubation in critically ill patients [abstract]. Anesthesiology 1995;83:A266.
(2) Wafai Y, Salem MR, Joseph NJ, et al. The self-inflating bulb for confirmation of tracheal intubation: incidence and demography of false negatives [abstract]. Anesthesiology 1994;81:A1304.
In the current study, the SIB failed three times, and a false-negative result was obtained by both examiners in one patient. This might be explained by the patient's body mass index of 34 kg/m2, because morbid obesity has been described as the most common factor causing false-negative results .2 Lang et al.  explained this phenomenon to be a consequence of the decreased functional residual capacity in morbid obesity and the collapse of large airways due to invagination of the posterior wall when subatmospheric pressure is generated by the SIB. An improvement of their results could be achieved when the SIB was compressed after connection to the tube, thereby maintaining the patency of the tracheobronchial tree and allowing the SIB to refill. However, in one of our patients who was not obese and who showed no signs of tracheobronchial obstruction, bronchospasm, or pulmonary edema, a wrong result was obtained. We hypothesize that the tip of the tube was impinging on the carina. Possibly, reinflation would have occurred if more time had been allowed (i.e., delayed reinflation). However, allowing 30 s for reinflation, as recommended by Zaleski et al. , seems inappropriate, especially in emergency situations.
Tracheal transillumination using a lighted stylet has been used for facilitation of endotracheal intubation, as well as for confirmation of tracheal tube placement . An intense, circumscribed midline glow in the region between the laryngeal prominence and the sternal notch is visible when the tube is correctly placed. In the case of esophageal intubation, the light is either absent or dull and diffuse. The potential advantage of this method lies in its independence from hemodynamic and/or pulmonary impairment. Besides its dependence on ambient light conditions, it may fail in the presence of neck tumors or swellings and in obese patients with short and thick necks [19,20]. In our study, rather high error rates (13% and 16%) were observed with transillumination, independent of the user's experience. Patients in whom incorrect determinations of tube position were made had a higher mean body weight, but the differences were not statistically significant. Regarding our results, this technique seems unreliable for the assessment of tube position, even under favorable light conditions.
The current study further shows that the assessment of tracheal tube position by auscultation was accurate when performed by experienced examiners. False-positive results by auscultation have been described by several authors [8,21-25]. In these cases, air movement through the esophagus was mistakenly identified as breath sounds. Andersen et al.  showed that auscultation of the epigastrium and the right and left axilla is more reliable for detecting tube position than chest auscultation.
Our data suggest that capnography is the most reliable method for detecting tube position, independent of the examiner's experience. Investigator experience, however, seems to be an important factor influencing the reliability of auscultation. Using the SIB led to correct results in 96% of cases, whereas transillumination was not trustworthy, even under optimal conditions.
The authors thank Ramez Salem, MD, Department of Anesthesiology, Illinois Masonic Medical Center, University of Illinois College of Medicine, Chicago, IL, for carefully reviewing our manuscript and for helpful comments.
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