This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Gucev, G. Articles by Glass, G. Search for Related Content PubMed PubMed Citation Articles by Gucev, G. Articles by Glass, G. Related Collections Airway Pediatrics

---

PEDIATRIC ANESTHESIA

Pediatric Airway and Esophageal Profiles with Acoustic Reflectometry

Gligor Gucev, MD^*, David T. Raphael, MD, PhD^*, Shlomo Elspas, MD^*, and Gary Glass

From the *Department of Anesthesiology, Keck School of Medicine, University of Southern California, Los Angeles, California; and E. Benson Hood Laboratories, Pembroke, Massachusetts.

Address correspondence to David Raphael, MD, PhD and reprint requests to Gligor Gucev, MD, Department of Anesthesiology, Keck School of Medicine, University of Southern California, 1200 N. State St., Room 14-901, Los Angeles, CA 90033. Address e-mail to draphael{at}usc.edu and gucev{at}usc.edu.

	Abstract

Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 
Acoustic reflectometry is a technique by which the dimensions of a cavity can be estimated in the form of an area–distance profile. We conducted a pilot study to obtain the acoustic reflectometry (AR) images associated with breathing tube (endotracheal tube, ETT) placement (inner diameter 4.5–6 mm) and positioning in 21 (n = 21) children, aged 2–12 yr. Characteristic AR profiles, as previously noted in adults, were obtained for tracheal and esophageal intubations in children. Both types of profiles showed constant area throughout the ETT length, followed distally by either a rapid area increase (tracheal) or an area decrease to a near zero value (esophageal). Relative to a tracheal profile, a bronchial intubation exhibits a decrease in area distal to the carina position. With deeper ETT insertion, abutment of the ETT against the bronchial wall can occur, with a possible profound area decrease. The occurrence of ETT abutment in children and neonates, and its possible AR detection and treatment, is discussed.

	Introduction

Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 
Acoustic reflectometry (AR) uses an acoustic time-domain signal processing technique, the Gopillaud-Ware-Aki algorithm (1,2), to create a one-dimensional image of a cavity, such as the airway and lung, with the image displayed as an area-distance curve (3–5) (total cross-sectional area versus axial distance into the cavity). The AR area-distance profiles of an endotracheal and an esophageal intubation are characteristic and distinctive (6). When a reflectometer is used to explore an adult endotracheal tube (ETT)-airway cavity, the adult profile shows constant cross-sectional area throughout the length of the ETT, followed by a rapid rise in the area past the carina (7). For an esophageal intubation, the AR profile shows constant cross-sectional area throughout the length of the ETT, followed by a sudden decrease in the cross-sectional area to a near zero value. The sudden decrease in the AR esophageal profile occurs because the soft, non-rigid esophagus collapses around the distal end of the ETT. These observations were confirmed in a study of 200 adult patients (8), with identification success rates of 100% for esophageal intubation and 99% for tracheal intubation. Identification of the site of ETT placement with AR could usually be made in 1.5–2.5 s.

An AR study of the pediatric airway needs to be undertaken to determine whether the characteristic morphology seen in adult AR profiles is maintained even with smaller airway diameters. Hence a pilot study was conducted to ascertain the AR images associated with breathing tube placement in 21 (n = 21) children, 2–12 yr of age.

	METHODS

Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 
Subjects
With the approval of the University of Southern California IRB, written informed parental consent was obtained on 21 children, aged 2–12 yr, who required an ETT for anesthesia and surgery.

The Two-Microphone Reflectometer
Reflectometers are available in the form of one-microphone and two-microphone systems. The single-microphone reflectometer is limited by the requirement that the wavetube must be as long as the cavity to be studied, which can be problematic in a cramped anesthesia workplace. Advances in theoretical acoustics (9–11) paved the way for the creation of a two-microphone system, which can function with a much shorter wavetube; indeed, the wavetube can be made almost arbitrarily short. For extended application of this acoustic technology to young children, we made use of a custom E. Benson Hood Labs (Pembroke, Mass) third- generation pediatric two-microphone reflectometer, which features improved image processing and a narrower wavetube than that used in adults. (Fig. 1) It was customized to measure areas up to a maximal axial distance of 35 cm from the distal end of the wavetube. In order to minimize the mismatch in area between the AR wavetube and the pediatric ETT, and thus to reduce the turbulence and acoustic impedance mismatch at their interface, the inner diameter of the wavetube was chosen to be 4.5 mm, an intermediate diameter between a neonatal ETT (3.0 mm) and that of a small adult (6.0 mm). The inner diameter of the connecting piece is that of the standard 15-mm ID connector, which attaches to an ETT adaptor. A truncated sinc function, sinc (x) = (sin x)/x, was used to produce a band-limited impulse of 2 ms duration. The adult low-pass filter spectral range, 200–5000 Hz, was truncated at its upper frequency to 3500 Hz to correct for minor oscillations and to minimize profile instability.

View larger version (67K): [in this window] [in a new window]   Figure 1. An acoustic reflectometer attached to an endotracheal tube.

An ensemble of 10 pulses is repeated at the rate of 5 per second (0.2 s). Within 0.2 s of the manual triggering of a sample acquisition, the first area-distance profile appears on the monitor screen. The computer-based reflectometer system was cleared for electrical safety prior to operating room usage.

The reflectometer profile is a plot of area versus distance. The vertical coordinate of the profile, indicated in square centimeter, corresponds to the total cavity cross-sectional area at a given axial length into the cavity. The horizontal coordinate in the profile corresponds to axial distance, in cm, with the origin taken to be the distal end of the wavetube, and with the axial direction taken to be parallel to the wavetube.

Induction of Anesthesia and Performance of Intubation
Each patient was routinely monitored with electrocardiography, pulse oximetry, and measurement of end-tidal carbon dioxide. ETT diameter size (in cm) was chosen on the basis of the formula [Age (yr)/4 + 4]. General anesthesia was induced and neuromuscular blocking drugs were given. There were no restrictions on the choice of drugs used for anesthetic induction or neuromuscular blockade, nor on the airway adjunct devices used to facilitate tracheal intubation. The intubations were performed by anesthesia faculty and residents, nurse anesthesia trainees, paramedic trainees, and medical students. Conventional oral breathing tubes with a Murphy eye were used.

Reflectometry Sample Acquisition and the Operator's Profile Interpretation
After the intubation attempt was made, the ETT cuff (if present) was inflated, and the wavetube with a distal connecting piece was attached to the breathing tube adaptor. The reflectometer operator was blinded to information concerning the intubation attempt; specifically, no information was given to the reflectometer operator as to whether the intubator visualized glottic structures, and as to whether the intubator observed passage of the breathing tube through the vocal cords.

An acoustic profile was generated when the reflectometer operator pressed a computer keyboard button. The reflectometer operator, seated a short distance away, used the acoustic area-distance profile alone to decide where the tube was placed. The operator then made a verbal declaration as to the placement of the breathing tube, i.e., whether the tube was in the trachea or the esophagus.

Distinguishing Tracheal from Esophageal Intubation
For interpretive purposes, the reflectometer operator was instructed to focus on the post-ETT segment immediately beyond the tip of the breathing tube. Using the conclusions from the adult study, the reflectometer operator was instructed to interpret an upgoing rise in the post-ETT segment as a tracheal intubation, whereas a marked depression of the area in the segment was to be interpreted as an esophageal intubation. The AR interpretation was consistently made within 3 s.

Upon disconnection of the reflectometer wavetube, the breathing circuit was attached to the breathing tube, and ventilation with the manual reservoir bag was initiated. Capnography, the "gold standard" method of comparison, was performed. The breathing tube was determined to be in the trachea if three or more successive capnometer CO₂ waveforms were observed. Incorrect placement in the esophagus was concluded if no CO₂ waveform was observed.

Detection of an Endobronchial Intubation (EBI)
After airway placement was confirmed, the remaining question was the possibility of an EBI, which is usually diagnosed by the auscultation of unilateral breath sounds. In a rabbit study (12) and a case report/model study (13), it was shown that the AR features of an EBI follow simply from the physics of AR in cylindrical ducts, in that a narrowing of the duct at any given axial distance always causes a corresponding decrease in the AR profile area, and conversely, a wider duct always causes a larger AR area. With AR, as the ETT is slowly withdrawn, the transition from a narrow bronchus (smaller area) to a wider trachea (larger area) would be expected to be manifested on the monitor screen as a sudden increase in the profile area distal to the ETT. This method was used to determine an endobronchial ETT position.

	RESULTS

Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 
Twenty-one children (14 boys, 7 girls) were studied. Patient data were collected with respect to age (mean ± sd, 6 ± 3; range, 2–12, median 5 yr), height (112 ± 19 cm; range, 78–143 cm), and weight (22 ± 9 kg; range, 9–40 kg). Subjects were intubated with a Murphy-eye ETT of inner diameter (5.2 ± 0.5 mm; range, 4.5–6 mm). There were seven cuffed ETTs (6 inflated to no leak), and 14 uncuffed ETTs (left with leaks of at least 12 cm H₂O pressure). The youngest subjects, three 2-yr-old children, were all intubated with an ETT of 4.5 mm ID.

There were 21 tracheal intubations, two of which were initially EBI, plus four esophageal intubations. In 20 pediatric patients, all ETT placements, tracheal and esophageal, were correctly identified initially with AR and confirmed immediately afterwards with capnography. ETT placement recognition (esophageal versus tracheal) could be made typically within 3 s after appearance of the AR profile on the monitor screen. There was one additional case of a presumed ETT abutment against a wall, which initially mimicked an esophageal intubation (a false negative), but which was promptly recognized with AR and capnography to be an EBI.

	DISCUSSION

Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 
In summary, an AR two-microphone device with a 4.5-mm ID wavetube allowed the correct determination of the ETT placement in 20 pediatric patients in the age range 2–12 yr. In Figure 2, we show three individual pairs of pediatric tracheal and esophageal profiles. With progressively narrower diameter ducts, such as those encountered in small children and neonates, frictional viscosity becomes more important. A theoretical concern could be raised that AR may no longer be valid in such small ducts. The results of this study indicate otherwise, inasmuch as the characteristic AR pediatric profiles (tracheal, esophageal) were of the same form as those seen previously in adults. Considering the Jarreau et al. AR neonatal study (14) (discussed later), we assert that AR is usable for the entire range of human tracheal diameters.

View larger version (19K): [in this window] [in a new window]   Figure 2. Pairs of acoustic reflectometry tracheal and esophageal profiles from three children: (top) 5-yr-old girl, (middle) 11-year-old girl, and (bottom) 2-year-old boy.

For uncuffed ETTs, the gauge pressure at which a leak was detected, yet providing ventilation, was used, with a minimum pressure of 12 cm H₂O. In our study of 200 adult patients, the effect of a fully inflated ETT cuff (in response to an audible leak) was to produce a slight reduction in the AR area distal to the ETT cuff. The effect was not as prominent in the pediatric patients studied herein, but we shall explore this further in a subsequent study with a higher-resolution AR device.

Applications of AR include situations where capnography is not available to confirm proper ETT placement after attempted intubation, and in the cardiac arrest setting when capnography may fail. AR has an advantage over capnography in that it does not require an obligatory manually delivered breath in order to determine ETT placement immediately after intubation. In the presence of an incorrect esophageal placement, AR would spare the patient from unnecessary gastric distention as a result of these customary manual breaths. This would have implications in full-stomach and difficult intubation scenarios.

In Figure 3, we show four superimposed AR profiles obtained in a 5-yr-old, 27-kg child with a 5-mm ID ETT (with a Murphy eye) with an initial insertion distance of 17 cm at the incisors. The initial AR profile was interpreted as an esophageal intubation, because the AR trace distal to the ETT was near zero (Fig. 3, "abutment"). However, upon delivery of a few breaths positive for CO₂, a 1-cm withdrawal of the ETT was followed by an incremental increase in the AR area, consistent with a deep EBI (Fig. 3, "deep endobronchial"). The ETT was withdrawn an additional 1 cm. On auscultation, breath sounds were decreased over the left chest, indicating a continuing endobronchial ETT tip position, as confirmed by the AR profile (Fig. 3, "bronchial"). Hence the ETT was withdrawn 2 cm more. Breath sounds were now equal and a higher-valued AR tracing was evident, compatible with a tracheal AR profile (Fig. 3, "tracheal"). We interpret the lowermost of the four profiles, which mimicked an esophageal intubation, as an ETT abutment against a bronchial wall. The intrabronchial location is evident from the successive rises in the AR profile area upon successive withdrawals of the ETT. In the differential diagnosis, one should also consider the possibility of a mucus plug, which we consider to be less likely in this case because there was no mucus present at the tip of the ETT upon extubation.

View larger version (37K): [in this window] [in a new window]   Figure 3. Four superimposed acoustic reflectometry (AR) profiles associated with the endotracheal tube (ETT) presumptively abutting initially against a bronchial wall ("Abutment"). Successive ETT position withdrawals result in AR profiles corresponding to "Deep Endobronchial," "Endobronchial," and "Tracheal" positions of the ETT tip. The child is a 5-yr-old, 27-kg boy.

In an earlier case report (13), AR was used to detect an endoscopically confirmed EBI in the presence of equal breath sounds. For the EBI in this study, as the ETT was slowly withdrawn, the reflectometer profile showed successive increases in area, which was consistent with its mid-course bronchial and final intratracheal positions (Fig. 4). Animal studies (12), the prior case report (13), and this pediatric EBI suggest that AR can be used to identify a malpositioned ETT in the bronchus, from which position it can be withdrawn and optimally positioned in the mid-trachea.

View larger version (26K): [in this window] [in a new window]   Figure 4. Simulation of an endobronchial intubation with acoustic reflectometry (AR) in a bifurcating glass model. As a 7.0-mm inner diameter endotracheal tube is successively withdrawn from a deep "endobronchial" intubation in the model, the AR profiles show successive area increases at the mid-course "bronchial" and "tracheal" positions. Model branch dimensions ("segmental bronchus," "mainstem bronchus," "trachea") were, respectively, of diameter (1.75, 2.0, 2.5 mm ID) and of length (8.25, 7.5, 10 cm) [modified from Raphael and Lee (13)].

Sugiyama et al. (15) showed that an adult ETT with an inflated cuff can be steadily advanced into the trachea and past the carina, such that no unilateral auscultory change is observed until an ETT with a Murphy eye is inserted 2.0 ± 0.4 cm past the carina. Breath sounds disappeared when the tube tip was advanced 3.2 ± 0.3 cm past the carina. Therefore, complete ETT cuff sealing of a mainstem bronchus is required to produce complete absence of contralateral breath sounds. If the cuff is inadequately inflated, the result is a circumferential leak, which makes possible the transmission of an audible acoustic disturbance (breath sounds) to the contralateral side, but without any significant lateral ventilation of that side. For pediatric patients, in whom uncuffed ETTs are often used, an increased frequency of EBIs with equal breath sounds should be expected.

A significant limitation of this study is the small sample size. The blinding of the AR operator in the operating room cannot be considered complete, owing to the presence of cues suggested by the nearby intubator's relative ease or difficulty associated with the intubation attempt. A larger study needs to be undertaken to document the utility of AR technology in children and neonates. This requires the determination of specificity and sensitivity of AR-determined ETT placement (tracheal versus esophageal). Moreover, the changes in AR profile morphology associated with an EBI needs to be studied further, with fiberoptic confirmation as the gold standard. Two dynamic ETT optimal positioning approaches to consider are 1) deep initial placement of the ETT followed by a steady withdrawal into the trachea, with a safe distance maintained relative to the carina, which can be identified with AR by its rapid profile takeoff, and 2) preglottic positioning of the ETT tip slightly beyond or at the glottic opening, and calculating the mid-tracheal position to be half the axial distance to the carina. This preliminary view into the trachea could be used to size the tracheal diameter, and to choose the proper size ETT.

We interpret the successive AR profiles of Figure 3 as follows: Upon ETT tip placement at a deep endobronchial location, the AR trace suddenly went to a near zero value as a result of abutment of the ETT orifice against the endobronchial wall. Two successive ETT withdrawals of 1 cm led to positions that were still endobronchial, as indicated by the successive increases in the AR area immediately distal to the ETT tip. However, upon a further 2-cm withdrawal, the AR trace increased further, which suggested a tracheal position for the ETT tip, and in which position the tube was ultimately left throughout the case.

Obstruction of the ETT with positional changes of the head has been detected in neonates with a 3.5-mm ID AR device (12). The obstruction was attributed to the ETT orifice abutting the tracheal wall, which occurs when the infant's head is positioned on one side, but not on the other (side-position-related ETT obstruction). In this AR neonatal study, the presence of a lateral hole (such as a Murphy eye) did not prevent side-position-related ETT obstruction.

For a suspected ETT abutment situation, and to absolutely exclude the possibility of an esophageal intubation, we recommend the following maneuvers: 1) manual ventilation with a few breaths to separate the ETT tip from the bronchial wall, as was done in this study, to observe the effect on the AR profile, and 2) withdraw the ETT by 1 cm and observe the distal AR profile. If the AR area increases upon ETT withdrawal, as in this study, this is suggestive of a deep EBI. If this happens, one should withdraw the ETT further still. On the other hand, if the post-ETT area remains near zero and is unaffected by the ETT withdrawal, the ETT position is confirmed as esophageal.

In summary, the children we studied exhibited the same characteristic type of AR profiles for EBI, tracheal, and esophageal intubations, as previously seen in adults. The variant profile associated with the presumed abutment of an ETT against an airway wall is discussed.

	ACKNOWLEDGMENTS

 
We thank E. Benson Hood Laboratories for providing us with a customized pediatric acoustic reflectometer for performing this study. We appreciate the assistance of Ms. Diane McIntee for her graphics assistance.

	Footnotes

 
Accepted for publication August 1, 2006.

Presented at the Fall 2004 Meeting of the American Society of Anesthesiologists (Poster presentation), Las Vegas, NV.

This work is attributed to the Department of Anesthesiology, Keck School of Medicine, University of Southern California, Los Angeles, CA.

	REFERENCES

Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 

1. Gopillaud PL. An approach to inverse filtering of near-surface layer effects from seismic records. Geophysics 1961;26:754–60.
2. Ware JA, Aki K. Continuous and discrete inverse-scattering problems in a stratified elastic medium. I: Plane waves at normal incidence. J Acoust Soc Am 1969;45:911–21.[Medline]
3. Fredberg JJ, Wohl MEB, Glass GM, Dorkin HL. Airway area by acoustic reflections measured at the mouth. J Appl Physiol 1980;48:749–58.[Abstract/Free Full Text]
4. Fredberg JJ. Acoustic determinants of respiratory system properties. Ann Biomed Engl 1981;9:463–73.
5. Jackson AC, Butler JP, Miller EJ, et al. Airway geometry by analysis of acoustic pulse response measurements. J Appl Physiol 1977;43:523–36.[Abstract/Free Full Text]
6. Mansfield JP, Lyle RP, Voorhees WD, Wodicka GR. An acoustical guidance and position monitoring system for endotracheal tubes. IEEE Trans Biomed Engl 1993;40:1330–5.
7. Raphael DT. Acoustic reflectometry profiles of endotracheal and esophageal intubation. Anesthesiology 2000;92:1293–9.[Web of Science][Medline]
8. Raphael DT, Benbassat M, Arnaudov D, et al. Validation study of two-microphone acoustic reflectometry for determination of breathing tube placement in 200 adult patients. Anesthesiology 2002;97:1371–7.[Web of Science][Medline]
9. Schroeder MR. Determination of the geometry of the human vocal tract by acoustic measurements. J Acoust Soc Am 1967;41: 1002–10.
10. Louis B, Glass G, Kresen B, Fredberg J. Airway area by acoustic reflection: the two-microphone method. Trans ASME 1993;115:278–85.
11. Louis B, Glass G, Fredberg JJ. Pulmonary airway area by the two-microphone acoustic reflection method. J Appl Physiol 1994;76:2234–40.[Abstract/Free Full Text]
12. Mansfield JP, Shannon DC, Wodicka GR. Acoustic method to quantitatively assess the position and patency of infant endotracheal tubes: preliminary results in rabbits. Pediatr Pulmonol 1999;26:354–61.
13. Raphael DT, Lee H. Detection via acoustic reflectometry of an esophageal intubation, and of an endobronchial Intubation in a patient with equal breath sounds: a case report. J Clin Anesth 2003;15:41–7.[Web of Science][Medline]
14. Jarreau PH, Louis B, Dassieu G, et al. Estimation of inspiratory pressure drop in neonatal and pediatric endotracheal tubes. J Appl Physiol 1999;87:36–46.[Abstract/Free Full Text]
15. Sugiyama K, Yokoyama K, Satoh K, et al. Does the Murphy eye reduce the reliability of chest auscultation in detecting endobronchial intubation? Anesth Analg 1999;88:1380–3.[Abstract/Free Full Text]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Gucev, G. Articles by Glass, G. Search for Related Content PubMed PubMed Citation Articles by Gucev, G. Articles by Glass, G. Related Collections Airway Pediatrics