In Reply:—

I thank Dr. Maleck for giving me the opportunity to discuss other acoustical approaches for distinguishing endotracheal from esophageal intubation.

The intent of my article 1was to focus on the imaging of the upper airway and esophagus, as made possible by the development of the Hood Labs acoustic reflectometer (Pembroke, MA), which generates a “one-dimensional” image in the form of an area–distance profile. These area–distance profiles are intuitive in that the operator is able to assess the total cross-sectional area at any given distance into the cavity of interest, whether the trachea or esophagus. For a breathing tube in the trachea, the cross-sectional area is constant within the endotracheal tube (ETT), and increases further with deeper penetration into the lung. By comparison, if the esophagus is intubated, the area amplitude goes essentially to zero immediately beyond the ETT tip as a result of the nonrigid esophagus collapsing around the distal ETT.

Mansfield et al.  2conducted a similar and earlier investigation in dogs. The approach used the delivery of a series of sonic impulses into the airway, with a miniature microphone placed in the endotracheal tube wall to monitor sound pressure. The key to the Mansfield system is the following: When the incident impulse encounters a boundary where there is a sudden increase in area (e.g. , endotracheal intubation), the reflected wave approaches the absolute amplitude but is inverted in an amplitude-versus -time delay (A-TD) plot. In contrast, if the incident wave encounters a large decrease in amplitude (e.g. , esophageal intubation), the reflection is large but is not inverted in the resulting A-TD plot. The presence of this deflection at the ETT tip allowed discrimination between esophageal intubation (upward deflection) and endotracheal intubation (downward deflection). Endotracheal intubation was confirmed by the presence of additional negative airway deflections in an A-TD plot. My concern, however, is that this approach is not intuitive and user-friendly, because the operator must look for the presence or absence of a key reflection amidst a series of undulations in a plot. By comparison, when a Hood Labs reflectometer area–distance profile is used, even an inexperienced operator can, in an instant, distinguish easily between endotracheal and esophageal intubation. Nonetheless, the Mansfield et al.  2system, in principle, is workable and, with appropriate modifications and operator training, could lead to a useful device.

Akerson 3developed a technique that used resonant sound as the basis for distinguishing between endotracheal and esophageal intubation. In complex asymmetric branching structures, such as the lung, there is not one resonant frequency but multiple resonant peaks caused by the clumping of eigenvalues into clusters in the low-frequency range. 4In earlier publications, 5,6I developed explicit, closed-form mathematical solutions to calculate the expected resonance frequencies for symmetrical and asymmetrical branching structures exhibiting an arbitrary number of bifurcations in which the branch areas and lengths were known a priori . The inverse problem, that of predicting cavity volumes and branch characteristics from the observed resonant frequencies, is a considerably more difficult mathematical problem that has not been solved. However, in the low-frequency range, the fundamental resonance frequency for such structures can be shown to be approximately inversely proportional to the cavity volume.

The rationale behind the Akerson device was to exploit the differences between the higher-valued fundamental resonant frequency associated with an esophageal intubation (smaller cavity) versus  the lower-valued fundamental resonant frequency associated with an endotracheal intubation (larger lung cavity). For configuration purposes, attention was paid to the resonant frequency characteristics of the ETT, considered as a cavity in itself. It was followed by the Sonomatic Confirmation of Tracheal Intubation (SCOTI) device, which required configuration to the individual ETT before its use. The device generated a series of numbers that were used to decide on proper ETT configuration and to determine correct endotracheal versus  esophageal placement. Although previous studies were promising in that the SCOTI device could determine most esophageal intubations, the device encountered several difficulties in that it could not determine tracheal intubations with the same level of success. 7The SCOTI device could not be configured reliably for ETTs with a diameter smaller than 6.0 mm and gave inconsistent results in cut ETTs. 8In patients who had been intubated already, the ETT position could not be checked without first removing the ETT from the patient for the required configuration of the device. The conclusions of several studies indicated caution in the use of the device, and, ultimately, because of disappointing sales, the device was withdrawn from the market. As Dr. Maleck points out, the SCOTI device merits mention as another acoustic device that aimed to distinguish between endotracheal and esophageal intubation.

Unlike the SCOTI device, the acoustic reflectometer can be used in an already-intubated patient, regardless of whether the ETT is cut. The present acoustic reflectometer is intended for use in adults and is capable of reproducing accurate longitudinal area changes in adult ETTs with internal diameters as small as 6.0 mm without difficulty. A large-scale validation study is currently in progress to determine the specificity and sensitivity of the acoustic reflectometer in the detection of endotracheal and esophageal intubations.

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