LOS ANGELES—Standard methods for confirming placement of an endotracheal tube (ETT) include direct visualization, radiographic assessment, and capnography. However, direct visualization is not always possible, radiographic studies cannot always be rapidly performed, and results of end-tidal capnography may be unreliable when the pulmonary circulation is disrupted, as occurs in cardiopulmonary arrest. In a recent study, a newly developed rapid acoustic method identified 99% of correctly placed ETTs and 100% of misplaced (esophageal) tubes.[1]

Based on his prototype, primary author David T. Raphael, MD, PhD, envisions “a small compact device that will allow the dynamic real-time imaging of the placement and position of ETTs.” It would allow, within seconds, “confirmation of proper breathing tube placement in the trachea—or [misplacement] in the esophagus, [as well as] detection of endobronchial intubations.”

REFLECTOMETRY REVEALS LOCATION

The prototype reflectometer is a modified acoustic pharyngometer that attaches to a patient’s ETT adapter. “It emits a series of acoustic pulses, and those pulses are transmitted … through the endotracheal tube and then through either the trachea or esophagus,” said Dr. Raphael, Associate Professor of Anesthesiology at University of Southern California’s Keck School of Medicine in Los Angeles. Reflected signals are analyzed to produce a graph that compares the area of the cavity and the distance from the adapter. The tracing can quickly be interpreted in a clinical setting. For instance, explained Dr. Raphael, “within the ETT, you expect the area to be constant. If at the end of the ETT, [the area] rapidly goes up because of the exponential increase in area [corresponding to] the lung, you know that you’re in the trachea.” He added, “In the esophagus, however, the elastic walls … close around the distal end of the ETT, and the air trace very quickly goes to zero. So, you can distinguish between the two very quickly—medical students or trainees can learn how to do this in two minutes.”

Dr. Raphael and colleagues used acoustic reflectometry to assess 214 patients who had undergone intubation; capnography was then used to confirm correct or incorrect tube placement. Reflectometry accurately identified tracheal intubation in 198 of 200 adult patients and all of the preceding 14 esophageal intubations. One of the two false readings resulted from damaged equipment; the other was obtained in a morbidly obese patient with asthma.

METHOD BROADLY USEFUL

The technique can also differentiate tracheal intubation from bronchial placement. With tracheal intubation, a characteristic rise in the area corresponding to both lungs occurs at the end of the trace. Dr. Raphael noted, “If, however, you’re in one of the bronchi, the area’s cut roughly in half.” A finding of a markedly diminished area increase, therefore, indicates the need to adjust the tube’s placement.

“The nice thing about it is you don’t have to pull out a fiberoptic scope and introduce another foreign body into the airway—you can simply attach a reflectometer,” Dr. Raphael remarked. Additionally, the device “will be useful in the detection of mucous plugs within the ETT in burn and ICU patients,” he said. “You can detect a decrease in the area of the ETT lumen and therefore intervene preemptively, rather than have to respond to an emergency with an obstructed airway crisis.”

Said Dr. Raphael, “We are in the process now of miniaturizing the instrument for use not only in the hospital setting but also in the field, so that a paramedic who runs out to a patient in cardiopulmonary arrest where there’s little CO2 being exhaled” can assess placement within three to four seconds.

—Mimi Zucker, PhD

Reference
1. 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;97:1371-1377.

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