Breathing Easy

A new device may help to quickly detect correct placement of an endotracheal tube.

by Karen Heyman

Many people have innocently swallowed a tidbit of food, only to wind up choking when it moves down the wrong pipe. This misdirection happens because the trachea, which leads to the lungs, and the esophagus, which leads to the stomach, are close together at the back of the throat. They are so close that the procedure of tracheal intubation—placing a tube down the trachea and into the lungs to facilitate breathing—can prove challenging, even to veteran anesthesiologists.

To help his colleagues more easily perform this critical procedure, David T. Raphael, M.D., Ph.D., associate professor of anesthesiology at the Keck School of Medicine of USC, investigated a device to immediately detect whether an endotracheal tube has been correctly placed in the trachea to deliver oxygen to patients.

Several techniques already exist to ensure correct placement, but they are not appropriate for all circumstances, Raphael says. They include simple visual inspection of the tube through the vocal chords, which may be impossible in patients with swollen airways. Physicians also use fiber-optic bronchoscopy, which consists of a fiber-optic tube attached to a camera and video monitor. While readily available in operating rooms and hospital settings, it is not for use in emergency settings.

The gold standard technique for correctly assessing placement is called capnography. A sensor at the patient’s mouth detects carbon dioxide concentration during exhalation and displays detailed information about each breath. But it cannot work when a patient is in cardiopulmonary arrest, since there may not be any exhalations. Yet, Raphael explains, this is exactly when a physician needs to be certain that a patient is receiving life-saving oxygen. He believes there is a serious need for a device that can help determine immediately if an endotracheal tube has been properly placed, especially in emergency situations.

Raphael found an answer outside the medical field. To determine where a tube is placed, he used a method geophysicists employ to find oil: seismic acoustic wave reflection. Explosive charges are set off in the ground, resulting in seismic waves that travel down and back through the layers of the earth, similar to how an echo is produced. The seismic waves return at different times depending on the density of the layers; a geophysicist determines which layer contains oil based on the rate of return.

Raphael knew that the dimensions of the trachea, esophagus and first bronchial branches also create different characteristic wave profiles. He thought that if he could use the acoustic reflection device to send out sound pulses into the throat, the same physics would help him know whether a breathing tube had been inserted correctly in the trachea. Such a device would have the additional advantage of being able to detect obstructive mucus plugs in the endotracheal tube, a concern for burn patients or those in intensive care.

Raphael customized an existing device to work with an endotracheal tube. In a prototype study, Raphael set up and ran an acoustic reflectometer on 200 patients. All were correctly intubated, which was confirmed by capnography.

The instrument itself is a nearly foot-long, metal rec-tangular box. On one end, it links to a computer, while on the other, it attaches to the adapter end of an endotracheal tube. The design spares patients from having another tube placed down their throats. When activated, the reflectometer sends sound pulses down the endotracheal tube, in the same way geophysicists send sound waves through the earth. Raphael developed an alternative algorithm to analyze the returning waves, with the results converted into a graph on a monitor. The signal should be constant throughout the endotracheal tube, and then show a widening as the tube enters the lungs. If instead it shows the space getting tighter, the tube has been incorrectly placed in the esophagus. The patterns are so distinct, it is easy to tell with only the briefest of training whether they describe an esophagus or a trachea. In a comparison study, Raphael says, the acoustic reflectometer gave the same results as capnography in all but two patients. One patient was both morbidly obese and asthmatic, and in the other case, there was an equipment glitch.

Raphael admits that the current acoustic reflectometer is too bulky and too tethered for field use, where it could be critical for saving the lives of soldiers and victims of accidents and fires. Nearly 2 percent of patients who require an endotracheal tube outside of a hospital may suffer from having the tube placed in the esophagus in rushed emergency situations.

He says, “My next step is to develop a smaller, portable version of the acoustic reflectometer that could help save lives in any setting.”

Karen Heyman is a freelance writer in Los Angeles. Carol Chaplin contributed to this article.