Ultrasound-based Endotracheal Tube


May 8, 1998

Experts in emergency medicine and ultrasound have teamed up at the University of Rochester to better one of the most vital tools in the paramedic's array of equipment: endotracheal tubes, the flexible tubes inserted in the throats of hundreds of thousands of unconscious patients each year to ensure continuous delivery of oxygen to their lungs. The new device, soon to receive a U.S. patent, takes advantage of the same ultrasound technology now used by physicians to observe babies developing in the womb.

Endotracheal tubes are relied upon by thousands of doctors, surgeons, and ambulance crews every day, but today's devices sometimes do more harm than good. Amid distractions like sirens and flashing lights, dangerous debris, and the cries of victims, rescuers can actually cause life-threatening complications by unwittingly inserting the tube in a patient's esophagus rather than the trachea, sending oxygen to the stomach instead of the lungs. Most experts estimate this occurs in 2 to 5 percent of patients, though a few doctors have suggested that the problem is much more common. If they're not found and corrected, such errors can lead not only to insufficient ventilation but also to stomach rupture, airway trauma, irreversible damage to the heart or brain, or even death.

The new endotracheal tube sidesteps these perils with "smarts" derived from ultrasound transducers smaller than a pencil's eraser -- miniature versions of the transducers in ultrasound scanners currently used by doctors to monitor fetal development, cardiac disease, and tumors. These transducers, which are attached to the lower end of the foot-long tubes, emit ultrasonic pulses that are detected by a small receiver on the front of the patient's neck only if the device is correctly inserted.

"This development takes advantage of human anatomy as well as one of the fundamental properties of ultrasonic waves -- that they don't travel through air," says Jack Mottley, associate professor of electrical engineering. "If a transducer-equipped endotracheal tube is accidentally placed in the esophagus, the ultrasonic wave it emits won't reach a receiver mounted on the front of the throat. That's because the esophagus is located behind the trachea in the neck, and the ultrasonic wave is blocked by the air in the trachea."

In tests on human cadavers, Mottley and collaborator Randy Lipscher, a former resident in emergency medicine at Rochester now affiliated with several hospitals in the Austin, Tex., area, found that their design can also serve to pinpoint the location of the end of the endotracheal tube within the trachea -- a critical concern. "If the lower end of the tube is just a half- inch too deep in the trachea, only one lung will receive oxygen," Mottley says. "But if our endotracheal tube shifts within the trachea, you'll know immediately, because the ultrasonic signal will no longer reach the receiver."

The Rochester airway is the first to offer continuous monitoring of its own location within the trachea. A green light on the system's hand-held battery-powered unit indicates that the endotracheal tube is correctly inserted in the trachea, and an alarm sounds if the tube shifts too much within the trachea at any time during treatment or transport to the hospital.

"Some patients are intubated by anesthetists in the O.R. before an operation, and in this quiet, well-lit setting, faulty intubation is rare," says Linda Spillane, an assistant professor of emergency medicine at Rochester. "But much more often it's done by paramedics in the field -- a noisy, uncontrolled setting where intubation may be greatly complicated by patients who are bloody or vomiting, or who have trauma to the face or throat. There's also sufficient jostling en route to the hospital to dislodge even the sturdiest airway."

Emergency care providers and anesthetists have become increasingly attuned to the pitfalls of intubation in recent years, and as a result there have been a number of attempts to make endotracheal tubes more user-friendly. One design incorporates a small light bulb mounted within the tube that can be seen shining through the thin skin on the front of the neck, but the bulb's light remains visible only in dimly lit surroundings. All endotracheal tube designs also feature a radio- opaque stripe that can be observed using conventional X-rays, but these can't be used to check the airway until the patient reaches a hospital.

Paramedics and emergency physicians got their first glimpse of the new endotracheal tube at last summer's annual meeting of the National Association of EMS Physicians. "There was a great deal of interest in the device," says Spillane, who was not involved in the research.

A patent for Mottley and Lipscher's endotracheal tube has been allowed by the U.S. Patent Office and will issue soon; the University is now seeking a corporate partner to produce the device. Adding tiny transducers and wiring to the disposable tubes should add little to their cost, Mottley says. He anticipates that transducer-equipped tubes could sell for slightly more than the $7 now charged. The system's battery unit would represent an additional one-time expense of $50 to $100 for an ambulance squad or hospital.

The research was funded by the departments of electrical engineering and emergency medicine at the University.

CONTACT: Steve Bradt, (716) 273-4726.