It's an exceptionally busy day at the Minneapolis neonatal intensive care unit at Children's Hospitals and Clinics of Minnesota. But even in the midst of a harried shift, the staff tries to keep the unit as quiet as possible for their struggling newborns.
At the center of the room is an infant who will spend months in here. She was born three months early and is no bigger than her nurse's hands.
"She's actually doing very, very well," Dr. Ellen Bendel-Stenzel whispers in the dimly lit patient room.
Taped to the side of the newborn's mouth is a clear plastic tube that pumps air directly into her fragile lungs. The ventilator, a new model getting a test run, is ultra-quiet.
Even though the ventilator sounds quiet to the adults in the room, and the baby is inside a box-like Plexiglas incubator, the ventilator tube could be conveying a significant amount of noise into the baby's body.
"Normally you think, well who cares if sound goes into a tube into a baby's lungs? That doesn't affect their hearing or their sensation of hearing. But, in fact, it does because it's being conducted up the gum line to the skull bones to the inner ear," said Dr. James Sidman, an ear, nose and throat specialist at Children's.
Sidman knows about what's called "bone conduction" because of his work with hearing-impaired patients. Some of the hearing aids he prescribes amplify skull or tooth vibrations to help patients hear better.
But until now, no one had really researched whether this same sound path could expose newborns to too much ventilator noise. So Sidman, author of the ventilator sound study, decided to measure the sound levels of five different ventilators at the points where the tubing is secured on the baby's head. That's where the tubes transfer vibrations like this to the skull.
When Sidman got his data back, he was surprised to find that that ventilator tubes produce sound pressure levels anywhere from 60 to 115 decibels in the spots where they make contact with facial bones.
"One hundred fifteen decibels is the guy working under the airplane directing the airplane at the airport," he said. "And that's what our babies are living with as long as they're on the ventilator, which could be weeks or occasionally months."
A lot more research will be needed to prove Sidman's theory that bone conduction is a source of hearing loss. But he says that shouldn't stop manufacturers from doing what they can now to minimize ventilator noise as much as possible.
Several factors other than bone conduction are known contributors to hearing loss in babies, including some medications, low birth weight itself, and a lack of oxygen during birth. Studies show that only between 2 and 10 percent of very low birth weight babies end up with permanent hearing loss, and ventilator noise may not be the culprit.
It's important to address Sidman's findings and lower the risk of hearing damage as much as possible, but parents should also know that the odds of good hearing are still in their baby's favor, Bendel-Stenzel said.
She called Sidman's findings hopeful.
"I don't think it should cause alarm and concern in our families that our graduates are coming out of our NICUs deaf, because they aren't," she said "Most of them are coming out with normal hearing."