“What I was worried most about was holding my breath if I panicked,” said the 64-year-old diver, because that could have caused a fatal embolism in his lungs. “Panic is what really kills people.”

Begnoche and other volunteers dove Sept. 18 to capture shipwreck footage off the coast of Grand Marais, Mich. After he was pulled onto the boat, he started feeling prickly sensations in his body and he lost control of his arms and legs. His rapid ascent caused nitrogen bubbles to form in his bloodstream, cutting off blood to his spinal cord and other parts of his body.

This decompression sickness was so severe that saving Begnoche required a helicopter rescue to Marquette, Mich., and then a low-altitude flight to Hennepin County Medical Center in Minneapolis — where doctors tried a rare treatment protocol that they looked up in the U.S. Navy Diving Manual.

Along with a nurse, Begnoche spent a record 53 hours last weekend in HCMC’s hyperbaric chamber, a room in which air pressure can be adjusted, to give his body a chance to slowly clear out the damaging nitrogen bubbles.

“It’s really rare to have to do that protocol for a couple reasons,” said Dr. Chris Logue, medical director of HCMC’s center for hyperbaric medicine. “One is that most people die. They never make it to treatment.”

HCMC nurse Kimberlee Nerling helped wheel Terry Begnoche into a hyperbaric chamber on Friday. Nerling spent two days with Begnoche in the chamber as he recovered from an accident.

Begnoche was like a “human soda bottle” that had been shaken up, Logue said, and caregivers needed to figure out how to deflate the bubbles.

Most of the time, HCMC’s hyperbaric chamber simulates a depth pressure of 45 feet to 60 feet below the surface and promotes healing by creating an oxygen-rich environment for patients suffering severe wounds or conditions such as anemia or carbon monoxide poisoning.

In this instance, Logue needed to simulate a depth of 165 feet — only 10 feet above the chamber’s maximum limit — to mimic Begnoche’s actual dive depth. That would drive the nitrogen in his body from bubbles into a solution and allow it to escape his body slowly and safely.

That’s what would have happened naturally if Begnoche ascended slowly from the lake as planned, with three or four decompression stops on the way to the surface.

Nurse Kimberlee Nerling, who was in the chamber with Begnoche the entire time, said she noticed immediate movement in the man’s arms when the chamber reached its maximum depth.

Begnoche has since regained control of his arms, and concerns about his lung functioning have been resolved. But the semiretired environmental manager and university lecturer cannot yet walk or move his legs.

He is receiving additional daily treatments in the hyperbaric chamber in the hopes that they will stimulate healing and reduce inflammation in his spinal cord. Whether he can fully recover is unclear.

“It’s such a rare event, it’s impossible to tell,” Logue said. “What we’ve gained back, I’m thrilled about.”

Begnoche, who lives in Michigan, had movies to entertain him during two-plus days in the chamber. He struggled to sleep and replayed how the dive went wrong.

Loaded with underwater lighting and a video camera to examine the recently discovered Nelson, a three-masted schooner that sank off the coast of Grand Marais, Mich., in 1899, Begnoche had neglected to put on a weight that would have otherwise controlled his ascent. The project was part of his work with the Great Lakes Shipwreck Museum.

Given the amount of oxygen in his tank, he had only 20 minutes to spend by the ship. At 19 minutes, just to play it safe, he started to rise and pointed the video camera back down at the ship on the lake’s floor. That’s when he noticed how rapidly he rose past another diver.

With 40 years of experience and numerous deep-water dives, Begnoche knew he couldn’t stop his momentum.

“I was doing everything I could do to push any air out of the suit or any buoyancy devices that I had,” he said, but with the pressure decreasing and the gasses in his suit and his body expanding, it was “like a self-fulfilling prophecy.”

When he reached the surface, Begnoche immediately submerged a few feet to try to decompress and then returned to the boat when his oxygen tank was nearly empty. A Coast Guard helicopter was dispatched along with a boat from the Pictured Rocks National Lakeshore Rangers.

Flying Begnoche to Minneapolis was complicated, because a high-altitude flight would have worsened his decompression sickness. So the pilot was ordered to keep the plane below 800 feet.

Nerling, the nurse, was in for an unusual shift, having never spent more than three hours in the hyperbaric chamber before. The 53 hours in the chamber was necessary for her as well. Bringing them out of a simulated depth of 165 feet any faster would have caused her to suffer decompression sickness.

Terry Begnoche equalized his ear pressure as he laid in a hyperbolic chamber at HCMC in Minneapolis, Minn., on Friday, September 26, 2014, after a diving incident in Lake Superior that almost killed him. ] RENEE JONES SCHNEIDER • reneejones@startribune.com

So Nerling made sure her daughter could stay with her father and grabbed a textbook she needed to study for an upcoming nurse practitioner exam.

“It’s me and you,” she told Begnoche in the chamber.

Once the air pressure in the chamber rose back to a simulated depth of 60 feet, additional caregivers were allowed in so that Nerling could rest.

Begnoche said he is grateful for the help he received in the lake, for the national Divers Alert Network that assessed his injuries and referred him to HCMC, and for the proximity of the hyperbaric chamber in Minneapolis.

He hopes that continued hyperbaric treatments along with physical therapy will help him regain strength, but he’s also worried. In the water, his expertise helped him focus and stay calm. In the hospital, he is burdened with the knowledge of just how devastating a rapid ascent can be to the body.

“It’s scary,” he said, “especially when you know what’s going on.”