If necessity is the mother of invention, whose parent is sudden, life-altering, bone-numbing tragedy?

When engineer Marie Johnson's husband fell to an unforeseen heart attack nine years ago, it sparked a passionate pursuit of a medical device to eliminate sudden cardiac death.

Johnson's company, AUM Cardiovascular, is nearing the threshold of FDA approval for a handheld device that can quickly spot blockages in the blood vessels around the heart with a few measurements.

Her husband, Robert Guion, was a 41-year-old engineer at Lockheed Martin who was studying to become a minister. Johnson was working on her Ph.D. at the University of Minnesota and working with 3M Co. on a tool to detect heart murmurs.

She had listened to her husband's heart months before his death, gathering data for her study.

Then one day she went to meet him at a YMCA where he had been swimming. With a young daughter and infant son in tow, she had just pulled into the parking lot when she saw an ambulance -- and a body covered with a sheet. Police told her it was her husband.

"You are not going to fully believe how different your life is after that," she said. "Everything changed. Everything."

Before her husband died, Johnson believed she'd heard a heart murmur. Three months after his death, she went to work -- developing an algorithm that identified a problem that had not been medically diagnosed. "I found a signature associated with something," she said.

In the beginning, the project used sound to detect blockages in the blood vessels around the heart -- using an electronic stethoscope. Now, she said, "we pick up subsonic pressure waves" through pressure sensors. By picking up changes in pressure within the blood vessel, Johnson said, the CADence can detect "clinically relevant lesions." It is much like how stream currents change as water rushes around rocks.

Those lesions can lead to heart attacks.

Said Johnson: "Our current studies say we're more accurate than a treadmill stress test."

A 2006 clinical study presented to the American Heart Association showed promise. After two postdoctoral studies in Italy and at Stanford University, Johnson became director of the University of Minnesota's Medical Devices Fellows Program.

The project won a $244,000 Qualifying Therapeutic Discovery Project grant from the IRS in 2010. It was a turning point, Johnson said. "All of a sudden, I had this money and I thought, 'Well, I should do something.'"

She left the U and went to work on CADence full time. In September, AUM won the 2011 Minnesota Cup competition, beating out more than 1,000 participants. She said she is now completing another round of financing for $2.5 million. Johnson is the only full-time employee. AUM has a contract employee and uses 15 consultants.

Still ahead: a March 2012 clinical trial. Johnson said CADence could become available in October 2013, pending FDA approval.

Stephen Parente, director of the University of Minnesota's Medical Industry Leadership Institute, has worked closely with Johnson over the years. He said she combines the relentless drive of an engineer with the passion of someone dedicated to a cause. "Marie has endless energy. There is never a wall that she will find that can thwart her," he said.

Johnson has no doubt whatsover where her path is leading. "It says that God will stand behind you and whisper go to the right, go to the left," she said of her belief in divine guidance. "I just know."

James Walsh • 612-673-7428