On April 1, Trost’s heart was operated on by Arthur Ng, a cardiac surgeon with an independent surgery group based in Hackensack, New Jersey. Trost was in the hospital for a week before going home to recover.

Within weeks, the couple began receiving the types of bills you’d expect from this type of experience: A few hundred for a hospitalist, a few hundred more for a urologist. Insurance pitched in, and Rosalsky wrote checks for the rest.

But the biggest bill was eye-popping: $32,845 for Ng’s services. United Healthcare offered to pay just $4,000 of that total.

Rosalsky provided a copy of the bill to The Atlantic, but Ng’s office did not return requests for comment.

The couple had encountered what’s known as a “balance bill”: The term for a situation in which an entire hospital stay or procedure is covered by insurance, but one of the specialists involved is out-of-network and bills the patient separately.

“Mike's life was in jeopardy,” Rosalsky said. “There was little time to think rationally. It would never have occurred to us to ask every individual, ‘Are you in my network?’"

Not all surprise medical bills are balance bills. However, nearly one-third of privately insured people have received a surprise medical bill in which their insurer paid less than they expected, according to a nationwide survey of more than 2,000 people conducted by the Consumer Reports National Research Center earlier this year. Balance bills for hospital visits sometimes occur when the doctor sends the patient a second, separate bill, on top of the one from the hospital.

Surprise bills might be likelier to occur in situations like Trost’s, where the patient lives in one state, is covered by a plan in a different state, and is treated by a provider based in a third state. About 5.2 million people commute to a different state for work, according to the Census Bureau.

“In the New York City metro area for example, there are hundreds of thousands of people who commute across the ‘tri-state’ area of New York, New Jersey and Connecticut,” said Chuck Bell, program director for Consumers Union, the policy and advocacy arm of Consumer Reports. “Ultimately, the varying state laws and regulations can be extremely confusing for consumers.”

Balance billing may become more common for all Americans as insurers narrow their networks, according to Sandy Ahn, a research fellow at Georgetown University's Health Policy Institute.

Rosalsky appealed Ng’s bill with her insurance, but the plan denied her appeal in a letter, which she also provided to The Atlantic. Rosalsky began “freaking out” and “panicking,” she said. She searched online for information and discovered that New York had recently passed a law that prohibits balance billing.

“I thought, ‘Holy shit, does this apply to us?’"

But then she took a closer look at her insurance-denial letter. “Please be advised that the New York State Surprise Bill Act is for New York State residents,” it read.