Average Medicare charges for emergency medicine and anesthesiology, the most common sources of surprise medical bills, grew faster than all other specialties, a new analysis finds.

The analysis, released Friday by the Brookings Institution, comes as Congress debates how to handle the problem of surprise medical bills.

Average charges to Medicare for emergency medicine grew by 6.7% annually from 2012 to 2017. Payments for anesthesiology increased by 4.4% annually over the same time period. In comparison, the average annual growth rate for all other specialties’ Medicare charges was 1.6% from 2012 to 2017.

“Interestingly, though, while still at high levels relative to other specialties, radiology and pathology charges relative to Medicare rates have grown roughly in line with other specialties since 2012,” the study found.

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Emergency medicine and anesthesiology are the most common sources of surprise billing because they are often staffed by outside contractors who could be outside of a patient’s insurance network. So while a patient goes to a hospital that is in their network, they could be treated by an anesthesiologist that isn’t carried by their insurance.

That patient would then face a high out-of-network charge.

“Because charges are unilaterally set and patients typically have no choice over the specific provider who treats them regardless of cost, charges for emergency and ancillary clinicians are subject to minimal market constraint,” the analysis said.

The analysis, based on federal data on charges for providers treating Medicare patients, comes as progress to tackle surprise billing in Congress has stalled.

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Provider groups and private equity firms responsible for staffing anesthesiologists and emergency medicine doctors have launched massive ad campaigns against legislation that uses a benchmark median rate for out-of-network charges. Providers charge that insurers would be able to game the system and charge higher rates.

Providers want instead to use a “baseball-style” arbitration process to handle out-of-network charges in which the insurer and provider submit a number to an independent arbiter, who then chooses one.