“There’s a saying that if you’ve seen one emergency medical system, you’ve seen one emergency medical system — no two are alike,” said Dr. Robert E. O’Connor, a vice president of the American College of Emergency Medicine and chairman of the department at the University of Virginia. Charges and payments, he said, “are all over the place.” Fire departments, which don’t charge for driving to fire alarms, do charge for ambulance runs.

In such a fragmented system, it is hard to know how much high-priced ambulance transport contributes nationally to America’s $2.7 trillion health care bill. And total out-of-pocket expenditures by individuals are hard to tally.

But Medicare, the insurance program for the elderly, does tabulate its numbers and has become alarmed at its fast-rising expenditures for ambulance rides: nearly $6 billion a year, up from just $2 billion in 2002.

That is true even though Medicare’s fixed payments for ambulance rides — ranging from $289 to $481 in 2011 — are far lower than commercial rates. Ambulance companies complain that Medicare rates do not meet the costs of running what are essentially mobile emergency rooms staffed by highly trained professionals.

In a recent study, the federal Health and Human Services Department’s Office of the Inspector General noted that the Medicare ambulance services were “vulnerable to abuse and fraud,” in part because there were lax standards on when an ambulance was needed and how the trip should be billed. The number of transports paid for by Medicare increased 69 percent between 2002 and 2011, while the number of Medicare patients increased only 7 percent during that period. In the last year, two ambulance companies have pleaded guilty or settled claims for overbilling Medicare.

The Affordable Care Act requires policies to include some coverage for emergency care as an essential benefit, including ambulance transport. But the ambulance ride and the care are billed separately. Many Silver plans — a lower-tier plan — require patients to pay an initial copay of $250 for the emergency room and $250 more for the transport, for example.