Matthew Groff is a car surgeon who saves lives for a living. He is a Fairfax County firefighter, an apparatus technician assigned to Engine and Rescue 421 in Fair Oaks, to be precise.

What that means is that when there is an accident with people trapped in a vehicle, his job is to cut open the car quickly and get the injured out and to the hospital. When he is not answering calls at Fire Station 21, he's teaching recruits and other rescue personnel the skills of the trade.

For Groff, 34, and the hundreds of other rescue squad members in Northern Virginia -- as well as others across the country -- there is a new hazard in their already dangerous workplace: hybrid cars. The automobiles, powered by electricity and gas, can deliver a lethal jolt of juice if a rescue worker cuts in the wrong place before the engine is shut off or the power is isolated to the battery.

Hybrids have high-voltage nickel-metal hydride batteries, stored in the trunk, that are connected to the engine by cables that run along the underbelly of the chassis. Groff said the hybrids generally carry about 265 volts.

About 5,000 hybrids are registered in Virginia, most of them in Northern Virginia. J.D. Power and Associates has estimated that the number of hybrids on the nation's highways will increase from 40,000 last year to about a half-million by 2008. Hybrids can get 60 miles per gallon of gasoline.

The cars are equipped with safety systems that shut down the power between the battery and engine when the cars are involved in accidents or when the engine is turned off. But one problem with the hybrid is that the electric motor is so quiet, it is difficult to hear whether it is running. A light on the instrument panel indicates whether the engine is on, but rescue workers might not be able to see the light if the car is badly damaged or the panel is obstructed by debris or passengers.

"They are spooky," Benjamin Barksdale, assistant chief of operations for the Arlington County Fire Department, said of the hybrids. "When the gasoline engine shuts down, you don't hear that vehicle at all. There is no noise, which can present situations that can be hazardous if that electric motor is still running and you assume the car is off."

Although the popularity of hybrids has increased in the past two years, not many have been involved in serious accidents in Northern Virginia, so rescue squad members have not had much practice working on them.

Part of the training by instructors in Northern Virginia involves field trips to Honda or Toyota dealerships to become familiar with the hybrids.

"It gives us a great perspective to see the thing up on the lift and see where the cables are located," Groff said. He said his courses consist of classroom study in the morning and visits to a Fairfax Toyota dealership in the afternoon for a lecture by the head mechanic and a chance to eyeball a Prius, Toyota's hybrid model.

Terry Davis, manager of Ourisman Fairfax Toyota, said the dealers want to do everything they can to help the rescue squad members. He said the hybrids have identifying features, as well as distinctive license plates in Virginia.

Davis usually has a Prius for test drives that Groff uses for his class.

Davis said he believes the reason few, if any, hybrids have been involved in serious accidents in Northern Virginia is the nature of the car and driver. "The majority of people who have been buying the Prius are doing it for the gas mileage," he said, "and the electric motor is used at lower speeds. The gas mode kicks in at the higher speed. I think the people who buy these cars don't drive as fast."

Car manufacturers are planning to introduce more hybrids to the market. And unlike the Prius, which has a distinctive design, the new vehicles will be hybrid versions of existing models, such as the Honda Civic and Toyota Highlander.

That will pose problems for rescue workers when they have to determine quickly whether it is a gas-powered vehicle or hybrid involved in the accident.

"When we get to an accident scene," Groff said, "each member of the team has their own job that they have to do. At an auto extraction, the things you have to determine [are] how many patients are there, are they trapped and the significance of their injuries. And now we have to look at the vehicle and determine whether it is a hybrid or not."

Groff and other rescue squad personnel use cars from junkyards to practice their extractions. But he has yet to use his equipment on a hybrid in practice or on the job.

Fairfax County firefighter Matthew Groff holds the tool that is used to cut through a vehicle's sheet metal when victims are trapped inside crashed vehicles.