Car batteries are designed to provide a large amount of amperage, to turn the starter with a high amount of torque via an electric motor. Generally this is done at 12 or 24 volts.
The current recommendations for a defibrillator requires 200V, and a very small amount of amperage. This is one of three settings in a standard defib procedure. The second setting is 300V, and the final is 360V. The point of defibrillation is not to provide power to the heart, it is to disrupt the heart's built-in electrical (biological) circuitry, such that the heart can go to rest (not beating).
Many cardiac events involve the heart beating in an uncoordinated manner, such that many or all of the muscle fibers in the heart beat in a manner that doesn't provide the squeezing action on the chambers to push the blood forwards. Defibrillation seeks to correct such an uncoordinated beating by resetting the heart to rest, after which it will hopefully start beating again in a coordinated manner.
In short, attaching a car battery is more likely to cook the heart (if it even gets that deep) because it is providing too much power, and it is unlikely to disrupt the heart's electrical field because it is too low of a voltage. With this in mind, one could theoretically attach enough electrical transformation equipment to a car battery to use it as a power source for defibrillation. But there are other issues with such an idea.
Car batteries are not designed for the usage that would mesh well with a defibrillation machine. Car batteries are designed to be recharged. Your car battery basically drains pretty heavily upon starting a car, and then the alternator recharges it by harvesting mechanical power from the engine during your trip. This heavy drain / heavy charge cycling is hard on batteries, and car batteries have been specifically designed to endure in such a scenario.
A defibrillator machine has a different use scenario. Basically it sits idle for long periods of time, and when it does require charging, it gets electricity from a wall socket, at higher voltage (and generally lower amperage). This means it tends to charge more slowly, which in general means longer battery life. Also the battery is built differently, to hold charge better at the expense of not being recharged as quickly.
In short, a car battery is really the wrong tool for the job.