In the same way that the Watt is the unit of power, the joule is the unit of energy. As Steve pointed out, one joule is the amount of energy delivered at a 1-watt rate for one second.

The cardiologist doesn't really care about voltage or current; he just wants to know how much energy is being delivered to the patient. Hence, the use of the joule as the measurement.

In its simplest form, a defibrillator is a capacitor with a charging circuit, a discharge switch, and a network that produces an optimal discharge waveform. The capacitor is charged to the desired energy level, then discharged through the electrodes.

The energy level is related to the voltage to which the capacitor is charged by this equation:

E = C * V^2 / 2

where E is the energy in joules, C is the capacitance in farads, and V is the voltage.

The capacitor in a 200-joule defibrillator is around 100 uF, so it needs to be charged to 2 kV (!) to reach 200 joules:

E = (100e-6 * 2000^2 / 2) = 200 J

By the way, if you ever grab a 100 uF capacitor with 2 kV across it, it will be a memorable experience.

Notice that the energy increases as the

square of the voltage; charging the same capacitor to twice the voltage (4 kV) yields four times the energy (800 J, not 400 J). For the same reason, a car moving at 60 MPH has

four times the kinetic energy of one moving at 30 MPH.

...put some current through to stimulate or regulate or whatever.

Contrary to what many people think from watching TV, the main purpose of the shock is not to start the heart, but to

stop a life-threatening irregular heartbeat. It's a sort of great big reset pushbutton.

I hope everything turns out well for your friend.

Edit: I'll get these UBL codes right the first time some day...[This message has been edited by John Crighton (edited 02-21-2007).]