The VNS Pulse Generator is a constant current device (mA).

The resistance of the system is the resistance of the lead plus the resistance of the vagus nerve between the positive and negative electrode.

The resistance of the lead from patient to patient would be equal but the resistance of the vagus nerve from patient to patient would be different.

Since resistance is somewhat variable and the current can be adjusted, this means that the voltage must be regulated to deliver the desired current.

The generator has a converter inside that steps up or down the voltage.

First, the basic electrical units shall be defined as follows :

            a) Voltage » potential difference » energy per electrical charge

            b) Ampere » current » electrical charge per second

            c) Ohm » resistance

It’s quite simple and follows Ohm’s law nicely : V = IR

            a) The potential difference required to drive a given current through a given wire equals the                            product of the current strength and the resistance of the wire.

            b) Expressed algebraically V = IR, V = the potential difference or the voltage, I = the current                                    strength, and R = the magnitude of the resistance

            c) This can also be stated as volts = (amperes)(ohms)

The VNS device, when starting a stimulation cycle, ramps up to the programmed output current.

After reaching this current, it actually turns on and off many times during the stimulation. This “on and off” action is dictated by pre-programmed settings; the wavelength being the distance between two successive crests of a wave, the frequency being the number of crests that pass a given point in one second, and the unit of frequency being the Hz.

The frequency and the pulse width (characterized by its own ON time which is measured in microseconds) are characteristics of the square wave (which changes from one amplitude to the other in a short time compared with the wavelength). For example, if the device’s ON time is 30 seconds and the frequency is 20 Hz, the device will turn on and then off 20 times per second for the duration of the 30 seconds.

This in short is a basic outline behind the physics of the VNS Pulse Generator.