What is Constant Current Stimulation?
All constant current stimulators are designed to maintain the stimulus current at a preset value during stimulation, irrespective of the resistance of the stimulation pathway. In situations where the resistance is continuously changing, the stimulator increases or decrease the voltage supplied in order to to keep the current flow at the required level. The voltage source in a constant current stimulator is known as the “compliance voltage” and this maximum voltage is usually specified in the product data sheet.
So what does “Out of Compliance” mean?
Anyone who routinely uses electrical stimulation should already be familiar with Ohm’s Law (V=IR). In constant current stimulation, Ohm’s law allows you to predict what voltage will be needed to deliver a specific current if the resistance of the preparation/electrode is known. For instance, if you want to deliver a 10mA stimulus through a 10kohm resistance you will need a stimulator with a voltage source of 100V. If the stimulator only has a compliance limit of 50V, the maximum current delivered will only be 5mA, even if the device is set to 10mA. If the actual current being delivered is of importance, it is critically important that out of compliance issues are avoided.
How will I know when my stimulator is out of compliance?
For many of our constant current devices (e.g the NL800A, DS3 and DS4), there is no indication that current you have requested has not been delivered, so it is of great importance that the resistance of your stimulation pathway is routinely measured with an electrode impedance checker (e.g. our D175) or multimeter, in order to check that the voltage available will be adequate. It is also important to add stimulator battery checks to your weekly lab routine, as the compliance voltage of all battery powered devices will reduce as the batteries are used.
Do any Digitimer devices offer out of compliance warnings?
Yes, our mains powered DS5, DS7A/AH and DS8R (in development) all feature out of compliance warnings. In the case of the DS7A and DS7AH there is a warning LED associated with the voltage control dial, which alerts the user that the device is out of compliance. If it does light up, the user should increase the voltage (if possible) or examine how the resistance at the skin/electrode interface can be reduced. The DS5 and DS8R have an out of compliance icon on the front panel display, which is only visible when the stimulator detects a stimulus that failed to meet the requested current.
How do I avoid out of compliance issues in the future?
Of prime importance is selection of a stimulator that has an adequate compliance voltage to meet your typical requirements. The 400V compliance of the DS7A is usually adequate for human transcutaneous stimulation, while the 90V of the DS3 is suitable for low current stimulation through high impedance electrodes. As mentioned above, it is important to know Ohm’s Law and apply it to your practical work. Check battery voltages routinely and get to know when units are likely to need their batteries replacing. Finally, try and keep electrode resistances as low as possible. In the case of in vitro or animal research, the electrode size and impedance is usually restricted by the size of the preparation, but in human application, impedance can be reduced massively by good skin preparation and larger surface area electrodes.