Electrotherapy Concepts.

As we saw in my recent article, (Electrotherapy, FES and it's historical roots) Electrotherapy and it’s cousin, electrical stimulation, have some deep and colourful roots. The stories of applying electrical energy to the body for therapeutic purposes have sometimes stretched the imagination, have sometimes inspired quackery but also produced some fantastic results along the way.

In simple terms, the aim is to deliver electrical energy to the tissues of the body in a precisely controlled manner, that will bring about physiological change and by this means achieve some therapeutic benefit.

Over the decades we have been learning how to precisely deliver this energy to control the effects and do so safely. if the technology can be set up to stimulate sensory nerves the result will be a sensory outcome and if the motor nerves are stimulated for example a motor effect will be the result.

In actual fact, whenever we introduce this energy to the tissues - typically through the surface of the skin via electrodes - we are “bathing” all of the tissues beneath with the energy, but by the choice of the stimulation waveform parameters we can choose to have a primary effect on one type of tissue - be it muscle or nerve.

What we know is far from complete. We know that the shape and various characteristics of the energy waveforms and how they vary over time (frequency, pulse shape and width, magnitude, dose etc) when presented to the tissues impacts upon the result. Attempts have been made to define “Therapeutic Windows”. This implies that we can optimise the therapeutic effect by how we vary the characteristics of the electrical energy. This is quite hard to do as some many factors are involved and they change over time.

A close look at electrotherapy and modern devices (such as the Stimulette Edition 5) reveals that the characteristics of the electrical energy can be programmed and delivered to

• encourage muscles that are innervated but weak to get stronger

• encourage muscles that are denervated to reinnervate or at least get stronger

• reduce various types of pain whether acute, chronic, neuropathic and so on

• relax muscles that are tense and in spasm

• encourage wound healing

We can take advantage of what we know of tissue physiology to either nudge or force therapeutic change.

If we take the example of using electrical energy to stimulate a motor nerve which results in a muscle contraction, the muscle doesn’t “know” the difference between the signal that comes via the brain and spinal cord and the signal that is delivered local to that muscle via electrical stimulation. The electrical stimulation changes the physiology in the same way as the bodies normal mechanisms do.

For example, nerves are like insulated cables with a potential voltage difference of some 70 mV from the outside to the inside. The nerve is said to have a “resting” membrane potential because this polarisation is present even when the nerve is in its resting state. This polarisation is achieved by the flow of ions across the membrane. This state is maintained naturally and the result is that the nerve is always ready to fire. A so called action potential (nerve impulse) is in effect a temporary change of state of the nerve membrane that flows sequentially along its length. Once the nerve impulse is initiated it follows an all or nothing behaviour.

The only difference between what happens in normal physiology and in electrical stimulation is that in the latter the action potential is “forced” by the external stimulus. The nerve fires so long as the stimulus is sufficiently large to push the membrane potential over it’s threshold value. It’s an “all or nothing” response. To use electrical stimulation this way results in a muscle contraction.

This can be achieved with a small current for a relatively longer time or a larger current for a shorter time. There is a certain minimum current needed to fire a nerve impulse - it’s referred to as the Rheobase. The relationship between strength of contraction and pulse duration might look something like this.

In future articles we will examine some of the most common applications for electrotherapy and available via devices such as the Stimulette Edition 5