why was the rectangle jealous of the triangle?

Hello it’s been awhile but we are back again J This time we thought we might share some specifics on the best type of stimulation for denervated muscle as well as how quickly to stimulate.  Just for some background information so you understand the difference once we get into this subject, nerves respond best to quick pulses of less than 1ms in a shorter square shaped waveform. As we told you before, long duration pulses are best muscle for stimulation.

But why is this the best for denervated muscle?

In the simplest terms possible, if the rate of the rise of the current is slow then it won’t provoke a nerve impulse because of the ionic balance across the nerve fiber membrane and is able to adjust itself so the threshold potential rises in response to the applied electric charge, which is known as accommodation (Low et al 1994). The ability to accommodate is much more marked in a nerve than in a muscle because muscle tissue needs a greater electrical charge at a slower spread as it is less excitable than nerves. 

So what pulse is best for this?

Triangular, or accommodation, pulses are most effective for denervated muscle.

There are two types of long duration pulses, accommodation and rectangular. A rectangular pulse is typically 1-600ms separated by pulse interval of 1ms-several seconds.  It is best for stimulation of motor and sensory nerves and denervated muscle.  An accommodation pulse can also be termed as triangular, trapezoidal, sawtooth, senate, slow-rising, shaped, and selective.  They are typically 300-1000ms separated by pulse intervals of ½ to several seconds..

So why is a triangular waveform better than a rectangular waveform if they both work for denervated muscle?

Because of the way that muscles accommodate, when the pulse is long and rectangular, a spike is felt at the beginning of the pulse, and the rest of the time, the muscle builds resistance to the current flowing at a constant voltage (Roberston et al 2006).  However, because a triangular waveform rises so slowly, it bypasses the nerve stimulation level and heads right to the correct stimulation for a muscle, as well as increases the demand on the muscle, rather than leave the stimulation constant, which generates the contraction. The muscle cannot accommodate, or accommodates very slowly, which allows the slow current flow to push the membranes to potential allowing for excitability and contraction of the muscle.

When would I start this stimulation?

Generally, it is best to start soon after the onset of denervation.  This is because the sooner it begins, the less of a chance of atrophy the muscle has.  This is still true even when the nerve might regenerate.  The nerve can take months to regenerate with a muscle, and years to fully heal, so the less atrophied the muscle is, the better chance of regeneration there is.  Just keep in mind the following: When a nerve is severed, both the distal and proximal ends are affected.  However, in some cases, the axon can regenerate, creating sprouts at the axonal tips.  The rate of regeneration is very slow, but studies show that electrical muscle stimulation can reduce or inhibit sprouting (Robinson et al 2008).

Want some more info? J

A study done by Kern et al 2002 showed that biphasic pulses are not necessarily better functionally, but are safer than monophasic pulses.  Monophasic pulses let the ions build up under the skin for hours at a time and can cause burns. So, stick to the biphasic waveforms when doing such a long treatment.

We will leave you to absorb all that. Until Next Time!

Team North America Signing off-

-Megs and Brooke

References:

Bardy, G, Ivey, T., Allen, M., Johnson, G, Mehra, R., Greene, H. (1989) A prospective randomized evaluation of biphasic versus monophasic waveform pulses on defibrillation efficacy in humans. Journal of the American College of Cardiology. 14:728-733.

Low, J., Reed, A. (1994) Electrotherapy Explained: Principles and Practice (2^nd Ed).  China: Elsevier Limited. Great Britain: Clays Ltd.

Roberston, V., Robertson, V., Low, J., Ward, A., Reed, A. (2006) Electrotherapy Explained: Principles and Practice (4^th Ed).  Oxford: Elsevier Limited.

Robinson, A., Snyder-Mackler, L. (2008) Clinical Electrophysiology: Electrotherapy and Electrophysiologic Testing. Maryland:  Lippincott Williams and Wilkins.