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Electricity guide

BASIC CONCEPTS FOR ELECTROTHERAPY TREATMENT This page is for healthcare staff with basic knowledge of the physiology of pain.


Current, voltage and resistance

Current is designated as I and measured in amperes (A). The current used in TENS, NMES and electroacupuncture is so weak that we measure it in milliamperes (mA). The current strength is also called amplitude. Resistance is designated as R and is measured in ohms (W). The skin’s resistance when treating with electrodes is 1,000–1,500 W. During electroacupuncture treatments, the skin resistance is only about 600 W because the skin is penetrated with a needle. Voltage is designated as U and is measured in volts (V). Voltage is the force that causes the flow of current.

The stimulator battery is its energy source. Cefar stimulators have a constant current generator, which means that when you set an amplitude of 20 mA, the stimulator constantly produces that amplitude regardless of the resistance. It also means that the voltage varies depending on the resistance. These three parameters – current, voltage and resistance – make up Ohm’s law: U = R x I

Wave form

Treatment with electrical stimulation requires a closed circuit for the current. This requires two electrodes in TENS or NMES, and two needles in electroacupuncture. Cefar stimulators have alternating current in a biphasic wave form. A biphasic wave form can be symmetrical or asymmetrical. TENS has a biphasic asymmetrical wave form, which means that polarity makes a difference. In practice, this means that there is greater activity under the black electrode, which can be of importance in deciding where to place the electrodes.
NMES and electroacupuncture have a biphasic symmetric wave form, which means that the activity under both electrodes/needles is the same. It makes no difference where you place the red and black electrodes/needles.

Frequency

Frequency means the number of pulses per second, and is measured in hertz (Hz). In electrotherapy, you can choose to treat patients with different frequencies. High-frequency TENS and electroacupuncture involve frequencies of 50–120 Hz. Cefar’s stimulators are often preset to 80 Hz.

High-frequency TENS 50–120 Hz

Low-frequency TENS and electroacupuncture are designed to generate current in bursts of eight pulses. This makes stimulation more pleasant to the patient. Low-frequency stimulation is usually at 2 Hz.

Low-frequency TENS 2 Hz in bursts (2 bursts per sec, 8 pulses per burst)

With modulated pulse duration high-frequency TENS and electroacupuncture, 80 Hz is often used. Modulation means that the pulse duration changes during a specific time interval. For example, it may drop from 180 µs to 70 µs, then go up to 180 µs again in 2 seconds.

Modified pulse duration high-frequency TENS 50–120 Hz

NMES usually uses frequencies of 20–120 Hz. The frequency varies depending on the purpose of the stimulation. Frequencies under 20 Hz provide heat, recovery or enhanced circulation through vibration. Higher frequencies (>20 Hz) are used to facilitate or to contract muscles.

Pulse duration

Each pulse lasts for a specific time. This is called the pulse duration and is measured in microseconds (µs) or milliseconds (ms). The pulse duration for TENS and electroacupuncture is usually 180 µs. In NMES, the pulse duration varies depending on how strong you want the stimulation to be. It might be 200 µs for smaller muscle groups and 400 µs for larger ones.

Pulse duration and amplitude

Amplitude is the force of the current and is measured in milliamperes, or thousandths of an ampere. The amplitude is always adjusted individually for each treatment, preferably by the patient him/herself. The pulse duration is a way to regulate the amount of energy (compare with amplitude). A small pulse duration is suitable for treating sensitive, nerve-rich areas like the face. With a smaller pulse duration, you can increase the amplitude higher, which makes it easier to find the right level without risking any pain for the patient with minimal increases in amplitude.

Current density of skin electrodes and needles

If you use small electrodes (such as 40 x 60 mm) with an amplitude of 20 mA, the stimulation will feel stronger than if you use larger electrodes. This is because the current “spreads out" on the larger electrode and isn’t so concentrated. If you treat the patient with one large and one small electrode, the patient will feel stronger stimulation under the smaller one. The current is more concentrated in electroacupuncture, since the needle has such a small area. A lower amplitude is usually used for this, around 5 mA.