Low-frequency pulsed currents. Classification, parameters, physiological and therapeutic effects. Application methods, dosage, indications, and contraindications

01Classification

Electrotherapy with pulses works because nerves fire in pulses too. A pulsed current sends a brief push, a pause, another push. If each push is strong enough and long enough, it fires the nerve once. How often, how strong and in what shape the pushes come decides what the patient feels and what the muscle does.

By polarity

By pulse shape

No shape is better in itself; its rise time and the charge it carries decide what it fires.

By modulation

Nerves stop responding to an unchanging stimulus: they adapt. Modulation keeps changing the stimulus to delay that.

• Unmodulated: steady pulses at one frequency. The book calls 80 to 150 Hz inhibitory and 1 to 50 Hz irritative.
• Burst (group) modulation: pulses come in trains with pauses, usually for muscle stimulation.
• Amplitude modulation: pulse strength rises and falls, usually for analgesia.
• Frequency modulation: pulse rate changes, usually for analgesia.

By clinical use (the book's list)

02Parameters

the low-frequency band

up to 1,000Hzthe low-frequency band

the traditional analgesic band (100 Hz the classic)

80–150Hzthe traditional analgesic band (100 Hz the classic)

a smooth tetanic contraction

50Hza smooth tetanic contraction

03Physiological and therapeutic effects

The gate control theory (Melzack and Wall, 1965) is the simplest picture of electroanalgesia. In the dorsal horn of the spinal cord, signals from thick, fast touch fibres switch on small inhibitory interneurons that turn down the message from thin pain fibres (Aδ and C). Rubbing a bumped elbow works the same way. Strong low-frequency stimulation adds a second, slower mechanism: the brain's own opioids.

04The currents one by one

Pulsed galvanic current (impulse galvanization)

A monophasic rectangular current: pulses of one polarity on top of, or instead of, a steady galvanic current. The book gives 100 to 150 Hz; the lecture 1 to 100 Hz with pulses of 0.1 to 10 ms. It joins galvanic effects (polarity, hyperaemia) with pulse effects (a fine "micro-massage" of tissue, analgesia), with less skin irritation than continuous galvanization. The more galvanic component, the more inhibitory; the more pulse component, the more irritative. Intensity at sensory level, 10 to 20 minutes. Technique, indications and contraindications are those of galvanization.

Tetanic (faradic) current

Short rectangular pulses (0.1 to 1 ms) at about 50 Hz, delivered in trains of a few seconds with rests between, so the muscle gives a smooth, fused (tetanic) contraction and then relaxes. The name faradic comes from Faraday's induction coil, which first produced such a current. It is used to strengthen innervated muscle and prevent disuse atrophy, not for pain. Electrodes go along the muscle, one above the other, or one over the motor point. The modern form of this current is NMES, where the parameters are covered in detail.

Exponential current, for denervated muscle

When a peripheral nerve is cut or crushed, the muscle loses its nerve: there is nothing to fire with short pulses, and the muscle fibres themselves must be fired. They need long pulses (their chronaxie is long), and they have lost most of their accommodation. A slowly rising pulse therefore fires the denervated fibres while healthy nerves and skin receptors, which accommodate, ignore it: the contraction is selective and less painful.

The book sets each parameter from electrodiagnosis, and the logic is the same for each: the worse the denervation, the more the current must be adapted.

As the nerve regrows, the pulse front is made steeper and shorter. The lecture gives pulses of 10 to 500 ms at 1 to 10 Hz, 10 to 20 minutes, electrodes along the muscle; the two ranges go together only one way round, because the longer the pulse, the fewer fit into a second (a 500 ms pulse allows at most about one a second).

Electroanalgesic currents

• TENS, the most used of all: biphasic pulses of 50 to 400 µs at 1 to 150 Hz, worn for long periods at home. It has its own topic.
• High-voltage pulsed current (HVPC). Very short twin-peaked monophasic pulses (about 50 to 200 µs) at 100 to 500 V (the book gives 150 V) and 1 to 120 Hz (the book gives 80 to 200 Hz for analgesia). The voltage is high, so it passes the skin easily, but each pulse is so short that the average current is tiny, so it hardly heats and hardly polarises. Used for pain, muscle re-education, oedema and, with some evidence, wound healing.
• Microcurrent. Current in microamperes (10 to 1,000 µA, below sensation), monophasic or biphasic, at low frequencies, applied over painful areas, trigger points or acupuncture points (micro-electroanalgesia in the book). It aims at tissue repair and pain without any sensation or contraction. Evidence is weak.

Diadynamic (Bernard) currents

Pierre Bernard, a French dentist, made these in the 1950s from mains current: 50 Hz alternating current, reduced in voltage and rectified so that it flows one way only. Keeping only one half of each wave gives 50 half-waves a second (MF); keeping both halves, flipped the same way, gives 100 (DF). By alternating and modulating them he made five classic forms.

1. 1

   DF, about 1 minute

   Strong, quick relief and lower skin resistance.
2. 2

   CP, a few minutes

   Longer relief; the change of rhythm delays adaptation.
3. 3

   LP, the longest part

   The longest-lasting relief.

The lecture's analgesic sequence is about 1 minute of DF, about 5 of CP and about 10 of LP. Many devices also let a small steady galvanic "base" current run under the pulses, which adds galvanic effects and the same burn risk.

Diadynamic currents are monophasic, so they have a direct-current component: wet pads, clean skin, polarity chosen as in galvanization (the painful point usually under the cathode in Bernard's original method, the electrodes often small and round over the painful point and along the nerve). They are quickly adapted to, so each session is short and a course is usually short too, about 6 to 10 daily sessions; the forms are changed to keep them working.

05Application methods

• Electrodes: carbon rubber with wet pads for monophasic currents (they need the pad's protection), self-adhesive gel electrodes for biphasic ones.
• Placement for pain: over or around the painful area, along the course of the nerve, over the nerve root paravertebrally (segmental), or over trigger or acupuncture points.
• Placement for muscle: one electrode over the motor point (where the nerve enters the muscle, the point that contracts it with the least current) and one over the muscle belly or tendon, or both along the muscle; a smaller active electrode localises the contraction.
• Placement for denervated muscle: both electrodes along the muscle belly, since there is no motor point to use.
• Monopolar or bipolar: a small active electrode with a large one elsewhere concentrates the effect under the small one; two equal electrodes spread it between them.

06Dosage

The intensity often needs a small cut in the first minutes, as skin resistance falls under wet pads. Patients adapt within minutes to a steady analgesic current, so modulation (or changing the diadynamic form) keeps it working rather than turning it up.

07Indications

Grouped by what the current does:

• Because it relieves pain: acute and chronic musculoskeletal pain (neck, low back, osteoarthritis), sprains, bruises, myofascial pain and spasm, neuralgias and radicular pain, post-traumatic and postoperative pain.
• Because it contracts muscle: disuse atrophy after immobilisation or surgery, weakness and inhibition, re-education; denervated muscle after peripheral nerve injury (exponential and triangular currents).
• Because contraction and vasodilation move fluid: oedema, poor local circulation.
• Because of polarity effects (monophasic currents): trophic problems, slow wounds (adjunct).

08Contraindications

The same reasoning as for every current through the body.