Electrotherapy Basics
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Direct answer
Faradic for innervated, galvanic for denervated — that one line organises clinical electrotherapy better than any other. Currents are applied to relieve pain (gate control and opiate-mediated mechanisms), to stimulate intact muscle through its nerve, to re-educate weak muscles, to promote tissue healing and to drive drugs across skin by iontophoresis. Every prescription is defined by four parameters — frequency, intensity, pulse duration and duty cycle — and the machine that heals can also harm when contraindications such as pacemakers and pregnancy are ignored.
What you must remember
- Current families: direct current (galvanic, unidirectional, with chemical effects at the electrodes), alternating current (bidirectional, no net chemical effect) and pulsed currents of which faradic-type stimulation is the classic clinical example.
- The four dosage parameters — frequency in hertz, intensity in milliamperes, pulse duration in microseconds to milliseconds, and duty cycle (the on–off ratio, commonly 1:5 for muscle work to delay fatigue).
- Faradic-type current: short pulses of roughly 0.1–1 millisecond, classically surged at around 50 hertz, producing a tetanic contraction in normally innervated muscle — and nothing useful in denervated muscle.
- Denervated muscle must be excited directly with long-duration triangular or exponential pulses, classically tens to hundreds of milliseconds at low frequencies; the triangular shape matters because its slow rise lets nerve fibres accommodate while the slowly responding denervated muscle fibre is still excited.
- Gate control theory (Melzack and Wall, 1965): activity in large-diameter A-beta fibres inhibits pain transmission in the dorsal horn — the rationale for sensory-level analgesic currents.
- Physiological effects of motor stimulation: the muscle pump improves venous and lymphatic return, disuse atrophy is limited, and wasted activation patterns are re-educated after injury and surgery.
- Iontophoresis uses continuous direct current to drive like-charged drug ions into tissue — positively charged drugs under the positive electrode.
- Standing contraindications: cardiac pacemakers and implanted electronics, pregnancy over the abdomen and pelvis, active malignancy, deep vein thrombosis, stimulation over the carotid sinus, broken or anaesthetic skin, and undiagnosed pain.
Matching the current to the patient
Three patients, three choices. First, a stroke patient with a dropped wrist but intact innervation: faradic-type neuromuscular electrical stimulation over the wrist extensors, sensory-motor intensity, 20–50 hertz, duty cycle about 1:5, twenty minutes per session — the nerve is present, so short pulses reach it easily.
Second, a radial nerve palsy with electromyography-confirmed denervation. Short pulses now achieve nothing, because the target is the muscle membrane itself, which needs durations many times longer — long triangular pulses at very low frequencies. This is the classic exam discrimination, and getting it wrong wastes months of therapy; re-innervation is also monitored with the same electrodiagnostics.
Third, a painful post-operative knee needing analgesia alone: sensory-level stimulation, intensity below motor threshold, using the gate mechanism. For each patient the reasoning runs nerve-versus-muscle, strength-versus-sensory threshold, and dose-versus-fatigue. Safety screening is identical for all three — implanted devices, pregnancy, thrombosis, carotid region, skin integrity — and electrodes must sit on clean, healthy skin with good contact gel, never over open wounds or tumours.
Where students slip
The faradic-versus-galvanic indication is the most-scrambled fact in electrotherapy; the cleanest memory hook is that a muscle stripped of its nerve needs direct, long-duration stimulation. Students also swap the duty cycle's purpose (rest periods prevent fatigue, not comfort), assign iontophoresis drugs to the wrong polarity, and quote gate control backwards — large-fibre input closes the gate on pain. Viva panels ask the year and authors of gate control as standard.
Frequently asked questions
Why can faradic current not stimulate denervated muscle?
Short pulses excite nerve, not muscle membrane; denervated muscle lacks functioning nerve and needs long-duration direct stimulation.
What is the duty cycle, and why is 1:5 used for muscle stimulation?
The on–off ratio of stimulation; the rest phase delays fatigue so a useful contraction lasts the session.
Which theory explains sensory-level electrical analgesia?
Gate control — Melzack and Wall, 1965 — large-diameter afferent activity inhibiting dorsal horn pain transmission.
Under which electrode is a positively charged drug placed during iontophoresis?
The positive (anode) electrode, since like charges repel.
List the standard contraindications to electrical stimulation.
Pacemakers and implanted devices, pregnancy over abdomen or pelvis, malignancy, deep vein thrombosis, carotid sinus area, and anaesthetic or broken skin.