Electrotherapy

In short

Electrotherapy is the therapeutic use of electrical currents, electromagnetic fields, sound waves, light and thermal agents to relieve pain, stimulate nerve and muscle, promote tissue healing and reduce inflammation or oedema. It is a core clinical subject of the BPT curriculum (usually Electrotherapy I and II in the second and third year) with both theory and practical/viva papers, and it decides a large part of the skill assessment in clinical postings. Every physiotherapy job interview - hospital, polyclinic, sports or home-care - tests parameter selection, contraindications and electrical safety, because a wrong setting on an SWD or a stimulator can burn a patient. Practice in India is regulated by the National Commission for Allied and Healthcare Professions Act, 2021, under which physiotherapy is a recognised allied-health profession with state registration councils; equipment safety follows the CDSCO/BIS and Medical Devices Rules, 2017 framework. Employers expect a new graduate to set up, dose and document TENS, IFT, ultrasound, SWD, muscle stimulation, LASER and thermal modalities independently from day one.

Explained

Electrical basics and the physics behind the modalities

• Current (I, ampere) is the flow of charge; clinical doses are in milliampere (mA) or microampere (µA). Voltage (V) drives it; resistance/impedance (R, Z, ohm) opposes it.
• Ohm's law governs every low-frequency modality: skin impedance is high and falls as frequency rises, which is why 4,000 Hz interferential current penetrates comfortably while 50 Hz faradic current stings.
• Current types: *Direct current (DC)* - unidirectional, continuous (galvanic) or interrupted; *Alternating current (AC)* - bidirectional, e.g. interferential, Russian; *Pulsed current* - trains of pulses separated by rest, e.g. TENS, faradic.
• Pulse parameters examined: pulse duration/width (µs-ms), amplitude (mA), frequency (Hz or pps), rise time, duty cycle, phase charge, on-off ratio, ramp (surge) time.
• Frequency bands: low frequency (0-1,000 Hz - TENS, faradic, galvanic), medium frequency (1-100 kHz - IFT, Russian current), high frequency (>100 kHz - SWD at 27.12 MHz, microwave at 2,450 MHz).
• Tissue effects: *electrolytic/chemical* (DC), *electrophysical* (ion movement, membrane depolarisation), *thermal* (high-frequency), *mechanical* (ultrasound).
• Electrodes and coupling: metal plate with wet lint, carbon-rubber with gel, self-adhesive; current density = current ÷ effective electrode area, so the small electrode (active) gives the strong sensation and the large one is dispersive.

Low-frequency currents: stimulation and pain relief

• Galvanic (continuous DC): no nerve stimulation at steady state; used for iontophoresis (driving ions of a drug through skin - e.g. dexamethasone, acetic acid for calcific tendinitis), hyperhidrosis and as a diagnostic current. Risk of chemical burn: acid (HCl) under the anode, alkali (NaOH) under the cathode - the alkali burn is worse.
• Interrupted/Faradic-type current: short pulses (0.1-1 ms) at 30-70 Hz producing tetanic contraction of innervated muscle; used for re-education, oedema reduction (muscle pump), preventing adhesions and Faradism-under-pressure.
• Interrupted galvanic / long-duration pulses: 10-300 ms pulses stimulate denervated muscle, which needs long pulse durations because it has lost the fast-accommodating nerve fibre.
• Russian current: 2,500 Hz AC burst-modulated at 50 Hz for strong, relatively comfortable quadriceps strengthening.
• TENS (Transcutaneous Electrical Nerve Stimulation) - the most used modality in Indian OPDs:
  • *Conventional/high-TENS:* 80-120 Hz, 50-80 µs, sensory-level tingling - fast relief via gate control theory (Aβ fibres inhibit C-fibre transmission in the dorsal horn); short-lasting.
  • *Acupuncture/low-TENS:* 2-5 Hz, 150-250 µs, strong but tolerable motor twitch - slower onset, longer relief via endogenous opioids (endorphin/enkephalin).
  • *Brief-intense:* 100-150 Hz with wide pulse width for painful procedures; *Burst TENS:* bursts of 2 pps containing 100 Hz pulses.
  • Typical session 20-30 min, daily; home TENS units are inexpensive (roughly ₹1,000-₹4,000) and are commonly prescribed.
• Diadynamic and high-voltage pulsed current (HVPC) - twin-peak monophasic pulses at up to 500 V for oedema and wound healing.
• Microcurrent (MENS): sub-sensory µA currents claimed to aid tissue repair; evidence is limited - state this honestly in vivas.

Medium-frequency and electrodiagnosis

• Interferential therapy (IFT): two medium-frequency circuits (e.g. 4,000 Hz and 4,100 Hz) cross in the tissue; their interference produces an amplitude-modulated beat frequency = difference of the two (here 100 Hz) deep in the tissue, with low skin impedance and little discomfort.
  • *Beat frequency selection:* 1-10 Hz for muscle twitch/circulation, 80-130 Hz for acute pain (gate control), 0-100 Hz sweep for chronic pain and to avoid accommodation.
  • *Electrode set-up:* quadripolar (four electrodes, crossed - true interference) or bipolar (two electrodes, pre-modulated inside the machine). Vector/rotating scan sweeps the field to cover a larger area.
• Electrodiagnosis - strength-duration (SD) curve: plot of minimum current amplitude against pulse duration needed to excite.
  • Rheobase: smallest current that excites with a very long pulse. Chronaxie: pulse duration needed at twice rheobase - normal nerve chronaxie is well under 1 ms; denervated muscle shows a shifted curve to the right with chronaxie often >10 ms.
  • Accommodation - nerve adapts to a slowly rising current; slope/accommodation ratio distinguishes nerve from muscle excitation.
• Reaction of Degeneration (RD): faradic response absent, galvanic (long-pulse) response present and sluggish/worm-like - indicates denervation.
• Other tests: chronaxie, SD curve, nerve conduction studies (NCV), EMG, H-reflex, F-wave, and FES/NMES for functional restoration (e.g. foot-drop stimulator).

High-frequency and actinotherapy

• Short-wave diathermy (SWD): 27.12 MHz, legally allotted ISM frequency; deep heating of joints and muscles.
  • *Condenser-field (capacitive) method:* patient is part of the dielectric between two plates - heats fat strongly; *Cable/induction-coil (inductothermy):* eddy currents heat muscle and high-water-content tissue preferentially.
  • *Dosage* by patient-reported warmth (Schliephake grades: no sensation → mild → comfortable → max tolerable), 15-20 min. No metal, no wet dressings, no pacemaker, no over pregnancy, not over growing epiphysis, never on a patient with impaired sensation.
  • *Pulsed SWD (PSWD)* gives non-thermal effects for acute oedema.
• Microwave diathermy (MWD): 2,450 MHz, superficial-to-moderate depth, directional - special caution for eyes and testes.
• Therapeutic ultrasound: 0.75-3 MHz (commonly 1 MHz deep ~3-5 cm, 3 MHz superficial ~1-2 cm). Produced by the reverse piezoelectric effect in a crystal transducer.
  • *Thermal effects* with continuous mode; *non-thermal* (cavitation, acoustic streaming, micromassage) with pulsed mode (1:4, 1:9).
  • *Intensity* 0.1-3 W/cm² (commonly 0.5-1.5); BNR (beam non-uniformity ratio) should be low (2-6) - the applicator must be kept moving. ERA (effective radiating area) defines actual treated area.
  • *Coupling:* gel, water-bath (immersion) for irregular surfaces, water-filled bag; phonophoresis drives drugs through skin.
  • *Contraindications:* over eye, pregnant uterus, heart, malignancy, thrombophlebitis, cemented/metal implants with caution, over epiphyseal plates, after deep X-ray therapy.
• LASER (Low-level laser therapy): monochromatic, coherent, collimated light - HeNe 632.8 nm (visible, ~1-2 mm penetration) and GaAs/GaAlAs 830-904 nm (infrared, deeper). Used for wound healing, trigger points, tendinopathy; dose in J/cm² (1-4 J/cm² typical). Mandatory protective goggles and no beam toward the eye.
• Ultraviolet radiation (UVR): UVA/UVB/UVC; dosed by test dose for minimal erythemal dose (E1-E4); used for psoriasis and vitiligo (with PUVA) and wound sterilisation.
• Infrared radiation (IRR): luminous and non-luminous generators, superficial heating up to ~10 mm; inverse square law applies to dosage.

Thermal, mechanical and newer agents

• Superficial heat: hot packs, paraffin wax bath (45-50 °C, dip/glove/paint techniques - low specific heat and poor conductivity make it safe for hands), moist heat, fluidotherapy, hydrocollator. Effects: vasodilatation, analgesia, reduced muscle spasm, increased tissue extensibility.
• Cryotherapy: ice packs, ice massage, cold compression, vapocoolant spray; stages of sensation Cold → Burning → Aching → Numbness (CBAN); 10-20 min; part of PEACE & LOVE (updating the older RICE) in acute injury. Contraindicated in cold urticaria, Raynaud's, cryoglobulinaemia, impaired circulation or sensation.
• Contrast bath: alternating hot (38-42 °C) and cold (10-18 °C), typically 3:1 min ratio for ~20 min, for sub-acute oedema and RSD/CRPS.
• Traction: cervical (static/intermittent, 7-10% of body weight upward, 20-25° flexion) and lumbar (25-50% of body weight, needs a split table); indicated for radiculopathy and disc prolapse, contraindicated in instability, infection, malignancy, cauda equina signs and osteoporosis.
• Biofeedback (EMG): surface electrodes convert muscle activity into visual/auditory signals for re-education of weak or hyperactive muscles, pelvic floor and post-stroke relearning.
• Shockwave therapy (ESWT), magnetotherapy (PEMF), whole-body vibration, diathermy-based tecar are increasingly common in Indian private clinics; evidence strength varies by condition and should be quoted cautiously.

Safety, standards and Indian practice

• Professional regulation: the National Commission for Allied and Healthcare Professions Act, 2021 created the NCAHP and state councils; a physiotherapist must be registered on the state/central register to practise. Many older state Acts and the IAP's code of ethics continue to apply alongside; registration requirements vary by state - confirm with the state council.
• Device regulation: electrotherapy equipment is a medical device under the Medical Devices Rules, 2017 (Drugs and Cosmetics Act framework), licensed by CDSCO; most electrotherapy units fall in risk Class A or B. Devices must carry manufacturer and import licence details.
• Electrical safety standard: medical electrical equipment follows IS/IEC 60601 series (general requirements for basic safety and essential performance), with specific parts for nerve/muscle stimulators, SWD and ultrasound equipment. Class I earthed or Class II double-insulated construction, type BF/CF applied parts, periodic earth-continuity and leakage-current checks.
• Clinic wiring follows the National Electrical Code of India, SP 30 and IS 732 for wiring installations; earthing to IS 3043; RCCB/ELCB protection is essential where mains-powered modalities touch patients.
• Universal precautions before any modality: check skin sensation (hot/cold discrimination) and circulation, inspect skin, remove metal, explain the expected sensation, obtain consent, test the machine on yourself/output test, place the call-bell within reach, never leave an SWD patient unattended, document parameters.
• Absolute/general contraindications across electrotherapy: pacemakers and implanted electronic devices, pregnancy (abdomen/pelvis/low back), malignancy, active TB or acute infection, DVT/thrombophlebitis, haemorrhage, impaired sensation or cognition, over the carotid sinus, anterior neck, eyes, gonads and growing epiphyses, and on broken/irradiated skin.
• Documentation and billing: record modality, parameters, duration, electrode placement, patient response and progress; private clinic charges per session in India typically range about ₹200-₹800 depending on city and modality - these vary widely and are not fixed by any schedule. Health-care services by a clinical establishment are GST-exempt, while the equipment purchase attracts GST at the applicable slab.

Key points

• Electrotherapy - therapeutic use of electrical, electromagnetic, sonic, light and thermal energy for pain, healing and neuromuscular re-education.
• Current density - current per unit electrode area; decides sensation intensity and burn risk, not total current.
• Gate control theory - large-diameter Aβ afferent input closes the spinal "gate" to nociceptive C-fibre traffic - basis of high-frequency TENS.
• Opioid mechanism - low-frequency, high-intensity stimulation releases endorphins/enkephalins - slower onset but longer-lasting analgesia.
• Faradic current - short pulses (0.1-1 ms), 30-70 Hz, tetanic contraction of innervated muscle.
• Interrupted galvanic - long pulses (10-300 ms) needed for denervated muscle.
• Reaction of degeneration - no faradic response, sluggish galvanic response = denervation.
• Rheobase - minimum current at infinite (very long) pulse duration; chronaxie - duration required at twice rheobase.
• Accommodation - nerve's adaptation to slow-rising current; absent in denervated muscle, hence trapezoidal/triangular currents selectively stimulate muscle.
• Interferential beat frequency - equals the difference between the two medium-frequency carriers; deep effect with low skin resistance.
• SWD at 27.12 MHz - deep heat; capacitive field heats fat, inductothermy heats muscle; metal, moisture and pacemakers are absolute bars.
• Ultrasound - 1 MHz for deep (3-5 cm), 3 MHz for superficial (1-2 cm); pulsed mode for non-thermal, continuous for thermal effects.
• BNR - beam non-uniformity ratio; a high BNR with a static head causes hot spots and periosteal pain.
• LASER dose - energy density in J/cm² = power (W) × time (s) ÷ area (cm²); goggles mandatory.
• Wax bath - 45-50 °C is safe because paraffin has low specific heat and low thermal conductivity and gives off latent heat slowly.
• CBAN - cold, burning, aching, numbness - the sequence confirming adequate cryotherapy.
• Traction - cervical ~7-10% body weight, lumbar 25-50% with a split table; avoid in instability, cord signs or malignancy.
• Regulation - NCAHP Act, 2021 for the profession; Medical Devices Rules, 2017 (CDSCO) and IS/IEC 60601 for equipment safety.

Formulas and examples

Example 1 - Current density and safety. A galvanic active electrode measures 4 cm × 5 cm and the machine reads 6 mA.

• Area A = 4 × 5 = 20 cm².
• J = 6 ÷ 20 = 0.3 mA/cm² - well within the comfortable range for DC.
• If the therapist swaps to a 2 cm × 2.5 cm electrode (5 cm²) at the same 6 mA, J = 6 ÷ 5 = 1.2 mA/cm² - four times higher, approaching the burn threshold. Same meter reading, very different risk.

Example 2 - IFT beat frequency and sweep. Circuit 1 is fixed at 4,000 Hz; circuit 2 is set to 4,090 Hz.

• f_beat = 4,090 − 4,000 = 90 Hz - suitable for acute pain (gate control).
• To treat chronic pain, circuit 2 is swept from 4,001 to 4,100 Hz → beat sweeps 1-100 Hz, which prevents accommodation.
• For a muscle-pump effect on post-traumatic oedema, set circuit 2 to 4,010 Hz → 10 Hz beat.

Example 3 - Ultrasound dosage and time. A supraspinatus tendinopathy over an area of 12 cm² is treated with a 1 MHz head of ERA 5 cm², pulsed 1:4, intensity 1.0 W/cm², allowing 2 minutes per ERA-sized zone.

• Number of zones = 12 ÷ 5 = 2.4.
• Treatment time = 2.4 × 2 = 4.8 min ≈ 5 minutes.
• Spatial-average energy delivered = 1.0 W/cm² × 5 cm² × 288 s = 1,440 J nominal; with a 1:4 duty cycle (20% on) the actual acoustic energy = 0.2 × 1,440 = 288 J, i.e. an effective intensity of 0.2 W/cm² - which is why pulsed mode is chosen when heating is unwanted.

Example 4 - LASER exposure time. A GaAlAs 830 nm probe delivers 100 mW continuous over a spot of 0.25 cm²; the prescription is 4 J/cm².

• Required energy = 4 J/cm² × 0.25 cm² = 1.0 J.
• Time = Energy ÷ Power = 1.0 ÷ 0.1 W = 10 seconds per point.
• For 6 trigger points, total "on" time = 60 s and total energy = 6 J.

Example 5 - Iontophoresis dose and traction load. (a) Dexamethasone iontophoresis is prescribed at 60 mA·min. At 4 mA tolerance, time = 60 ÷ 4 = 15 minutes. If the patient tolerates only 3 mA, time = 60 ÷ 3 = 20 minutes for the same dose. (b) A 70 kg adult needs lumbar traction at 30% body weight → 0.30 × 70 = 21 kgf; the same patient for cervical traction at 8% → 0.08 × 70 = 5.6 kgf, usually begun lower (about 3-4 kgf) and progressed.

Where it is used

• Hospital and corporate physiotherapy departments (AIIMS, PGIMER, CMC Vellore, Apollo, Fortis, Manipal) - OPD pain clinics, post-operative orthopaedics, ICU chest physiotherapy, neuro-rehab with NMES and biofeedback.
• Private and polyclinic practice - the commonest first job: IFT, TENS, ultrasound, SWD, wax and traction for low back pain, cervical spondylosis, frozen shoulder and knee osteoarthritis.
• Sports and fitness - IPL and state academies, Sports Authority of India centres, sports clinics: cryotherapy, ESWT, LASER, compression and NMES in return-to-play protocols.
• Equipment industry and sales/application roles - Indian manufacturers and suppliers of electrotherapy units (Technomed, Acculife, Zealmax, Enraf-Nonius/Chattanooga distributors): product demonstration, clinical application training, service and CDSCO/BIS compliance.
• Community, geriatric and home-care physiotherapy - portable TENS and home programmes under schemes and insurance panels; wound care with HVPC/LASER in diabetic foot clinics.
• Academics, research and rehabilitation councils - BPT/MPT teaching posts, clinical trials on modality effectiveness, NCAHP-linked curriculum and registration work.

Interview questions

• Q: Why does interferential therapy feel more comfortable than TENS at the same depth of effect? A: Its 4 kHz carrier meets much lower skin impedance than a low-frequency current, so the sensory receptors in the skin are stimulated less while the beat frequency acts deep in the tissue.
• Q: How do you decide between faradic and interrupted galvanic current? A: Faradic (short pulses) for innervated muscle re-education; interrupted galvanic with long pulses for denervated muscle, because denervated fibres need a much longer pulse duration and do not accommodate.
• Q: What is the reaction of degeneration? A: Loss of response to faradic current with a retained but sluggish, worm-like response to long-duration galvanic current - evidence of denervation of the muscle.
• Q: Which ultrasound frequency would you choose for de Quervain's tenosynovitis and why? A: 3 MHz, because the structure is superficial (1-2 cm) and 3 MHz is absorbed more rapidly in the superficial layers; 1 MHz would deposit most energy deeper than the target.
• Q: Why must the ultrasound head always be kept moving? A: The beam is non-uniform (BNR of 2-6), so a static head creates hot spots that cause unstable cavitation, periosteal pain and tissue damage.
• Q: Why is a metal implant a problem for SWD but handled differently for ultrasound? A: Metal concentrates the high-frequency electromagnetic field and causes localised burns, so SWD is contraindicated; ultrasound is a mechanical wave and metal implants are a relative caution, though reflection and cemented prostheses need care.
• Q: Which TENS parameters would you set for chronic low back pain not responding to conventional TENS? A: Switch to acupuncture-like TENS - about 2-4 Hz with wide pulse width (150-250 µs) at a strong but comfortable motor-level intensity for 30 minutes, to recruit the endogenous opioid mechanism.
• Q: What is the first thing you check before any heating modality? A: Thermal (hot/cold) sensation and circulation in the area, plus skin condition - impaired sensation turns a therapeutic dose into a burn.
• Q: How is a LASER dose calculated and what safety rule is non-negotiable? A: Energy density (J/cm²) = power × time ÷ spot area; both patient and therapist must wear wavelength-specific goggles and the beam must never be directed at the eye.
• Q: Why is paraffin wax at 50 °C tolerable when water at 50 °C would scald? A: Wax has a low specific heat and low thermal conductivity and releases its latent heat slowly, so the rate of heat transfer to the skin is much lower than from water.
• Q: Which electrode is "active" and why does it matter? A: The smaller electrode - it carries a higher current density, produces the therapeutic sensation or contraction, and is also where burns occur if it is poorly applied or the skin is broken.
• Q: Under which law do you register to practise physiotherapy in India, and under which rules is your IFT machine regulated? A: Registration is under the National Commission for Allied and Healthcare Professions Act, 2021 through the central/state council; the equipment is a medical device under the Medical Devices Rules, 2017 administered by CDSCO, built to the IS/IEC 60601 safety series.

Common mistakes

• Quoting the machine's total current instead of current density, and then using a small electrode at the same mA - the commonest cause of electrical burns.
• Treating a denervated muscle with faradic current and recording "no response" as muscle death instead of switching to long-duration pulses.
• Confusing the carrier frequency of IFT (4,000 Hz) with the beat frequency (1-150 Hz) in exam answers, or forgetting that the beat equals the difference of the two carriers.
• Choosing 1 MHz ultrasound for a superficial lesion (or 3 MHz for a deep one) and holding the head stationary over a bony prominence.
• Skipping the sensation and circulation test and metal check before SWD, hot packs or wax - and leaving an SWD patient without a call bell.
• Reversing polarity in iontophoresis: the drug ion must be placed under the electrode of the same sign so it is repelled into the tissue.
• Writing "contraindicated in pregnancy, malignancy, pacemaker" as a blanket list without stating which site or modality - examiners expect the reason, and for TENS the bar is the anterior neck, carotid sinus and pelvis rather than every application.

Related topics

Exercise therapy · Orthotics and prosthetics · Musculoskeletal and sports physiotherapy · Neurological rehabilitation · Human anatomy and neurophysiology of pain · Clinical assessment and electrodiagnosis (EMG/NCV)