Introduction
This topic covers the medical use of constant (direct) current and of low-frequency pulsed currents: galvanisation, iontophoresis, and the electrical stimulation of nerves and muscles. It rests on two ideas: the body conducts current through its ionic fluids, and excitable tissue responds only to a stimulus of sufficient strength and duration.
Physical basis in the tissue
Tissue is a mixed conductor. Current flows mainly through the ionic fluid of blood, muscle and extracellular space, and the resistance of each tissue differs, so current concentrates in the more conductive pathways. The cell membrane behaves as a capacitor in series with a resistance, which is why a steady current has its main effect at the boundaries of the tissues and at the electrodes. Under a direct current, the ions accumulate at the electrodes and polarisation of the tissue develops. Current strength follows Ohm's law:
Galvanisation and iontophoresis
Galvanisation applies a steady low current, usually a few milliamperes for several minutes, through electrodes on the skin. It is used to improve local blood flow, reduce oedema and pain, and support healing. The anode and the cathode produce different local effects because the polarity drives ions in opposite directions, so the electrode arrangement is chosen according to the aim of treatment.
Iontophoresis uses direct current to move charged drug ions through the skin. The electrode carrying the same charge as the drug repels it into the tissue, so the drug is delivered locally with fewer systemic effects. Examples are dexamethasone for local inflammation, and pilocarpine, which is delivered by iontophoresis in the sweat test used to diagnose cystic fibrosis.